= Google JavaScript Style Guide :sectlinks: Revision 2.93 Aaron Whyte + Bob Jervis + Dan Pupius + Erik Arvidsson + Fritz Schneider + Robby Walker + Table of Contents [width="100%",cols="50%,50%",] |========================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================= a| link:#javascript-language-rules[JavaScript Language Rules] a| link:#var[var] link:#constants[Constants] link:#semicolons[Semicolons] link:#nested-functions[Nested functions] link:#function-declarations-within-blocks[Function Declarations Within Blocks] link:#exceptions[Exceptions] link:#custom-exceptions[Custom exceptions] link:#standards-features[Standards features] link:#wrapper-objects-for-primitive-types[Wrapper objects for primitive types] link:#Multi-level_prototype_hierarchies[Multi-level prototype hierarchies] link:#method-and-property-definitions[Method and property definitions] link:#delete[delete] link:#closures[Closures] link:#eval[eval()] link:#with___%7B%7D[with() \{}] link:#this[this] link:#for-in-loop[for-in loop] link:#associative-arrays[Associative Arrays] link:#multiline-string-literals[Multiline string literals] link:#array-and-object-literals[Array and Object literals] link:#modifying-prototypes-of-builtin-objects[Modifying prototypes of builtin objects] link:#internet-explorer-s-conditional-comments[Internet Explorer's Conditional Comments] a| link:#javascript-style-rules[JavaScript Style Rules] a| link:#naming[Naming] link:#custom-tostring-methods[Custom toString() methods] link:#deferred-initialization[Deferred initialization] link:#explicit-scope[Explicit scope] link:#code-formatting[Code formatting] link:#parentheses[Parentheses] link:#strings[Strings] link:#visibility--private-and-protected-fields[Visibility (private and protected fields)] link:#javascript-types[JavaScript Types] link:#comments[Comments] link:#Providing_Dependencies_With_goog.provide[Providing Dependencies With goog.provide] link:#compiling[Compiling] link:#tips-and-tricks[Tips and Tricks] |========================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================= [[Important_Note]] == Important Note === Displaying Hidden Details in this Guide This style guide contains many details that are initially hidden from view. They are marked by the triangle icon, which you see here on your left. Click it now. You should see "Hooray" appear below. Hooray! Now you know you can expand points to get more details. Alternatively, there's a "toggle all" at the top of this document. [[Background]] == Background JavaScript is the main client-side scripting language used by many of Google's open-source projects. This style guide is a list of __do__s and __don't__s for JavaScript programs. [[JavaScript_Language_Rules]] == JavaScript Language Rules === var Declarations with `var`: Always Decision: When you fail to specify `var`, the variable gets placed in the global context, potentially clobbering existing values. Also, if there's no declaration, it's hard to tell in what scope a variable lives (e.g., it could be in the Document or Window just as easily as in the local scope). So always declare with `var`. === Constants * Use `NAMES_LIKE_THIS` for constant __values__. * Use `@const` to indicate a constant (non-overwritable) _pointer_ (a variable or property). * Never use the https://developer.mozilla.org/en/JavaScript/Reference/Statements/const[`const` keyword] as it's not supported in Internet Explorer. Decision: Constant values If a value is intended to be _constant_ and __immutable__, it should be given a name in `CONSTANT_VALUE_CASE`. `ALL_CAPS` additionally implies `@const` (that the value is not overwritable). Primitive types (`number`, `string`, `boolean`) are constant values. `Objects`' immutability is more subjective — objects should be considered immutable only if they do not demonstrate observable state change. This is not enforced by the compiler. Constant pointers (variables and properties) The `@const` annotation on a variable or property implies that it is not overwritable. This is enforced by the compiler at build time. This behavior is consistent with the https://developer.mozilla.org/en/JavaScript/Reference/Statements/const[`const` keyword] (which we do not use due to the lack of support in Internet Explorer). A `@const` annotation on a method additionally implies that the method cannot not be overridden in subclasses. A `@const` annotation on a constructor implies the class cannot be subclassed (akin to `final` in Java). Examples Note that `@const` does not necessarily imply `CONSTANT_VALUES_CASE`. However, `CONSTANT_VALUES_CASE` _does_ imply `@const`. ------------------------------------------ /** * Request timeout in milliseconds. * @type {number} */ goog.example.TIMEOUT_IN_MILLISECONDS = 60; ------------------------------------------ The number of seconds in a minute never changes. It is a constant value. `ALL_CAPS` also implies `@const`, so the constant cannot be overwritten. The open source compiler will allow the symbol to be overwritten because the constant is _not_ marked as `@const`. -------------------------------------------------- /** * Map of URL to response string. * @const */ MyClass.fetchedUrlCache_ = new goog.structs.Map(); -------------------------------------------------- ----------------------------------- /** * Class that cannot be subclassed. * @const * @constructor */ sloth.MyFinalClass = function() {}; ----------------------------------- In this case, the pointer can never be overwritten, but value is highly mutable and not constant (and thus in `camelCase`, not `ALL_CAPS`). === Semicolons Always use semicolons. Relying on implicit insertion can cause subtle, hard to debug problems. Don't do it. You're better than that. There are a couple places where missing semicolons are particularly dangerous: --------------------------------------------------------------------------------- // 1. MyClass.prototype.myMethod = function() { return 42; } // No semicolon here. (function() { // Some initialization code wrapped in a function to create a scope for locals. })(); var x = { 'i': 1, 'j': 2 } // No semicolon here. // 2. Trying to do one thing on Internet Explorer and another on Firefox. // I know you'd never write code like this, but throw me a bone. [ffVersion, ieVersion][isIE](); var THINGS_TO_EAT = [apples, oysters, sprayOnCheese] // No semicolon here. // 3. conditional execution a la bash -1 == resultOfOperation() || die(); --------------------------------------------------------------------------------- So what happens? 1. JavaScript error - first the function returning 42 is called with the second function as a parameter, then the number 42 is "called" resulting in an error. 2. You will most likely get a 'no such property in undefined' error at runtime as it tries to call `x[ffVersion, ieVersion][isIE]()`. 