JavaScript is case-sensitive,
use an indent of 2 spaces,
no trailing whitespace,
use JSHint to check most of the rules.

Strict mode

All scripts should be using the strict mode by adding "use strict"; at the top of the closure.


(function ($, Drupal, _) {

  "use strict";
  
  // Rest of the code.

})(jQuery, Drupal, _);

Variable declaration

It is not recommended not to mix assignment and no-assignment variables under a single var statement. Additionally, it is preferred to use multiple var statements in combination with new-lines to avoid having variables accidentally leak into the global scope.


// confusing
var foo = 'bar', x, y, meh = 'bla';

// error-prone
var foo = 'bar',
    x,
    y,
    meh = 'bla';

// Recomended
var x, y;
var foo = 'bar';
var meh = 'bla';

String Concatenation

Always use a space between the + and the concatenated parts to improve readability.


var string = 'Foo' + bar;
string = bar + 'foo';
string = bar() + 'foo';
string = 'foo' + 'bar';

When using the concatenating assignment operator ('+='), use a space on each side as with the assignment operator:


var string += 'Foo';
string += bar;
string += baz();

CamelCasing

Multi-word variables and functions in JavaScript should be lowerCamelCased. The first letter of each variable or function should be lowercase, while the first letter of subsequent words should be capitalized. There should be no underscores between the words. Constructors should start with a capital letter.

Semi-colons

JavaScript allows any expression to be used as a statement and uses semi-colons to mark the end of a statement. However, it attempts to make this optional with "semi-colon insertion", which can mask some errors and will also cause JS aggregation to fail. All statements should be followed by ; except for the following: for, function, if, switch, try, while The exceptions to this are functions declared like


  Drupal.behaviors.tableSelect = function (context) {
    // Statements...
  };

and


  do {
    // Statements...
  } while (condition);

These should all be followed by a semi-colon. In addition the return value expression must start on the same line as the return keyword in order to avoid semi-colon insertion. If the "Optimize JavaScript files" performance option in Drupal 6 is enabled, and there are missing semi-colons, then the JS aggregation will fail. It is therefore very important that semi-colons are used.

Control Structures

These include if, for, while, switch, etc. Here is an example if statement, since it is the most complicated of them:


if (condition1 || condition2) {
  action1();
}
else if (condition3 && condition4) {
  action2();
}
else {
  defaultAction();
}

Control statements should have one space between the control keyword and opening parenthesis, to distinguish them from function calls. You are strongly encouraged to always use curly braces even in situations where they are technically optional. Having them increases readability and decreases the likelihood of logic errors being introduced when new lines are added.

switch

For switch statements:


switch (condition) {
  case 1:
    action1();
    break;

  case 2:
    action2();
    break;

  default:
    defaultAction();
}

try

The try class of statements should have the following form:


  try {
    // Statements...
  }
  catch (error) {
    // Error handling...
  }
  finally {
    // Statements...
  }

for in

The for in statement allows for looping through the names of all of the properties of an object. Unfortunately, all of the members which were inherited through the prototype chain will also be included in the loop. This has the disadvantage of serving up method functions when the interest is in data members. To prevent this, the body of every for in statement should be wrapped in an if statement that does filtering. It can select for a particular type or range of values, or it can exclude functions, or it can exclude properties from the prototype. For example:


for (var variable in object) {
  if (filter) {
    // Statements...
  }
}

You can use the hasOwnProperty method to distinguish the true members of the object (but always put it inside the loop, not on the same line:


for (var index in object) {
  if (object.hasOwnProperty(index)) {
    // Statements...
  }
}

and NOT this:


for (var variable in object) if (object.hasOwnProperty(variable)) {
  // Statements...
}

Functions

Function and method names


Drupal.behaviors.tableDrag = function (context) {
  for (var base in Drupal.settings.tableDrag) {
    if (!$('#' + base + '.tabledrag-processed', context).size()) {
      $('#' + base).filter(':not(.tabledrag-processed)').each(addBehavior);
      $('#' + base).addClass('tabledrag-processed');
    }
  }
};

Functions and methods should be named in lowerCamelCase.
Function names should begin with the name of the module or theme declaring the function to avoid collisions.

Function Declarations


Drupal.behaviors.tableDrag = function (context) {
  ...
  div.onclick = function (e) {
    return false;
  };
  ...
};

function funStuff (field, settings) {
  settings = settings || Drupal.settings;
  alert("This JS file does fun message popups.");
  return field;
}

There is always a space between the function declaration and the following left parenthesis.
Define optional arguments (using default values) at the end of the function signature.
Always attempt to return a meaningful value from a function if one is appropriate.

Note: The above examples code are lacking JSDoc and comments, only for clarity.

Function Calls

Functions should be called with no spaces between the function name, the opening parenthesis, and the first parameter; spaces between commas and each parameter, and no space between the last parameter, the closing parenthesis, and the semicolon. Here's an example:


foobar = foo(bar, baz, quux);

Variables and Arrays

All variables should be declared with var before they are used and should only be declared once. Doing this makes the program easier to read and makes it easier to detect undeclared variables that may become implied globals. Variables should never be defined in the global scope; try to define them in a local function scope at all costs. All variables should be declared at the beginning of a function.

