Skip to main content

Principles of Writing Consistent, Idiomatic JavaScript

This is a living document and new ideas for improving the code around us are always welcome. Contribute: fork, clone, branch, commit, push, pull request.​

All code in any code-base should look like a single person typed it, no matter how many people contributed.​

The following list outlines the practices that I use in all code that I am the original author of; contributions to projects that I have created should follow these guidelines.​

I do not intend to impose my style preferences on other people's code or projects; if an existing common style exists, it should be respected.​

"Arguments over style are pointless. There should be a style guide, and you should follow it"​

Rebecca Murphey

 

"Part of being a good steward to a successful project is realizing that writing code for yourself is a Bad Idea™. If thousands of people are using your code, then write your code for maximum clarity, not your personal preference of how to get clever within the spec."​

Idan Gazit

Translations​

Important, Non-Idiomatic Stuff:​

Code Quality Tools, Resources & References​

Get Smart​

Annotated ECMAScript 5.1​

EcmaScript Language Specification, 5.1 Edition​

The following should be considered 1) incomplete, and 2) REQUIRED READING. I don't always agree with the style written by the authors below, but one thing is certain: They are consistent. Furthermore, these are authorities on the language.

Build & Deployment Process​

Projects should always attempt to include some generic means by which source can be linted, tested and compressed in preparation for production use. For this task, grunt by Ben Alman is second to none and has officially replaced the "kits/" directory of this repo.

Test Facility​

Projects must include some form of unit, reference, implementation or functional testing. Use case demos DO NOT QUALIFY as "tests". The following is a list of test frameworks, none of which are endorsed more than the other.

Table of Contents​

Preface​

The following sections outline a reasonable style guide for modern JavaScript development and are not meant to be prescriptive. The most important take-away is the law of code style consistency. Whatever you choose as the style for your project should be considered law. Link to this document as a statement of your project's commitment to code style consistency, readability and maintainability.

Idiomatic Style Manifesto​

  1. Whitespace
  • Never mix spaces and tabs.
  • When beginning a project, before you write any code, choose between soft indents (spaces) or real tabs, consider this law.
    • For readability, I always recommend setting your editor's indent size to two characters — this means two spaces or two spaces representing a real tab.
  • If your editor supports it, always work with the "show invisibles" setting turned on. The benefits of this practice are:
    • Enforced consistency
    • Eliminating end of line whitespace
    • Eliminating blank line whitespace
    • Commits and diffs that are easier to read
  • Use Editorconfig when possible. It supports most IDEs and handles most whitespace settings.

  1. Beautiful Syntax

    A. Parens, Braces, Linebreaks


    // if/else/for/while/try always have spaces, braces and span multiple lines
    // this encourages readability

    // 2.A.1.1
    // Examples of really cramped syntax

    if(condition) doSomething();

    while(condition) iterating++;

    for(var i=0;i<100;i++) someIterativeFn();


    // 2.A.1.1
    // Use whitespace to promote readability

    if ( condition ) {
      // statements
    }

    while ( condition ) {
      // statements
    }

    for ( var i = 0; i < 100; i++ ) {
      // statements
    }

    // Even better:

    var i,
      length = 100;

    for ( i = 0; i < length; i++ ) {
      // statements
    }

    // Or...

    var i = 0,
      length = 100;

    for ( ; i < length; i++ ) {
      // statements
    }

    var prop;

    for ( prop in object ) {
      // statements
    }


    if ( true ) {
      // statements
    } else {
      // statements
    }

    B. Assignments, Declarations, Functions ( Named, Expression, Constructor )


    // 2.B.1.1
    // Variables
    var foo = "bar",
      num = 1,
      undef;

    // Literal notations:
    var array = [],
      object = {};


    // 2.B.1.2
    // Using only one `var` per scope (function) or one `var` for each variable,
    // promotes readability and keeps your declaration list free of clutter.
    // Using one `var` per variable you can take more control of your versions
    // and makes it easier to reorder the lines.
    // One `var` per scope makes it easier to detect undeclared variables
    // that may become implied globals.
    // Choose better for your project and never mix them.

    // Bad
    var foo = "",
      bar = "";
    var qux;

    // Good
    var foo = "";
    var bar = "";
    var qux;

    // or..
    var foo = "",
      bar = "",
      qux;

    // or..
    var // Comment on these
    foo = "",
    bar = "",
    quux;

    // 2.B.1.3
    // var statements should always be in the beginning of their respective scope (function).