3. `die` is always called since the array minus 1 is `NaN` which is never equal to anything (not even if `resultOfOperation()` returns `NaN`) and `THINGS_TO_EAT` gets assigned the result of `die()`. Why? JavaScript requires statements to end with a semicolon, except when it thinks it can safely infer their existence. In each of these examples, a function declaration or object or array literal is used inside a statement. The closing brackets are not enough to signal the end of the statement. Javascript never ends a statement if the next token is an infix or bracket operator. This has really surprised people, so make sure your assignments end with semicolons. Clarification: Semicolons and functions Semicolons should be included at the end of function expressions, but not at the end of function declarations. The distinction is best illustrated with an example: ------------------------ var foo = function() { return true; }; // semicolon here. function foo() { return true; } // no semicolon here. ------------------------ === Nested functions Yes Nested functions can be very useful, for example in the creation of continuations and for the task of hiding helper functions. Feel free to use them. === Function Declarations Within Blocks No Do not do this: ------------------- if (x) { function foo() {} } ------------------- While most script engines support Function Declarations within blocks it is not part of ECMAScript (see http://www.ecma-international.org/publications/standards/Ecma-262.htm[ECMA-262], clause 13 and 14). Worse implementations are inconsistent with each other and with future EcmaScript proposals. ECMAScript only allows for Function Declarations in the root statement list of a script or function. Instead use a variable initialized with a Function Expression to define a function within a block: -------------------------- if (x) { var foo = function() {}; } -------------------------- === Exceptions Yes You basically can't avoid exceptions if you're doing something non-trivial (using an application development framework, etc.). Go for it. === Custom exceptions Yes Without custom exceptions, returning error information from a function that also returns a value can be tricky, not to mention inelegant. Bad solutions include passing in a reference type to hold error information or always returning Objects with a potential error member. These basically amount to a primitive exception handling hack. Feel free to use custom exceptions when appropriate. === Standards features Always preferred over non-standards features For maximum portability and compatibility, always prefer standards features over non-standards features (e.g., `string.charAt(3)` over `string[3]` and element access with DOM functions instead of using an application-specific shorthand). === Wrapper objects for primitive types No There's no reason to use wrapper objects for primitive types, plus they're dangerous: ------------------------------ var x = new Boolean(false); if (x) { alert('hi'); // Shows 'hi'. } ------------------------------ Don't do it! However type casting is fine. ---------------------------------------------- var x = Boolean(0); if (x) { alert('hi'); // This will never be alerted. } typeof Boolean(0) == 'boolean'; typeof new Boolean(0) == 'object'; ---------------------------------------------- This is very useful for casting things to `number`, `string` and `boolean`. === Multi-level prototype hierarchies Not preferred Multi-level prototype hierarchies are how JavaScript implements inheritance. You have a multi-level hierarchy if you have a user-defined class D with another user-defined class B as its prototype. These hierarchies are much harder to get right than they first appear! For that reason, it is best to use `goog.inherits()` from http://code.google.com/closure/library/[the Closure Library] or a similar library function. --------------------------------- function D() { goog.base(this) } goog.inherits(D, B); D.prototype.method = function() { ... }; --------------------------------- === Method and property definitions `/** @constructor */ function SomeConstructor() { this.someProperty = 1; } Foo.prototype.someMethod = function() { ... };` While there are several ways to attach methods and properties to an object created via "new", the preferred style for methods is: -------------------------------- Foo.prototype.bar = function() { /* ... */ }; -------------------------------- The preferred style for other properties is to initialize the field in the constructor: ------------------- /** @constructor */ function Foo() { this.bar = value; } ------------------- Why? Current JavaScript engines optimize based on the "shape" of an object, https://developers.google.com/v8/design#prop_access[adding a property to an object (including overriding a value set on the prototype) changes the shape and can degrade performance.] === delete Prefer `this.foo = null`. ------------------------------------ Foo.prototype.dispose = function() { this.property_ = null; }; ------------------------------------ Instead of: ------------------------------------ Foo.prototype.dispose = function() { delete this.property_; }; ------------------------------------ In modern JavaScript engines, changing the number of properties on an object is much slower than reassigning the values. The delete keyword should be avoided except when it is necessary to remove a property from an object's iterated list of keys, or to change the result of `if (key in obj)`. === Closures Yes, but be careful. The ability to create closures is perhaps the most useful and often overlooked feature of JS. Here is http://jibbering.com/faq/faq_notes/closures.html[a good description of how closures work]. One thing to keep in mind, however, is that a closure keeps a pointer to its enclosing scope. As a result, attaching a closure to a DOM element can create a circular reference and thus, a memory leak. For example, in the following code: ------------------------------------------------------ function foo(element, a, b) { element.onclick = function() { /* uses a and b */ }; } ------------------------------------------------------ the function closure keeps a reference to `element`, `a`, and `b` even if it never uses `element`. Since `element` also keeps a reference to the closure, we have a cycle that won't be cleaned up by garbage collection. In these situations, the code can be structured as follows: ------------------------------------------- function foo(element, a, b) { element.onclick = bar(a, b); } function bar(a, b) { return function() { /* uses a and b */ }; } ------------------------------------------- === eval() Only for code loaders and REPL (Read–eval–print loop) `eval()` makes for confusing semantics and is dangerous to use if the string being `eval()`'d contains user input. There's usually a better, clearer, and safer way to write your code, so its use is generally not permitted. For RPC you can always use JSON and read the result using `JSON.parse()` instead of `eval()`. Let's assume we have a server that returns something like this: -------------------------------------- { "name": "Alice", "id": 31502, "email": "looking_glass@example.com" } -------------------------------------- ------------------------------ var userInfo = eval(feed); var email = userInfo['email']; ------------------------------ If the feed was modified to include malicious JavaScript code, then if we use `eval` then that code will be executed. -------------------------------- var