Constants and Global Variables

lowerCamelCasing should be used for pre-defined constants. Unlike the PHP standards, you should use lowercase true, false and null as the uppercase versions are not valid in JS. Variables added through drupal_add_js() should also be lowerCamelCased, so that they can be consistent with other variables once they are used in JavaScript.


 array('basePath' => base_path())), 'setting');
?>

This variable would then be referenced:


drupalSettings.myModule.basePath;

Arrays

Arrays should be formatted with a space separating each element and assignment operator, if applicable:


someArray = ['hello', 'world'];

Note that if the line spans longer than 80 characters (often the case with form and menu declarations), each element should be broken into its own line, and indented one level: Note there is no comma at the end of the last array element. This is different from the PHP coding standards.. Having a comma on the last array element in JS will cause an exception to occur.

Comments

Inline documentation for source files should follow the Doxygen formatting conventions. Non-documentation comments are strongly encouraged. A general rule of thumb is that if you look at a section of code and think "Wow, I don't want to try and describe that", you need to comment it before you forget how it works. Comments can be removed by JS compression utilities later, so they don't negatively impact on the file download size. Non-documentation comments should use capitalized sentences with punctuation. All caps are used in comments only when referencing constants, e.g., TRUE. Comments should be on a separate line immediately before the code line or block they reference. For example:


// Unselect all other checkboxes.

If each line of a list needs a separate comment, the comments may be given on the same line and may be formatted to a uniform indent for readability. C style comments (/* */) and standard C++ comments (//) are both fine.

JS code placement

JavaScript code should not be embedded in the HTML where possible, as it adds significantly to page weight with no opportunity for mitigation by caching and compression.

"with" statement

Do not use the with statement. Instead you should use the explicit version:


  var o = foo.bar.foobar;
  o.abc = true;
  o.xyz = true;

Operators

True or false comparisons

Always use strict comparaison using === and !==. The == and != operators do type coercion before comparing. This is bad because it causes: ' \t\r\n' == 0 to be true. This can mask type errors. When comparing to any of the following values, use the === or !== operators, which do not do type coercion: 0 '' undefined null false true

Comma Operator

The comma operator causes the expressions on either side of it to be executed in left-to-right order, and returns the value of the expression on the right, and should be avoided. Example usage is:


var x = (y = 3, z = 9);

This sets x to 9. This can be confusing for users not familiar with the syntax and makes the code more difficult to read and understand. So avoid the use of the comma operator except for in the control part of for statements. This does not apply to the comma separator (used in object literals, array literals, etc.)

Avoiding unreachable code

To prevent unreachable code, a return, break, continue, or throw statement should be followed by a } or case or default.

Constructors

Constructors are functions that are designed to be used with the new prefix. The new prefix creates a new object based on the function's prototype, and binds that object to the function's implied this parameter. JavaScript doesn't issue compile-time warning or run-time warnings if a required new is omitted. If you neglect to use the new prefix, no new object will be made and this will be bound to the global object (bad). Constructor functions should be given names with an initial uppercase and a function with an initial uppercase name should not be called unless it has the new prefix.

Use literal expressions

Use literal expressions instead of the new operator:

Instead of new Array() use []
Instead of new Object() use {}
Don't use the wrapper forms new Number, new String, new Boolean.

In most cases, the wrapper forms should be the same as the literal expressions. However, this isn't always the case, take the following as an example:


var literalNum = 0;
var objectNum = new Number(0);
if (literalNum) { } // false because 0 is a false value, will not be executed.
if (objectNum) { }  // true because objectNum exists as an object, will be executed.
if (objectNum.valueOf()) { } // false because the value of objectNum is 0.

eval is evil

eval() is evil. It effectively requires the browser to create an entirely new scripting environment (just like creating a new web page), import all variables from the current scope, execute the script, collect the garbage, and export the variables back into the original environment. Additionally, the code cannot be cached for optimization purposes. It is probably the most powerful and most misused method in JavaScript. It also has aliases. So do not use the Function constructor and do not pass strings to setTimeout() or setInterval() .

Preventing XSS

All output to the browser that has been provided by a user should be run through the Drupal.checkPlain() function first. This is similar to Drupal's PHP check_plain() and encodes special characters in a plain-text string for display as HTML.

Typeof

When using a typeof check, don't use the parenthesis for the typeof. The following is the correct coding standard:


if (typeof myVariable === 'string') {
  // ...
}

Drupal 6 (and later) Specific Stuff

Drupal 6 saw the introduction of JavaScript theming and translation of JavaScript files.

Theming

There is a theming mechanism for JavaScript code. Any modules containing JavaScript which produces HTML content must now provide default theme functions in the Drupal.theme namespace.

String Translation

All strings in JavaScript files should be wrapped in Drupal.t() which is an equivalent of the well-known t() function. Likewise, there is also an equivalent to format_plural(), named Drupal.formatPlural(). The parameter order is exactly like their server-side counterparts.

File closure

All js files should have a closure to avoid adding variables to the global namespace. The parameter of the closure correspond to the dependencies needed for the script to work. If a script declare jQuery, Drupal, drupalSettings and underscore as dependencies in hook_library_info() as so:

 array(
      'myjsfile.js' => array(),
    ),
    'dependencies' => array(
      array('system', 'jquery'),
      array('system', 'drupal'),
      array('system', 'drupalSettings'),
      array('system', 'underscore'),
      // The following dependencies do not declare a global
      // object but adds to the jQuery or Drupal object.
      // They will not be present in the closure declaration.
      array('system', 'jquery.once'),
      array('system', 'drupal.ajax'),
    ),
  );
  
  return $libraries;
}
?>

Then the js file should contain the following closure declaration:


(function ($, Drupal, drupalSettings, _) {
  
  "use strict";
  
  
})(jQuery, Drupal, drupalSettings, _);

When this script will be minified it will help replace all the global variables by short names inside the closure.