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COLLECTED BY
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string
●number
●boolean
●null
●undefined
var foo = 1; var bar = foo; bar = 9; console.log(foo, bar); // => 1, 9Complex: When you access a complex type you work on a reference to its value. ●
object
●array
●function
var foo = [1, 2]; var bar = foo; bar[0] = 9; console.log(foo[0], bar[0]); // => 9, 9
// bad var item = new Object(); // good var item = {};Don't use reserved words as keys. It won't work in IE8. More info.
// bad var superman = { class: 'superhero', default: { clark: 'kent' }, private: true }; // good var superman = { cssClass: 'superhero', defaults: { clark: 'kent' }, hidden: true };Use readable synonyms in place of reserved words.
// bad var superman = { class: 'alien' }; // bad var superman = { klass: 'alien' }; // good var superman = { type: 'alien' };
// bad var items = new Array(); // good var items = [];Use Array#push instead of direct assignment to add items to an array.
var someStack = []; // bad someStack[someStack.length] = 'abracadabra'; // good someStack.push('abracadabra');When you need to copy an array use Array#slice. jsPerf
var len = items.length; var itemsCopy = []; var i; // bad for (i = 0; i < len; i++) { itemsCopy[i] = items[i]; } // good itemsCopy = items.slice();To convert an array-like object to an array, use Array#slice.
function trigger() { var args = Array.prototype.slice.call(arguments); ... }
'' for strings.
// bad var name = "Bob Parr"; // good var name = 'Bob Parr'; // bad var fullName = "Bob " + this.lastName; // good var fullName = 'Bob ' + this.lastName;
// anonymous function as event handler $el.on('click', function () { return true; }); // optional name to aid debugging with stack traces $el.on('click', function elClick() { return true; }); // anonymous function as method MyObject.prototype.isTrue = function () { return true; }; // immediately-invoked function expression (IIFE) (function () { console.log('Welcome to the Internet. Please follow me.'); })();Function declarations:
// use a named declaration when appropriate mainly // for internal helper logic and organization function namedFunction() { return true; }Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. Note: ECMA-262 defines a
block as a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.
// bad if (currentUser) { function test() { console.log('Nope.'); } } // good var test; if (currentUser) { test = function test() { console.log('Yup.'); }; }Never name a parameter
arguments. This will take precedence over the arguments object that is given to every function scope.
// bad function nope(name, options, arguments) { // ...stuff... } // good function yup(name, options, args) { // ...stuff... }
var luke = { jedi: true, age: 28 }; // bad var isJedi = luke['jedi']; // good var isJedi = luke.jedi;Use subscript notation
[] when accessing properties with a variable.
var luke = { jedi: true, age: 28 }; function getProp(prop) { return luke[prop]; } var isJedi = getProp('jedi');
var to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.
// bad superPower = new SuperPower(); // good var superPower = new SuperPower();Use one
var declaration per variable.
It's easier to add new variable declarations this way, and you never have
to worry about swapping out a ; for a , or introducing punctuation-only
diffs.
// bad var items = getItems(), goSportsTeam = true, dragonball = 'z'; // bad // (compare to above, and try to spot the mistake) var items = getItems(), goSportsTeam = true; dragonball = 'z'; // good var items = getItems(); var goSportsTeam = true; var dragonball = 'z';Declare unassigned variables last. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables. Multiple unassigned variables can be declared in a single-line var declaration.
// good var items = getItems(); var goSportsTeam = true; var i, j, length;Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.
// bad function () { test(); console.log('doing stuff..'); //..other stuff.. var name = getName(); if (name === 'test') { return false; } return name; } // good function () { var name = getName(); test(); console.log('doing stuff..'); //..other stuff.. if (name === 'test') { return false; } return name; } // bad - unnecessary function call function () { var name = getName(); if (!arguments.length) { return false; } this.setFirstName(name); return true; } // good function () { var name; if (!arguments.length) { return false; } name = getName(); this.setFirstName(name); return true; }
// we know this wouldn't work (assuming there // is no notDefined global variable) function example() { console.log(notDefined); // => throws a ReferenceError } // creating a variable declaration after you // reference the variable will work due to // variable hoisting. Note: the assignment // value of `true` is not hoisted. function example() { console.log(declaredButNotAssigned); // => undefined var declaredButNotAssigned = true; } // The interpreter is hoisting the variable // declaration to the top of the scope, // which means our example could be rewritten as: function example() { var declaredButNotAssigned; console.log(declaredButNotAssigned); // => undefined declaredButNotAssigned = true; }Anonymous function expressions hoist their variable name, but not the function assignment.
function example() { console.log(anonymous); // => undefined anonymous(); // => TypeError anonymous is not a function var anonymous = function () { console.log('anonymous function expression'); }; }Named function expressions hoist the variable name, not the function name or the function body.
function example() { console.log(named); // => undefined named(); // => TypeError named is not a function superPower(); // => ReferenceError superPower is not defined var named = function superPower() { console.log('Flying'); }; } // the same is true when the function name // is the same as the variable name. function example() { console.log(named); // => undefined named(); // => TypeError named is not a function var named = function named() { console.log('named'); }; }Function declarations hoist their name and the function body.
function example() { superPower(); // => Flying function superPower() { console.log('Flying'); } }For more information refer to JavaScript Scoping & HoistingbyBen Cherry.
