Principles of Writing Consistent, Idiomatic JavaScript

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

Table of Contents

• Whitespace
• Syntax
• Type Checking (Courtesy jQuery Core Style Guidelines)
• Conditional Evaluation
• Practical Style
• Naming
• Misc
• Native & Host Objects
• Comments
• One Language Code

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Code styling

1. Syntax

   A. Whitespaces

   • Use always tabs.
   • Use always double quotes.
   • Trim trailing white spaces.

   B. Parenthesis, Braces and Linebreaks

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

   // Bad
   if ( condition ) doSomething();
   
   // Good
   if (condition) {
       doSomething();
   }
   
   // Bad
   for(var i=0;i<100;i++) someIterativeFn();
   
   // Good
   var i;
   
   for (i = 0; i < 100; i++) {
       someIterativeFn();
   }

   C. Assignments, Declarations

   • Do not expose local variables to the global scope.
   • Use only one var per scope (function).
   • var statements should always be in the beginning of their respective scope.
   • Prefer literal notation over object notation.

   // Bad
   var foo = "bar",
       num,
       arr = [1,2,3];
   
   // Bad
   var foo = "";
   var num = 3;
   var arr = [1,2,3];
   
   // Good
   var foo = "bar",
       num = 1,
       arr = [1,2,3];
   
   // also good
   var foo, num, arr;
   
   // Bad
   var array = new Array(),
       object = new Object();
   
   // Good
   var array = [],
       object = {};
   
   // Bad
   function foo() {
     // some statements here
   
     var bar = "";
   }
   
   // Good
   function foo() {
     var bar = "";
   
     // all statements after the variables declarations.
   }
   
   // Bad
   function foo() {
     bar = "";
     // some statements here
   }
   
   // Good
   function foo() {
     var bar = "";
     // some statements here
   }

   D. Functions ( Named, Expression, Constructor )

   // Named Function Declaration
   function foo(arg1, argN) {
   
   }
   
   // Usage
   foo(arg1, argN);
   
   // Really contrived continuation passing style
   function square(number, callback) {
     callback(number * number);
   }
   
   square(10, function(square) {
     // callback statements
   });
   
   // Function Expression
   var square = function(number) {
     // Return something valuable and relevant
     return number * number;
   };
   
   // UNAPPROVED
   // Useful for functions that call themself, but I think it can be
   // removed from here
   /*
   // 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);
   };
   */
   
   // Constructor Declaration
   function FooBar(options) {
     this.options = options;
   }
   
   // Usage
   var fooBar = new FooBar({a: "alpha"});
   fooBar.options;
   // { a: "alpha" }
   
   // Functions with callbacks
   foo(function() {
     // Note there is no extra space between the first paren
     // of the executing function call and the word "function"
   });

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:

    $.isArray(arrayLikeObject)
   

   Node:

    elem.nodeType === 1
   

   null:

    variable === null
   

   null or undefined (do no use the short version):

    variable == null                                // Bad
    variable === null || variable === undefined     // Good
   

   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
   
   // 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. 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) ...

   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
   
   // Falsy:
   "", 0, null, undefined, NaN, false, void 0
   
   // Any other return true

4. 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 = 0; 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

   
   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.
   
   ```javascript
   
   // 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 );
   }
   
   

5. 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 witnesses here as well: rwaldron#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";
   }

6. 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)

   <iframe src="http://blip.tv/play/g_Mngr6LegI.html" width="480" height="346" frameborder="0" allowfullscreen></iframe>

   http://blip.tv/jsconf/jsconf2011-andrew-dupont-everything-is-permitted-extending-built-ins-5211542

7. 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

8. One Language Code

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

Appendix

JSHint file

// code

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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.