userInfo = JSON.parse(feed); var email = userInfo['email']; -------------------------------- With `JSON.parse`, invalid JSON (including all executable JavaScript) will cause an exception to be thrown. === with() \{} No Using `with` clouds the semantics of your program. Because the object of the `with` can have properties that collide with local variables, it can drastically change the meaning of your program. For example, what does this do? ------------ with (foo) { var x = 3; return x; } ------------ Answer: anything. The local variable `x` could be clobbered by a property of `foo` and perhaps it even has a setter, in which case assigning `3` could cause lots of other code to execute. Don't use `with`. === this Only in object constructors, methods, and in setting up closures The semantics of `this` can be tricky. At times it refers to the global object (in most places), the scope of the caller (in `eval`), a node in the DOM tree (when attached using an event handler HTML attribute), a newly created object (in a constructor), or some other object (if function was `call()`ed or `apply()`ed). Because this is so easy to get wrong, limit its use to those places where it is required: * in constructors * in methods of objects (including in the creation of closures) === for-in loop Only for iterating over keys in an object/map/hash `for-in` loops are often incorrectly used to loop over the elements in an `Array`. This is however very error prone because it does not loop from `0` to `length - 1` but over all the present keys in the object and its prototype chain. Here are a few cases where it fails: --------------------------------------- function printArray(arr) { for (var key in arr) { print(arr[key]); } } printArray([0,1,2,3]); // This works. var a = new Array(10); printArray(a); // This is wrong. a = document.getElementsByTagName('*'); printArray(a); // This is wrong. a = [0,1,2,3]; a.buhu = 'wine'; printArray(a); // This is wrong again. a = new Array; a[3] = 3; printArray(a); // This is wrong again. --------------------------------------- Always use normal for loops when using arrays. ------------------------------- function printArray(arr) { var l = arr.length; for (var i = 0; i < l; i++) { print(arr[i]); } } ------------------------------- === Associative Arrays Never use `Array` as a map/hash/associative array Associative `Array`s are not allowed... or more precisely you are not allowed to use non number indexes for arrays. If you need a map/hash use `Object` instead of `Array` in these cases because the features that you want are actually features of `Object` and not of `Array`. `Array` just happens to extend `Object` (like any other object in JS and therefore you might as well have used `Date`, `RegExp` or `String`). === Multiline string literals No Do not do this: --------------------------------------------------------------------------- var myString = 'A rather long string of English text, an error message \ actually that just keeps going and going -- an error \ message to make the Energizer bunny blush (right through \ those Schwarzenegger shades)! Where was I? Oh yes, \ you\'ve got an error and all the extraneous whitespace is \ just gravy. Have a nice day.'; --------------------------------------------------------------------------- The whitespace at the beginning of each line can't be safely stripped at compile time; whitespace after the slash will result in tricky errors; and while most script engines support this, it is not part of ECMAScript. Use string concatenation instead: -------------------------------------------------------------------------- var myString = 'A rather long string of English text, an error message ' + 'actually that just keeps going and going -- an error ' + 'message to make the Energizer bunny blush (right through ' + 'those Schwarzenegger shades)! Where was I? Oh yes, ' + 'you\'ve got an error and all the extraneous whitespace is ' + 'just gravy. Have a nice day.'; -------------------------------------------------------------------------- === Array and Object literals Yes Use `Array` and `Object` literals instead of `Array` and `Object` constructors. Array constructors are error-prone due to their arguments. --------------------------------------------------------------------------- // Length is 3. var a1 = new Array(x1, x2, x3); // Length is 2. var a2 = new Array(x1, x2); // If x1 is a number and it is a natural number the length will be x1. // If x1 is a number but not a natural number this will throw an exception. // Otherwise the array will have one element with x1 as its value. var a3 = new Array(x1); // Length is 0. var a4 = new Array(); --------------------------------------------------------------------------- Because of this, if someone changes the code to pass 1 argument instead of 2 arguments, the array might not have the expected length. To avoid these kinds of weird cases, always use the more readable array literal. --------------------- var a = [x1, x2, x3]; var a2 = [x1, x2]; var a3 = [x1]; var a4 = []; --------------------- Object constructors don't have the same problems, but for readability and consistency object literals should be used. ---------------------- var o = new Object(); var o2 = new Object(); o2.a = 0; o2.b = 1; o2.c = 2; o2['strange key'] = 3; ---------------------- Should be written as: ------------------ var o = {}; var o2 = { a: 0, b: 1, c: 2, 'strange key': 3 }; ------------------ === Modifying prototypes of builtin objects No Modifying builtins like `Object.prototype` and `Array.prototype` are strictly forbidden. Modifying other builtins like `Function.prototype` is less dangerous but still leads to hard to debug issues in production and should be avoided. === Internet Explorer's Conditional Comments No Don't do this: --------------------------------------------------------- var f = function () { /*@cc_on if (@_jscript) { return 2* @*/ 3; /*@ } @*/ }; --------------------------------------------------------- Conditional Comments hinder automated tools as they can vary the JavaScript syntax tree at runtime. [[JavaScript_Style_Rules]] == JavaScript Style Rules === Naming In general, use `functionNamesLikeThis`, `variableNamesLikeThis`, `ClassNamesLikeThis`, `EnumNamesLikeThis`, `methodNamesLikeThis`, `CONSTANT_VALUES_LIKE_THIS`, `foo.namespaceNamesLikeThis.bar`, and `filenameslikethis.js`. Properties and methods * _Private_ properties and methods should be named with a trailing underscore. * _Protected_ properties and methods should be named without a trailing underscore (like public ones). For more information on _private_ and __protected__, read the section on link:#visibility--private-and-protected-fields[visibility]. Method and function parameter Optional function arguments start with `opt_`. Functions that take a variable number of arguments should have the last argument named `var_args`. You may not refer to `var_args` in the code; use the `arguments` array. Optional and variable arguments can also be specified in `@param` annotations. Although either convention is acceptable to the compiler, using both together is preferred. Getters and Setters EcmaScript 5 getters and