    // Bad
    function foo() {

      // some statements here

      var bar = "",
        qux;
    }

    // Good
    function foo() {
      var bar = "",
        qux;

      // all statements after the variables declarations.
    }

    // 2.B.1.4
    // const and let, from ECMAScript 6, should likewise be at the top of their scope (block).

    // Bad
    function foo() {
      let foo,
        bar;
      if ( condition ) {
        bar = "";
        // statements
      }
    }
    // Good
    function foo() {
      let foo;
      if ( condition ) {
        let bar = "";
        // statements
      }
    }

    // 2.B.2.1
    // Named Function Declaration
    function foo( arg1, argN ) {

    }

    // Usage
    foo( arg1, argN );


    // 2.B.2.2
    // Named Function Declaration
    function square( number ) {
      return number * number;
    }

    // Usage
    square( 10 );

    // Really contrived continuation passing style
    function square( number, callback ) {
      callback( number * number );
    }

    square( 10, function( square ) {
      // callback statements
    });


    // 2.B.2.3
    // Function Expression
    var square = function( number ) {
      // Return something valuable and relevant
      return number * number;
    };

    // Function Expression with Identifier
    // This preferred form has the added value of being
    // able to call itself and have an identity in stack traces:
    var factorial = function factorial( number ) {
      if ( number < 2 ) {
        return 1;
      }

      return number * factorial( number - 1 );
    };


    // 2.B.2.4
    // Constructor Declaration
    function FooBar( options ) {

      this.options = options;
    }

    // Usage
    var fooBar = new FooBar({ a: "alpha" });

    fooBar.options;
    // { a: "alpha" }

    C. Exceptions, Slight Deviations


    // 2.C.1.1
    // Functions with callbacks
    foo(function() {
      // Note there is no extra space between the first paren
      // of the executing function call and the word "function"
    });

    // Function accepting an array, no space
    foo([ "alpha", "beta" ]);

    // 2.C.1.2
    // Function accepting an object, no space
    foo({
      a: "alpha",
      b: "beta"
    });

    // Single argument string literal, no space
    foo("bar");

    // Expression parens, no space
    if ( !("foo" in obj) ) {
      obj = (obj.bar || defaults).baz;
    }

    D. Consistency Always Wins

    In sections 2.A-2.C, the whitespace rules are set forth as a recommendation with a simpler, higher purpose: consistency. It's important to note that formatting preferences, such as "inner whitespace" should be considered optional, but only one style should exist across the entire source of your project.


    // 2.D.1.1

    if (condition) {
      // statements
    }

    while (condition) {
      // statements
    }

    for (var i = 0; i < 100; i++) {
      // statements
    }

    if (true) {
      // statements
    } else {
      // statements
    }

    E. Quotes

    Whether you prefer single or double shouldn't matter, there is no difference in how JavaScript parses them. What ABSOLUTELY MUST be enforced is consistency. Never mix quotes in the same project. Pick one style and stick with it.

    F. End of Lines and Empty Lines

    Whitespace can ruin diffs and make changesets impossible to read. Consider incorporating a pre-commit hook that removes end-of-line whitespace and blanks spaces on empty lines automatically.

  2. Type Checking (Courtesy jQuery Core Style Guidelines)

    A. Actual Types

    String:

     typeof variable === "string"

    Number:

     typeof variable === "number"

    Boolean:

     typeof variable === "boolean"

    Object:

     typeof variable === "object"

    Array:

     Array.isArray( arrayLikeObject )
     (wherever possible)

    Node:

     elem.nodeType === 1

    null:

     variable === null

    null or undefined:

     variable == null

    undefined:

    Global Variables:

     typeof variable === "undefined"

    Local Variables:

     variable === undefined

    Properties:

     object.prop === undefined
     object.hasOwnProperty( prop )
     "prop" in object

    B. Coerced Types

    Consider the implications of the following...

    Given this HTML:


    <input type="text" id="foo-input" value="1">


    // 3.B.1.1

    // `foo` has been declared with the value `0` and its type is `number`
    var foo = 0;

    // typeof foo;
    // "number"
    ...