=== and !== over == and !=.
Conditional statements such as the ifstatement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
●Objects evaluate to true
●Undefined evaluates to false
●Null evaluates to false
●Booleans evaluate to the value of the boolean
●Numbers evaluate to falseif+0, -0, or NaN, otherwise true
●Strings evaluate to false if an empty string '', otherwise true
if ([0]) { // true // An array is an object, objects evaluate to true }Use shortcuts.
// bad if (name !== '') { // ...stuff... } // good if (name) { // ...stuff... } // bad if (collection.length > 0) { // ...stuff... } // good if (collection.length) { // ...stuff... }For more information see Truth Equality and JavaScript by Angus Croll.
// bad if (test) return false; // acceptable (only for very simple statements and early function return) if (test) return false; // good if (test) { return false; } // bad function () { return false; } // good function () { return false; }If you're using multi-line blocks with
ifand else, put else on the same line as your
if block's closing brace.
// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
/** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.
// bad // make() returns a new element // based on the passed in tag name // // @param {String} tag // @return {Element} element function make(tag) { // ...stuff... return element; } // good /** * make() returns a new element * based on the passed in tag name * * @param {String} tag * @return {Element} element */ function make(tag) { // ...stuff... return element; }Use
// for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment.
// bad var active = true; // is current tab // good // is current tab var active = true; // bad function getType() { console.log('fetching type...'); // set the default type to 'no type' var type = this._type || 'no type'; return type; } // good function getType() { console.log('fetching type...'); // set the default type to 'no type' var type = this._type || 'no type'; return type; }Prefixing your comments with
FIXMEorTODO helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME -- need to figure this outorTODO -- need to implement.
Use // FIXME: to annotate problems.
function Calculator() { // FIXME: shouldn't use a global here total = 0; return this; }Use
// TODO: to annotate solutions to problems.
function Calculator() { // TODO: total should be configurable by an options param this.total = 0; return this; }
// bad function () { ∙∙var name; } // bad function () { ∙∙∙∙var name; } // good function () { var name; }Place 1 space before the leading brace.
// bad function test(){ console.log('test'); } // good function test() { console.log('test'); } // bad dog.set('attr',{ age: '1 year', breed: 'Bernese Mountain Dog' }); // good dog.set('attr', { age: '1 year', breed: 'Bernese Mountain Dog' });Place 1 space before the opening parenthesis in control statements (
if, while etc.). Place no space before the argument list in function calls and declarations.
// bad if(isJedi) { fight (); } // good if (isJedi) { fight(); } // bad function fight () { console.log ('Swooosh!'); } // good function fight() { console.log('Swooosh!'); }Place 1 space before anonymous function parenthesis.
// bad function() { var name; } // good function () { var name; }Set off operators with spaces.
// bad var x=y+5; // good var x = y + 5;End files with a single newline character.
// bad (function (global) { // ...stuff... })(this);
// bad (function (global) { // ...stuff... })(this);↵ ↵
// good (function (global) { // ...stuff... })(this);↵Use indentation when making long method chains. Long method chains should be avoided except in cases of a performance benefit or significant readability benefit.
// bad $('#items').find('.selected').highlight().end().find('.open').updateCount(); // good $('#items') .find('.selected') .highlight() .end() .find('.open') .updateCount(); // bad var leds = stage.selectAll('.led').data(data).enter().append("svg:svg").class('led', true) .attr('width', (radius + margin) * 2).append("svg:g") .attr("transform", "translate(" + (radius + margin) + "," + (radius + margin) + ")") .call(tron.led); // good var leds = stage.selectAll('.led') .data(data) .enter().append("svg:svg") .class('led', true) .attr('width', (radius + margin) * 2) .append("svg:g") .attr("transform", "translate(" + (radius + margin) + "," + (radius + margin) + ")") .call(tron.led);
// bad var story = [ once , upon , aTime ]; // good var story = [ once, upon, aTime ]; // bad var hero = { firstName: 'Bob' , lastName: 'Parr' , heroName: 'Mr. Incredible' , superPower: 'strength' }; // good var hero = { firstName: 'Bob', lastName: 'Parr', heroName: 'Mr. Incredible', superPower: 'strength' };Additional trailing comma: Nope. This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 (source): Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.