setters for properties are discouraged. However, if they are used, then getters must not change observable state. --------------------------------------------------- /** * WRONG -- Do NOT do this. */ var foo = { get next() { return this.nextId++; } }; --------------------------------------------------- Accessor functions Getters and setters methods for properties are not required. However, if they are used, then getters must be named `getFoo()` and setters must be named `setFoo(value)`. (For boolean getters, `isFoo()` is also acceptable, and often sounds more natural.) Namespaces JavaScript has no inherent packaging or namespacing support. Global name conflicts are difficult to debug, and can cause intractable problems when two projects try to integrate. In order to make it possible to share common JavaScript code, we've adopted conventions to prevent collisions. Use namespaces for global code _ALWAYS_ prefix identifiers in the global scope with a unique pseudo namespace related to the project or library. If you are working on "Project Sloth", a reasonable pseudo namespace would be `sloth.*`. -------------------------- var sloth = {}; sloth.sleep = function() { ... }; -------------------------- Many JavaScript libraries, including http://code.google.com/closure/library/[the Closure Library] and http://www.dojotoolkit.org/[Dojo toolkit] give you high-level functions for declaring your namespaces. Be consistent about how you declare your namespaces. -------------------------- goog.provide('sloth'); sloth.sleep = function() { ... }; -------------------------- Respect namespace ownership When choosing a child-namespace, make sure that the owners of the parent namespace know what you are doing. If you start a project that creates hats for sloths, make sure that the Sloth team knows that you're using `sloth.hats`. Use different namespaces for external code and internal code "External code" is code that comes from outside your codebase, and is compiled independently. Internal and external names should be kept strictly separate. If you're using an external library that makes things available in `foo.hats.*`, your internal code should not define all its symbols in `foo.hats.*`, because it will break if the other team defines new symbols. --------------------------------- foo.require('foo.hats'); /** * WRONG -- Do NOT do this. * @constructor * @extends {foo.hats.RoundHat} */ foo.hats.BowlerHat = function() { }; --------------------------------- If you need to define new APIs on an external namespace, then you should explicitly export the public API functions, and only those functions. Your internal code should call the internal APIs by their internal names, for consistency and so that the compiler can optimize them better. --------------------------------------------------------------- foo.provide('googleyhats.BowlerHat'); foo.require('foo.hats'); /** * @constructor * @extends {foo.hats.RoundHat} */ googleyhats.BowlerHat = function() { ... }; goog.exportSymbol('foo.hats.BowlerHat', googleyhats.BowlerHat); --------------------------------------------------------------- Alias long type names to improve readability Use local aliases for fully-qualified types if doing so improves readability. The name of a local alias should match the last part of the type. -------------------------------------------------------------- /** * @constructor */ some.long.namespace.MyClass = function() { }; /** * @param {some.long.namespace.MyClass} a */ some.long.namespace.MyClass.staticHelper = function(a) { ... }; myapp.main = function() { var MyClass = some.long.namespace.MyClass; var staticHelper = some.long.namespace.MyClass.staticHelper; staticHelper(new MyClass()); }; -------------------------------------------------------------- Do not create local aliases of namespaces. Namespaces should only be aliased using link:#goog-scope[goog.scope]. ---------------------------------------------------------- myapp.main = function() { var namespace = some.long.namespace; namespace.MyClass.staticHelper(new namespace.MyClass()); }; ---------------------------------------------------------- Avoid accessing properties of an aliased type, unless it is an enum. ---------------------------------------- /** @enum {string} */ some.long.namespace.Fruit = { APPLE: 'a', BANANA: 'b' }; myapp.main = function() { var Fruit = some.long.namespace.Fruit; switch (fruit) { case Fruit.APPLE: ... case Fruit.BANANA: ... } }; ---------------------------------------- -------------------------------------------- myapp.main = function() { var MyClass = some.long.namespace.MyClass; MyClass.staticHelper(null); }; -------------------------------------------- Never create aliases in the global scope. Use them only in function blocks. Filenames Filenames should be all lowercase in order to avoid confusion on case-sensitive platforms. Filenames should end in `.js`, and should contain no punctuation except for `-` or `_` (prefer `-` to `_`). === Custom toString() methods Must always succeed without side effects. You can control how your objects string-ify themselves by defining a custom `toString()` method. This is fine, but you need to ensure that your method (1) always succeeds and (2) does not have side-effects. If your method doesn't meet these criteria, it's very easy to run into serious problems. For example, if `toString()` calls a method that does an `assert`, `assert` might try to output the name of the object in which it failed, which of course requires calling `toString()`. === Deferred initialization OK It isn't always possible to initialize variables at the point of declaration, so deferred initialization is fine. === Explicit scope Always Always use explicit scope - doing so increases portability and clarity. For example, don't rely on `window` being in the scope chain. You might want to use your function in another application for which `window` is not the content window. === Code formatting Expand for more information. We follow the link:cppguide.xml#Formatting[C++ formatting rules] in spirit, with the following additional clarifications. Curly Braces Because of implicit semicolon insertion, always start your curly braces on the same line as whatever they're opening. For example: ---------------- if (something) { // ... } else { // ... } ---------------- Array and Object Initializers Single-line array and object initializers are allowed when they fit on a line: --------------------------------------------------------------- var arr = [1, 2, 3]; // No space after [ or before ]. var obj = {a: 1, b: 2, c: 3}; // No space after { or before }. --------------------------------------------------------------- Multiline array initializers and object initializers are indented 2 spaces, with the braces on their own line, just like blocks. -------------------------------------------------------- // Object initializer. var inset = { top: 10, right: 20, bottom: 15, left: 12 }; // Array initializer. this.rows_ = [ '"Slartibartfast" ', '"Zaphod Beeblebrox" ', '"Ford Prefect" ', '"Arthur Dent" ', '"Marvin the Paranoid Android" ', 'the.mice@magrathea.com' ]; // Used in a method call. goog.dom.createDom(goog.dom.TagName.DIV, { id: 