    // Somewhere later in your code, you need to update `foo`
    // with a new value derived from an input element

    foo = document.getElementById("foo-input").value;

    // If you were to test `typeof foo` now, the result would be `string`
    // This means that if you had logic that tested `foo` like:

    if ( foo === 1 ) {

      importantTask();

    }

    // `importantTask()` would never be evaluated, even though `foo` has a value of "1"


    // 3.B.1.2

    // You can preempt issues by using smart coercion with unary + or - operators:

    foo = +document.getElementById("foo-input").value;
    //    ^ unary + operator will convert its right side operand to a number

    // typeof foo;
    // "number"

    if ( foo === 1 ) {

      importantTask();

    }

    // `importantTask()` will be called

    Here are some common cases along with coercions:


    // 3.B.2.1

    var number = 1,
      string = "1",
      bool = false;

    number;
    // 1

    number + "";
    // "1"

    string;
    // "1"

    +string;
    // 1

    +string++;
    // 1

    string;
    // 2

    bool;
    // false

    +bool;
    // 0

    bool + "";
    // "false"
    // 3.B.2.2

    var number = 1,
      string = "1",
      bool = true;

    string === number;
    // false

    string === number + "";
    // true

    +string === number;
    // true

    bool === number;
    // false

    +bool === number;
    // true

    bool === string;
    // false

    bool === !!string;
    // true
    // 3.B.2.3

    var array = [ "a", "b", "c" ];

    !!~array.indexOf("a");
    // true

    !!~array.indexOf("b");
    // true

    !!~array.indexOf("c");
    // true

    !!~array.indexOf("d");
    // false

    // Note that the above should be considered "unnecessarily clever"
    // Prefer the obvious approach of comparing the returned value of
    // indexOf, like:

    if ( array.indexOf( "a" ) >= 0 ) {
      // ...
    }
    // 3.B.2.4


    var num = 2.5;

    parseInt( num, 10 );

    // is the same as...

    ~~num;

    num >> 0;

    num >>> 0;

    // All result in 2


    // Keep in mind however, that negative numbers will be treated differently...

    var neg = -2.5;

    parseInt( neg, 10 );

    // is the same as...

    ~~neg;

    neg >> 0;

    // All result in -2
    // However...

    neg >>> 0;

    // Will result in 4294967294




  3. Conditional Evaluation


    // 4.1.1
    // When only evaluating that an array has length,
    // instead of this:
    if ( array.length > 0 ) ...

    // ...evaluate truthiness, like this:
    if ( array.length ) ...


    // 4.1.2
    // When only evaluating that an array is empty,
    // instead of this:
    if ( array.length === 0 ) ...

    // ...evaluate truthiness, like this:
    if ( !array.length ) ...


    // 4.1.3
    // When only evaluating that a string is not empty,
    // instead of this:
    if ( string !== "" ) ...

    // ...evaluate truthiness, like this:
    if ( string ) ...


    // 4.1.4
    // When only evaluating that a string _is_ empty,
    // instead of this:
    if ( string === "" ) ...

    // ...evaluate falsy-ness, like this:
    if ( !string ) ...


    // 4.1.5
    // When only evaluating that a reference is true,
    // instead of this:
    if ( foo === true ) ...

    // ...evaluate like you mean it, take advantage of built in capabilities:
    if ( foo ) ...


    // 4.1.6
    // When evaluating that a reference is false,
    // instead of this:
    if ( foo === false ) ...

    // ...use negation to coerce a true evaluation
    if ( !foo ) ...

    // ...Be careful, this will also match: 0, "", null, undefined, NaN
    // If you _MUST_ test for a boolean false, then use
    if ( foo === false ) ...


    // 4.1.7
    // When only evaluating a ref that might be null or undefined, but NOT false, "" or 0,
    // instead of this:
    if ( foo === null || foo === undefined ) ...

    // ...take advantage of == type coercion, like this:
    if ( foo == null ) ...

    // Remember, using == will match a `null` to BOTH `null` and `undefined`
    // but not `false`, "" or 0
    null == undefined

    ALWAYS evaluate for the best, most accurate result - the above is a guideline, not a dogma.


    // 4.2.1
    // Type coercion and evaluation notes

    // Prefer `===` over `==` (unless the case requires loose type evaluation)

    // === does not coerce type, which means that:

    "1" === 1;
    // false

    // == does coerce type, which means that:

    "1" == 1;
    // true


    // 4.2.2
    // Booleans, Truthies & Falsies

    // Booleans:
    true, false

    // Truthy:
    "foo", 1

    // Falsy:
    "", 0, null, undefined, NaN, void 0

  4. Practical Style


    // 5.1.1
    // A Practical Module

    (function( global ) {
      var Module = (function() {

        var data = "secret";

        return {
          // This is some boolean property
          bool: true,
          // Some string value
          string: "a string",
          // An array property
          array: [ 1, 2, 3, 4 ],
          // An object property
          object: {
            lang: "en-Us"
          },
          getData: function() {
            // get the current value of `data`
            return data;
          },
          setData: function( value ) {
            // set the value of `data` and return it
            return ( data = value );
          }
        };
      })();