```javascript
// bad
var hero = {
firstName: 'Kevin',
lastName: 'Flynn',
};
var heroes = [
'Batman',
'Superman',
];
// good
var hero = {
firstName: 'Kevin',
lastName: 'Flynn'
};
var heroes = [
'Batman',
'Superman'
];
```
// bad (function () { var name = 'Skywalker' return name })() // good (function () { var name = 'Skywalker'; return name; })(); // good (guards against the function becoming an argument when two files with IIFEs are concatenated) ;(function () { var name = 'Skywalker'; return name; })();Read more.
// => this.reviewScore = 9; // bad var totalScore = this.reviewScore + ''; // good var totalScore = '' + this.reviewScore; // bad var totalScore = '' + this.reviewScore + ' total score'; // good var totalScore = this.reviewScore + ' total score';Use
parseInt for Numbers and always with a radix for type casting.
var inputValue = '4'; // bad var val = new Number(inputValue); // bad var val = +inputValue; // bad var val = inputValue >> 0; // bad var val = parseInt(inputValue); // good var val = Number(inputValue); // good var val = parseInt(inputValue, 10);If for whatever reason you are doing something wild and
parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.
// good /** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */ var val = inputValue >> 0;Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
2147483647 >> 0 //=> 2147483647 2147483648 >> 0 //=> -2147483648 2147483649 >> 0 //=> -2147483647Booleans:
var age = 0; // bad var hasAge = new Boolean(age); // good var hasAge = Boolean(age); // good var hasAge =!!age;
// bad function q() { // ...stuff... } // good function query() { // ..stuff.. }Use camelCase when naming objects, functions, and instances.
// bad var OBJEcttsssss = {}; var this_is_my_object = {}; var o = {}; function c() {} var u = new user({ name: 'Bob Parr' }); // good var thisIsMyObject = {}; function thisIsMyFunction() {}; var user = new User({ name: 'Bob Parr' });Use PascalCase when naming constructors or classes.
// bad function user(options) { this.name = options.name; } var bad = new user({ name: 'nope' }); // good function User(options) { this.name = options.name; } var good = new User({ name: 'yup' });Use a leading underscore
_ when naming private properties.
// bad this.__firstName__ = 'Panda'; this.firstName_ = 'Panda'; // good this._firstName = 'Panda';When saving a reference to
this use self.
// bad function () { var that = this; return function () { console.log(that); }; } // bad function () { var _this = this; return function () { console.log(_this); }; } // good function () { var self = this; return function () { console.log(self); }; }Name your functions. This is helpful for stack traces.
// bad var log = function (msg) { console.log(msg); }; // good var log = function log(msg) { console.log(msg); };
// bad dragon.age(); // good dragon.getAge(); // bad dragon.age(25); // good dragon.setAge(25);If the property is a boolean, use isVal() or hasVal().
// bad if (!dragon.age()) { return false; } // good if (!dragon.hasAge()) { return false; }It's okay to create get() and set() functions, but be consistent.
function Jedi(options) { options || (options = {}); var lightsaber = options.lightsaber || 'blue'; this.set('lightsaber', lightsaber); } Jedi.prototype.set = function (key, val) { this[key] = val; }; Jedi.prototype.get = function (key) { return this[key]; };
function Jedi() { console.log('new jedi'); } // bad Jedi.prototype = { fight: function fight() { console.log('fighting'); }, block: function block() { console.log('blocking'); } }; // good Jedi.prototype.fight = function fight() { console.log('fighting'); }; Jedi.prototype.block = function block() { console.log('blocking'); };Methods can return
this to help with method chaining.
// bad Jedi.prototype.jump = function () { this.jumping = true; return true; }; Jedi.prototype.setHeight = function (height) { this.height = height; }; var luke = new Jedi(); luke.jump(); // => true luke.setHeight(20); // => undefined // good Jedi.prototype.jump = function () { this.jumping = true; return this; }; Jedi.prototype.setHeight = function (height) { this.height = height; return this; }; var luke = new Jedi(); luke.jump() .setHeight(20);It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.
function Jedi(options) { options || (options = {}); this.name = options.name || 'no name'; } Jedi.prototype.getName = function getName() { return this.name; }; Jedi.prototype.toString = function toString() { return 'Jedi - ' + this.getName(); };
$.
// bad var sidebar = $('.sidebar'); // good var $sidebar = $('.sidebar');Cache jQuery lookups.
// bad function setSidebar() { $('.sidebar').hide(); // ...stuff... $('.sidebar').css({ 'background-color': 'pink' }); } // good function setSidebar() { var $sidebar = $('.sidebar'); $sidebar.hide(); // ...stuff... $sidebar.css({ 'background-color': 'pink' }); }For DOM queries use Cascading
$('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf
Use find with scoped jQuery object queries.
// bad $('ul', '.sidebar').hide(); // bad $('.sidebar').find('ul').hide(); // good $('.sidebar ul').hide(); // good $('.sidebar > ul').hide(); // good (slower) $sidebar.find('ul'); // good (faster) $($sidebar[0]).find('ul');
function () { return true; }
root = true from any file that isn't the root.