'foo', className: 'some-css-class', style: 'display:none' }, 'Hello, world!'); -------------------------------------------------------- Long identifiers or values present problems for aligned initialization lists, so always prefer non-aligned initialization. For example: ---------------------------- CORRECT_Object.prototype = { a: 0, b: 1, lengthyName: 2 }; ---------------------------- Not like this: -------------------------- WRONG_Object.prototype = { a : 0, b : 1, lengthyName: 2 }; -------------------------- Function Arguments When possible, all function arguments should be listed on the same line. If doing so would exceed the 80-column limit, the arguments must be line-wrapped in a readable way. To save space, you may wrap as close to 80 as possible, or put each argument on its own line to enhance readability. The indentation may be either four spaces, or aligned to the parenthesis. Below are the most common patterns for argument wrapping: ------------------------------------------------------------------------------- // Four-space, wrap at 80. Works with very long function names, survives // renaming without reindenting, low on space. goog.foo.bar.doThingThatIsVeryDifficultToExplain = function( veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo, tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) { // ... }; // Four-space, one argument per line. Works with long function names, // survives renaming, and emphasizes each argument. goog.foo.bar.doThingThatIsVeryDifficultToExplain = function( veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo, tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) { // ... }; // Parenthesis-aligned indentation, wrap at 80. Visually groups arguments, // low on space. function foo(veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo, tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) { // ... } // Parenthesis-aligned, one argument per line. Emphasizes each // individual argument. function bar(veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo, tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) { // ... } ------------------------------------------------------------------------------- When the function call is itself indented, you're free to start the 4-space indent relative to the beginning of the original statement or relative to the beginning of the current function call. The following are all acceptable indentation styles. ---------------------------------------------- if (veryLongFunctionNameA( veryLongArgumentName) || veryLongFunctionNameB( veryLongArgumentName)) { veryLongFunctionNameC(veryLongFunctionNameD( veryLongFunctioNameE( veryLongFunctionNameF))); } ---------------------------------------------- Passing Anonymous Functions When declaring an anonymous function in the list of arguments for a function call, the body of the function is indented two spaces from the left edge of the statement, or two spaces from the left edge of the function keyword. This is to make the body of the anonymous function easier to read (i.e. not be all squished up into the right half of the screen). ---------------------------------------------------------------------- prefix.something.reallyLongFunctionName('whatever', function(a1, a2) { if (a1.equals(a2)) { someOtherLongFunctionName(a1); } else { andNowForSomethingCompletelyDifferent(a2.parrot); } }); var names = prefix.something.myExcellentMapFunction( verboselyNamedCollectionOfItems, function(item) { return item.name; }); ---------------------------------------------------------------------- Aliasing with goog.scope https://docs.google.com/document/pub?id=1ETFAuh2kaXMVL-vafUYhaWlhl6b5D9TOvboVg7Zl68Y[`goog.scope`] may be used to shorten references to namespaced symbols in programs using http://code.google.com/closure/library/[the Closure Library]. Only one `goog.scope` invocation may be added per file. Always place it in the global scope. The opening `goog.scope(function() {` invocation must be preceded by exactly one blank line and follow any `goog.provide` statements, `goog.require` statements, or top-level comments. The invocation must be closed on the last line in the file. Append `// goog.scope` to the closing statement of the scope. Separate the comment from the semicolon by two spaces. Similar to C++ namespaces, do not indent under goog.scope declarations. Instead, continue from the 0 column. Only alias names that will not be re-assigned to another object (e.g., most constructors, enums, and namespaces). Do not do this (see below for how to alias a constructor): ---------------------------- goog.scope(function() { var Button = goog.ui.Button; Button = function() { ... }; ... ---------------------------- Names must be the same as the last property of the global that they are aliasing. -------------------------------------------------------- goog.provide('my.module.SomeType'); goog.require('goog.dom'); goog.require('goog.ui.Button'); goog.scope(function() { var Button = goog.ui.Button; var dom = goog.dom; // Alias new types after the constructor declaration. my.module.SomeType = function() { ... }; var SomeType = my.module.SomeType; // Declare methods on the prototype as usual: SomeType.prototype.findButton = function() { // Button as aliased above. this.button = new Button(dom.getElement('my-button')); }; ... }); // goog.scope -------------------------------------------------------- Indenting wrapped lines Except for link:#array-and-object-literals[array literals, object literals], and anonymous functions, all wrapped lines should be indented either left-aligned to a sibling expression above, or four spaces (not two spaces) deeper than a parent expression (where "sibling" and "parent" refer to parenthesis nesting level). ---------------------------------------------------------------------------- someWonderfulHtml = '' + getEvenMoreHtml(someReallyInterestingValues, moreValues, evenMoreParams, 'a duck', true, 72, slightlyMoreMonkeys(0xfff)) + ''; thisIsAVeryLongVariableName = hereIsAnEvenLongerOtherFunctionNameThatWillNotFitOnPrevLine(); thisIsAVeryLongVariableName = siblingOne + siblingTwo + siblingThree + siblingFour + siblingFive + siblingSix + siblingSeven + moreSiblingExpressions + allAtTheSameIndentationLevel; thisIsAVeryLongVariableName = operandOne + operandTwo + operandThree + operandFour + operandFive * ( aNestedChildExpression + shouldBeIndentedMore); someValue = this.foo( shortArg, 'Some really long string arg - this is a pretty common case, actually.', shorty2, this.bar()); if (searchableCollection(allYourStuff).contains(theStuffYouWant) && !ambientNotification.isActive() && (client.isAmbientSupported() || client.alwaysTryAmbientAnyways())) { ambientNotification.activate(); } ---------------------------------------------------------------------------- Blank lines Use newlines to group logically related pieces of code. For example: ---------------------- doSomethingTo(x); doSomethingElseTo(x); andThen(x); nowDoSomethingWith(y); andNowWith(z); ---------------------- Binary and Ternary Operators Always put the operator on the preceding line. Otherwise, line breaks and indentation follow the same rules as in other Google style