      // Other things might happen here

      // expose our module to the global object
      global.Module = Module;

    })( this );


    // 5.2.1
    // A Practical Constructor

    (function( global ) {

      function Ctor( foo ) {

        this.foo = foo;

        return this;
      }

      Ctor.prototype.getFoo = function() {
        return this.foo;
      };

      Ctor.prototype.setFoo = function( val ) {
        return ( this.foo = val );
      };


      // To call constructor's without `new`, you might do this:
      var ctor = function( foo ) {
        return new Ctor( foo );
      };


      // expose our constructor to the global object
      global.ctor = ctor;

    })( this );

  5. Naming

    A. You are not a human code compiler/compressor, so don't try to be one.

    The following code is an example of egregious naming:


    // 6.A.1.1
    // Example of code with poor names

    function q(s) {
      return document.querySelectorAll(s);
    }
    var i,a=[],els=q("#foo");
    for(i=0;i<els.length;i++){a.push(els[i]);}

    Without a doubt, you've written code like this - hopefully that ends today.

    Here's the same piece of logic, but with kinder, more thoughtful naming (and a readable structure):


    // 6.A.2.1
    // Example of code with improved names

    function query( selector ) {
      return document.querySelectorAll( selector );
    }

    var idx = 0,
      elements = [],
      matches = query("#foo"),
      length = matches.length;

    for ( ; idx < length; idx++ ) {
      elements.push( matches[ idx ] );
    }

    A few additional naming pointers:


    // 6.A.3.1
    // Naming strings

    `dog` is a string


    // 6.A.3.2
    // Naming arrays

    `dogs` is an array of `dog` strings


    // 6.A.3.3
    // Naming functions, objects, instances, etc

    camelCase; function and var declarations


    // 6.A.3.4
    // Naming constructors, prototypes, etc.

    PascalCase; constructor function


    // 6.A.3.5
    // Naming regular expressions

    rDesc = //;


    // 6.A.3.6
    // From the Google Closure Library Style Guide

    functionNamesLikeThis;
    variableNamesLikeThis;
    ConstructorNamesLikeThis;
    EnumNamesLikeThis;
    methodNamesLikeThis;
    SYMBOLIC_CONSTANTS_LIKE_THIS;

    B. Faces of this

    Beyond the generally well known use cases of call and apply, always prefer .bind( this ) or a functional equivalent, for creating BoundFunction definitions for later invocation. Only resort to aliasing when no preferable option is available.


    // 6.B.1
    function Device( opts ) {

      this.value = null;

      // open an async stream,
      // this will be called continuously
      stream.read( opts.path, function( data ) {

        // Update this instance's current value
        // with the most recent value from the
        // data stream
        this.value = data;

      }.bind(this) );

      // Throttle the frequency of events emitted from
      // this Device instance
      setInterval(function() {

        // Emit a throttled event
        this.emit("event");

      }.bind(this), opts.freq || 100 );
    }

    // Just pretend we've inherited EventEmitter ;)

    When unavailable, functional equivalents to .bind exist in many modern JavaScript libraries.

    // 6.B.2

    // eg. lodash/underscore, _.bind()
    function Device( opts ) {

      this.value = null;

      stream.read( opts.path, _.bind(function( data ) {

        this.value = data;

      }, this) );

      setInterval(_.bind(function() {

        this.emit("event");

      }, this), opts.freq || 100 );
    }

    // eg. jQuery.proxy
    function Device( opts ) {

      this.value = null;

      stream.read( opts.path, jQuery.proxy(function( data ) {

        this.value = data;

      }, this) );

      setInterval( jQuery.proxy(function() {

        this.emit("event");

      }, this), opts.freq || 100 );
    }

    // eg. dojo.hitch
    function Device( opts ) {

      this.value = null;

      stream.read( opts.path, dojo.hitch( this, function( data ) {

        this.value = data;

      }) );

      setInterval( dojo.hitch( this, function() {

        this.emit("event");

      }), opts.freq || 100 );
    }

    As a last resort, create an alias to this using self as an Identifier. This is extremely bug prone and should be avoided whenever possible.


    // 6.B.3

    function Device( opts ) {
      var self = this;

      this.value = null;

      stream.read( opts.path, function( data ) {

        self.value = data;

      });

      setInterval(function() {

        self.emit("event");

      }, opts.freq || 100 );
    }

    C. Use thisArg

    Several prototype methods of ES 5.1 built-ins come with a special thisArg signature, which should be used whenever possible


    // 6.C.1

    var obj;

    obj = { f: "foo", b: "bar", q: "qux" };

    Object.keys( obj ).forEach(function( key ) {

      // |this| now refers to `obj`

      console.log( this[ key ] );

    }, obj ); // <-- the last arg is `thisArg`

    // Prints...

    // "foo"
    // "bar"
    // "qux"

    thisArg can be used with Array.prototype.every, Array.prototype.forEach, Array.prototype.some, Array.prototype.map, Array.prototype.filter

  6. Misc

    This section will serve to illustrate ideas and concepts that should not be considered dogma, but instead exists to encourage questioning practices in an attempt to find better ways to do common JavaScript programming tasks.

    A. Using switch should be avoided, modern method tracing will blacklist functions with switch statements

    There seems to be drastic improvements to the execution of switch statements in latest releases of Firefox and Chrome. http://jsperf.com/switch-vs-object-literal-vs-module

    Notable improvements can be witnessed here as well: https://github.com/rwldrn/idiomatic.js/issues/13


    // 7.A.1.1
    // An example switch statement

    switch( foo ) {
      case "alpha":
        alpha();
        break;
      case "beta":
        beta();
        break;
      default:
        // something to default to
        break;
    }

    // 7.A.1.2
    // A alternate approach that supports composability and reusability is to
    // use an object to store "cases" and a function to delegate:

    var cases, delegator;

    // Example returns for illustration only.
    cases = {
      alpha: function() {
        // statements
        // a return
        return [ "Alpha", arguments.length ];
      },
      beta: function() {
        // statements
        // a return
        return [ "Beta", arguments.length ];
      },
      _default: function() {
        // statements
        // a return
        return [ "Default", arguments.length ];
      }
    };

    delegator = function() {
      var args, key, delegate;

      // Transform arguments list into an array
      args = [].slice.call( arguments );

      // shift the case key from the arguments
      key = args.shift();

      // Assign the default case handler
      delegate = cases._default;

      // Derive the method to delegate operation to
      if ( cases.hasOwnProperty( key ) ) {
        delegate = cases[ key ];
      }

      // The scope arg could be set to something specific,
      // in this case, |null| will suffice
      return delegate.apply( null, args );
    };

    // 7.A.1.3
    // Put the API in 7.A.1.2 to work:

    delegator( "alpha", 1, 2, 3, 4, 5 );
    // [ "Alpha", 5 ]

    // Of course, the `case` key argument could easily be based
    // on some other arbitrary condition.

    var caseKey, someUserInput;

    // Possibly some kind of form input?
    someUserInput = 9;

    if ( someUserInput > 10 ) {
      caseKey = "alpha";
    } else {
      caseKey = "beta";
    }

    // or...

    caseKey = someUserInput > 10 ? "alpha" : "beta";

    // And then...

    delegator( caseKey, someUserInput );
    // [ "Beta", 1 ]

    // And of course...

    delegator();
    // [ "Default", 0 ]


    B. Early returns promote code readability with negligible performance difference


    // 7.B.1.1
    // Bad:
    function returnLate( foo ) {
      var ret;

      if ( foo ) {
        ret = "foo";
      } else {
        ret = "quux";
      }
      return ret;
    }

    // Good:

    function returnEarly( foo ) {

      if ( foo ) {
        return "foo";
      }
      return "quux";
    }

  7. Native & Host Objects

    The basic principle here is:

    Don't do stupid shit and everything will be ok.​

    To reinforce this concept, please watch the following presentation:

    “Everything is Permitted: Extending Built-ins” by Andrew Dupont (JSConf2011, Portland, Oregon)​

    https://www.youtube.com/watch?v=xL3xCO7CLNM

  8. Comments

    Single line above the code that is subject​

    Multiline is good​

    End of line comments are prohibited!​

    JSDoc style is good, but requires a significant time investment​

  9. One Language Code

    Programs should be written in one language, whatever that language may be, as dictated by the maintainer or maintainers.

Appendix​

Comma First.​

Any project that cites this document as its base style guide will not accept comma first code formatting, unless explicitly specified otherwise by that project's author.

Creative Commons License
Principles of Writing Consistent, Idiomatic JavaScript by Rick Waldron and Contributors is licensed under a Creative Commons Attribution 3.0 Unported License.
Based on a work at github.com/rwldrn/idiomatic.js.