guides. This operator placement was initially agreed upon out of concerns about automatic semicolon insertion. In fact, semicolon insertion cannot happen before a binary operator, but new code should stick to this style for consistency. ------------------------------------------------------------------ var x = a ? b : c; // All on one line if it will fit. // Indentation +4 is OK. var y = a ? longButSimpleOperandB : longButSimpleOperandC; // Indenting to the line position of the first operand is also OK. var z = a ? moreComplicatedB : moreComplicatedC; ------------------------------------------------------------------ This includes the dot operator. ---------------------- var x = foo.bar(). doSomething(). doSomethingElse(); ---------------------- === Parentheses Only where required Use sparingly and in general only where required by the syntax and semantics. Never use parentheses for unary operators such as `delete`, `typeof` and `void` or after keywords such as `return`, `throw` as well as others (`case`, `in` or `new`). === Strings Prefer ' over " For consistency single-quotes (') are preferred to double-quotes ("). This is helpful when creating strings that include HTML: ------------------------------ var msg = 'This is some HTML'; ------------------------------ === Visibility (private and protected fields) Encouraged, use JSDoc annotations `@private` and `@protected` We recommend the use of the JSDoc annotations `@private` and `@protected` to indicate visibility levels for classes, functions, and properties. The --jscomp_warning=visibility compiler flag turns on compiler warnings for visibility violations. See http://code.google.com/p/closure-compiler/wiki/Warnings[Closure Compiler Warnings]. `@private` global variables and functions are only accessible to code in the same file. Constructors marked `@private` may only be instantiated by code in the same file and by their static and instance members. `@private` constructors may also be accessed anywhere in the same file for their public static properties and by the `instanceof` operator. Global variables, functions, and constructors should never be annotated `@protected`. ----------------------------------------------------------------------- // File 1. // AA_PrivateClass_ and AA_init_ are accessible because they are global // and in the same file. /** * @private * @constructor */ AA_PrivateClass_ = function() { }; /** @private */ function AA_init_() { return new AA_PrivateClass_(); } AA_init_(); ----------------------------------------------------------------------- `@private` properties are accessible to all code in the same file, plus all static methods and instance methods of that class that "owns" the property, if the property belongs to a class. They cannot be accessed or overridden from a subclass in a different file. `@protected` properties are accessible to all code in the same file, plus any static methods and instance methods of any subclass of a class that "owns" the property. Note that these semantics differ from those of C++ and Java, in that they grant private and protected access to all code in the same file, not just in the same class or class hierarchy. Also, unlike in C++, private properties cannot be overridden by a subclass. ---------------------------------------------------------------- // File 1. /** @constructor */ AA_PublicClass = function() { /** @private */ this.privateProp_ = 2; /** @protected */ this.protectedProp = 4; }; /** @private */ AA_PublicClass.staticPrivateProp_ = 1; /** @protected */ AA_PublicClass.staticProtectedProp = 31; /** @private */ AA_PublicClass.prototype.privateMethod_ = function() {}; /** @protected */ AA_PublicClass.prototype.protectedMethod = function() {}; // File 2. /** * @return {number} The number of ducks we've arranged in a row. */ AA_PublicClass.prototype.method = function() { // Legal accesses of these two properties. return this.privateProp_ + AA_PublicClass.staticPrivateProp_; }; // File 3. /** * @constructor * @extends {AA_PublicClass} */ AA_SubClass = function() { // Legal access of a protected static property. AA_PublicClass.staticProtectedProp = this.method(); }; goog.inherits(AA_SubClass, AA_PublicClass); /** * @return {number} The number of ducks we've arranged in a row. */ AA_SubClass.prototype.method = function() { // Legal access of a protected instance property. return this.protectedProp; }; ---------------------------------------------------------------- Notice that in JavaScript, there is no distinction between a type (like `AA_PrivateClass_`) and the constructor for that type. There is no way to express both that a type is public and its constructor is private (because the constructor could easily be aliased in a way that would defeat the privacy check). === JavaScript Types Encouraged and enforced by the compiler. When documenting a type in JSDoc, be as specific and accurate as possible. The types we support are based on the http://wiki.ecmascript.org/doku.php?id=spec:spec[EcmaScript 4 spec]. The JavaScript Type Language The ES4 proposal contained a language for specifying JavaScript types. We use this language in JsDoc to express the types of function parameters and return values. As the ES4 proposal has evolved, this language has changed. The compiler still supports old syntaxes for types, but those syntaxes are deprecated. [width="100%",cols="25%,25%,25%,25%",options="header",] |========================================================================================================================================================================================================================================== |Syntax Name |Syntax |Description |Deprecated Syntaxes |Primitive Type |There are 5 primitive types in JavaScript: `{null}`, `{undefined}`, `{boolean}`, `{number}`, and `{string}`. |Simply the name of a type. | |Instance Type a| `{Object}` + An instance of Object or null. `{Function}` + An instance of Function or null. `{EventTarget}` + An instance of a constructor that implements the EventTarget interface, or null. a| An instance of a constructor or interface function. Constructor functions are functions defined with the `@constructor` JSDoc tag. Interface functions are functions defined with the `@interface` JSDoc tag. By default, instance types will accept null. This is the only type syntax that makes the type nullable. Other type syntaxes in this table will not accept null. | |Enum Type |`{goog.events.EventType}` + One of the properties of the object literal initializer of `goog.events.EventType`. a| An enum must be initialized as an object literal, or as an alias of another enum, annotated with the `@enum` JSDoc tag. The properties of this literal are the instances of the enum. The syntax of the enum is defined link:#enums[below]. Note that this is one of the few things in our type system that were not in the ES4 spec. | |Type Application a| `{Array.}` + An array of strings. `{Object.}` + An object in which the keys are strings and the values are numbers. |Parameterizes a type, by applying a set of type arguments to that type. The idea is analogous to generics in Java. | |Type Union |`{(number|boolean)}` + A number or a boolean. a| Indicates that a value might have type A OR type B. The parentheses may be omitted at the top-level expression, but the parentheses should be included in sub-expressions to avoid ambiguity. + `{number|boolean}` + `{function(): (number|boolean)}` |`{(number,boolean)}`, + `{(number||boolean)}` |Nullable type |`{?number}` + A number or null. |Shorthand for the union of the null type with any other type. This is just syntactic sugar. |`{number?}` |Non-nullable type |`{!Object}` + An Object, but never the `null` value. |Filters null out of nullable types. Most often used with instance types, which are nullable by default. |`{Object!}` |Record Type |`{{myNum: number, myObject}}` + An anonymous type with the given type members. a| Indicates that the value has the specified members with the specified types. In this case, `myNum` with a type `number` and `myObject` with any type. Notice that the braces are part of the type syntax. For example, to denote an `Array` of objects that have a `length` property, you might write `Array.<{length}>`. | |Function Type |`{function(string, boolean)}` + A function that takes two arguments (a string and a boolean), and has an unknown return value. + |Specifies a function. | |Function Return Type |`{function(): number}` + A function that takes no arguments and returns a number. + |Specifies a function return type. | |Function `this` Type |`{function(this:goog.ui.Menu, string)}` + A function that takes one argument (a string), and executes in the context of a goog.ui.Menu. |Specifies the context type of a function type. | |Function `new` Type |`{function(new:goog.ui.Menu, string)}` + A constructor that takes one argument (a string), and creates a new instance of goog.ui.Menu when called with the 'new' keyword. |Specifies the constructed type of a constructor. | |Variable arguments |`{function(string, ...[number]): number}` + A function that takes one argument (a string), and then a variable number of arguments that must be numbers. |Specifies variable arguments to a function. | |Variable arguments (in `@param` annotations) |`@param {...number} var_args` + A variable number of arguments to an annotated function. |Specifies that the annotated function accepts a variable number of arguments. | |Function link:#optional[optional arguments] |`{function(?string=, number=)}` + A function that takes one optional, nullable string and one optional number as arguments. The `=` syntax is only for `function` type declarations. |Specifies optional arguments to a function. | |Function link:#optional[optional arguments] (in `@param` annotations) |`@param {number=} opt_argument` + An optional parameter of type `number`. |Specifies that the annotated function accepts an optional argument. | |The ALL type |`{*}` |Indicates that the variable can take on any type. | |The UNKNOWN type |`{?}` |Indicates that the variable can take on any type, and the compiler should not type-check any uses of it. | |========================================================================================================================================================================================================================================== Types in JavaScript [width="100%",cols="34%,33%,33%",options="header",] |=========================================================================================================================================================================================================================================== |Type Example |Value Examples |Description |number a| ------- 1 1.0 -5 1e5 Math.PI ------- | |Number a| ---------------- new Number(true) ---------------- |link:#wrapper-objects-for-primitive-types[Number object] |string a| ---------- 'Hello' "World" String(42) ---------- |String value |String a| ------------------- new String('Hello') new String(42) ------------------- |link:#wrapper-objects-for-primitive-types[String object] |boolean a| ---------- true false Boolean(0) ---------- |Boolean value |Boolean a| ----------------- new Boolean(true) ----------------- |link:#wrapper-objects-for-primitive-types[Boolean object] |RegExp a| ------------------- new RegExp('hello') /world/g ------------------- | |Date a| ---------- new Date new Date() ---------- | |null a| ---- null ---- | |undefined a| --------- undefined --------- | |void a| -------------- function f() { return; } -------------- |No return value |Array a| ------------------ ['foo', 0.3, null] [] ------------------ |Untyped Array |Array. a| ------------ [11, 22, 33] ------------ |An Array of numbers |Array.> a| ----------------------------------------- [['one', 'two', 'three'], ['foo', 'bar']] ----------------------------------------- |Array of Arrays of strings |Object a| --------------------------------- {} {foo: 'abc', bar: 123, baz: null} --------------------------------- | |Object. a| -------------- {'foo': 'bar'} -------------- |An Object in which the values are strings. |Object. a| --------------- var obj = {}; obj[1] = 'bar'; --------------- a| An Object in which the keys are numbers and the values are strings. Note that in JavaScript, the keys are always implicitly converted to strings, so `obj['1'] == obj[1]`. So the key will always be a string in for...in loops. But the compiler will verify the type of the key when indexing into the object. |Function a| ---------------- function(x, y) { return x * y; } ---------------- |link:#wrapper-objects-for-primitive-types[Function object] |function(number, number): number a| ---------------- function(x, y) { return x * y; } ---------------- |function value |SomeClass a| ----------------------- /** @constructor */ function SomeClass() {} new SomeClass(); ----------------------- | |SomeInterface a| --------------------------------------------- /** @interface */ function SomeInterface() {} SomeInterface.prototype.draw = function() {}; --------------------------------------------- | |project.MyClass a| -------------------------------- /** @constructor */ project.MyClass = function () {} new project.MyClass() -------------------------------- | |project.MyEnum a| ------------------------ /** @enum {string} */ project.MyEnum = { /** The color blue. */ BLUE: '#0000dd', /** The color red. */ RED: '#dd0000' }; ------------------------ a| Enumeration JSDoc comments on enum values are optional. |Element a| ----------------------------- document.createElement('div') ----------------------------- |Elements in the DOM. |Node a| ------------------------ document.body.firstChild ------------------------ |Nodes in the DOM. |HTMLInputElement a| --------------------------------------------- htmlDocument.getElementsByTagName('input')[0] --------------------------------------------- |A specific type of DOM element. |=========================================================================================================================================================================================================================================== Type Casts In cases where type-checking doesn't accurately infer the type of an expression, it is possible to add a type cast comment by adding a type annotation comment and enclosing the expression in parentheses. The parentheses are required. ------------------------- /** @type {number} */ (x) ------------------------- Nullable vs. Optional Parameters and Properties Because JavaScript is a loosely-typed language, it is very important to understand the subtle differences between optional, nullable, and undefined function parameters and class properties. Instances of classes and interfaces are nullable by default. For example, the following declaration ---------------------------------------- /** * Some class, initialized with a value. * @param {Object} value Some value. * @constructor */ function MyClass(value) { /** * Some value. * @type {Object} * @private */ this.myValue_ = value; } ---------------------------------------- tells the compiler that the `myValue_` property holds either an Object or null. If `myValue_` must never be null, it should be declared like this: ------------------------------------------------- /** * Some class, initialized with a non-null value. * @param {!Object} value Some value. * @constructor */ function MyClass(value) { /** * Some value. * @type {!Object} * @private */ this.myValue_ = value; } ------------------------------------------------- This way, if the compiler can determine that somewhere in the code `MyClass` is initialized with a null value, it will issue a warning. Optional parameters to functions may be undefined at runtime, so if they are assigned to class properties, those properties must be declared accordingly: ---------------------------------------------------- /** * Some class, initialized with an optional value. * @param {Object=} opt_value Some value (optional). * @constructor */ function MyClass(opt_value) { /** * Some value. * @type {Object|undefined} * @private */ this.myValue_ = opt_value; } ---------------------------------------------------- This tells the compiler that `myValue_` may hold an Object, null, or remain undefined. Note that the optional parameter `opt_value` is declared to be of type `{Object=}`, not `{Object|undefined}`. This is because optional parameters may, by definition, be undefined. While there is no harm in explicitly declaring an optional parameter as possibly undefined, it is both unnecessary and makes the code harder to read. Finally, note that being nullable and being optional are orthogonal properties. The following four declarations are all different: -------------------------------------------------------------------------------- /** * Takes four arguments, two of which are nullable, and two of which are * optional. * @param {!Object} nonNull Mandatory (must not be undefined), must not be null. * @param {Object} mayBeNull Mandatory (must not be undefined), may be null. * @param {!Object=} opt_nonNull Optional (may be undefined), but if present, * must not be null! * @param {Object=} opt_mayBeNull Optional (may be undefined), may be null. */ function strangeButTrue(nonNull, mayBeNull, opt_nonNull, opt_mayBeNull) { // ... }; -------------------------------------------------------------------------------- Typedefs Sometimes types can get complicated. A function that accepts content for an Element might look like: ----------------------------------------------------------------------- /** * @param {string} tagName * @param {(string|Element|Text|Array.|Array.)} contents * @return {!Element} */ goog.createElement = function(tagName, contents) { ... }; ----------------------------------------------------------------------- You can define commonly used type expressions with a `@typedef` tag. For example, -------------------------------------------------------------------- /** @typedef {(string|Element|Text|Array.|Array.)} */ goog.ElementContent; /** * @param {string} tagName * @param {goog.ElementContent} contents * @return {!Element} */ goog.createElement = function(tagName, contents) { ... }; -------------------------------------------------------------------- Template types The compiler has limited support for template types. It can only infer the type of `this` inside an anonymous function literal from the type of the `this` argument and whether the `this` argument is missing. -------------------------------------------------------------- /** * @param {function(this:T, ...)} fn * @param {T} thisObj * @param {...*} var_args * @template T */ goog.bind = function(fn, thisObj, var_args) { ... }; // Possibly generates a missing property warning. goog.bind(function() { this.someProperty; }, new SomeClass()); // Generates an undefined this warning. goog.bind(function() { this.someProperty; }); -------------------------------------------------------------- === Comments Use JSDoc We follow the link:cppguide.xml#Comments[C++ style for comments] in spirit. All files, classes, methods and properties should be documented with http://code.google.com/p/jsdoc-toolkit/[JSDoc] comments with the appropriate link:#jsdoc-tag-reference[tags] and link:#jstypes[types]. Textual descriptions for properties, methods, method parameters and method return values should be included unless obvious from the property, method, or parameter name. Inline comments should be of the `//` variety. Complete sentences are recommended but not required. Complete sentences should use appropriate capitalization and punctuation. Comment Syntax The JSDoc syntax is based on http://www.oracle.com/technetwork/java/javase/documentation/index-137868.html[JavaDoc]. Many tools extract metadata from JSDoc comments to perform code validation and optimizations. These comments must be well-formed. -------------------------------------------------------------- /** * A JSDoc comment should begin with a slash and 2 asterisks. * Inline tags should be enclosed in braces like {@code this}. * @desc Block tags should always start on their own line. */ -------------------------------------------------------------- JSDoc Indentation If you have to line break a block tag, you should treat this as breaking a code statement and indent it four spaces. ----------------------------------------------------------------------------- /** * Illustrates line wrapping for long param/return descriptions. * @param {string} foo This is a param with a description too long to fit in * one line. * @return {number} This returns something that has a description too long to * fit in one line. */ project.MyClass.prototype.method = function(foo) { return 5; }; ----------------------------------------------------------------------------- You should not indent the `@fileoverview` command. You do not have to indent the `@desc` command. Even though it is not preferred, it is also acceptable to line up the description. ----------------------------------------------------------------------------- /** * This is NOT the preferred indentation method. * @param {string} foo This is a param with a description too long to fit in * one line. * @return {number} This returns something that has a description too long to * fit in one line. */ project.MyClass.prototype.method = function(foo) { return 5; }; ----------------------------------------------------------------------------- HTML in JSDoc Like JavaDoc, JSDoc supports many HTML tags, like , 
, , ,