JavaScript Coding style

Coding style

prettier is the tool used to reformat the JavaScript code.

Methods and functions

In JavaScript, functions should use camelCase, but should not capitalize the first letter. Methods should not use the named function expression syntax, because our tools understand method names:

doSomething: function (aFoo, aBar) {

In-line functions should have spaces around braces, except before commas or semicolons:

function valueObject(aValue) { return { value: aValue }; }

JavaScript objects

var foo = { prop1: "value1" };

var bar = {
  prop1: "value1",
  prop2: "value2"

Constructors for objects should be capitalized and use Pascal Case:

function ObjectConstructor() { = "bar";


In JavaScript, overlong expressions not joined by && and || should break so the operator starts on the second line and starting in the same column as the beginning of the expression in the first line. This applies to ?:, binary arithmetic operators including +, and member-of operators. Rationale: an operator at the front of the continuation line makes for faster visual scanning, as there is no need to read to the end of line. Also there exists a context-sensitive keyword hazard in JavaScript; see {{bug(442099, “bug”, 19)}}, which can be avoided by putting . at the start of a continuation line, in long member expression.

In JavaScript, == is preferred to ===.

Unary keyword operators, such as typeof, should have their operand parenthesized; e.g. typeof("foo") == "string".


Double-quoted strings (e.g. "foo") are preferred to single-quoted strings (e.g. 'foo'), in JavaScript, except to avoid escaping embedded double quotes, or when assigning inline event handlers.


  • k=constant (e.g. kNC_child). Not all code uses this style; some uses ALL_CAPS for constants.

  • g=global (e.g. gPrefService)

  • a=argument (e.g. aCount)

  • JavaScript Specific Prefixes

    • _=member (variable or function) (e.g. _length or _setType(aType))

    • k=enumeration value (e.g. const kDisplayModeNormal = 0)

    • on=event handler (e.g. function onLoad())

    • Convenience constants for interface names should be prefixed with nsI:

      const nsISupports = Components.interfaces.nsISupports;
      const nsIWBN = Components.interfaces.nsIWebBrowserNavigation;

Other advices

  • Do not compare x == true or x == false. Use (x) or (!x) instead. x == true, is certainly different from if (x)! Compare objects to null, numbers to 0 or strings to "", if there is chance for confusion.

  • Make sure that your code doesn’t generate any strict JavaScript warnings, such as:

    • Duplicate variable declaration.

    • Mixing return; with return value;

    • Undeclared variables or members. If you are unsure if an array value exists, compare the index to the array’s length. If you are unsure if an object member exists, use "name" in aObject, or if you are expecting a particular type you may use typeof( == "function" (or whichever type you are expecting).

  • Use ['value1, value2'] to create a JavaScript array in preference to using new {{JSxRef("Array", "Array", "Syntax", 1)}}(value1, value2) which can be confusing, as new Array(length) will actually create a physically empty array with the given logical length, while [value] will always create a 1-element array. You cannot actually guarantee to be able to preallocate memory for an array.

  • Use { member: value, ... } to create a JavaScript object; a useful advantage over new {{JSxRef("Object", "Object", "", 1)}}() is the ability to create initial properties and use extended JavaScript syntax, to define getters and setters.

  • If having defined a constructor you need to assign default properties, it is preferred to assign an object literal to the prototype property.

  • Use regular expressions, but use wisely. For instance, to check that aString is not completely whitespace use /\S/.{{JSxRef("RegExp.test", "test(aString)", "", 1)}}. Only use {{JSxRef(“”, “”)}} if you need to know the position of the result, or {{JSxRef(“String.match”, “aString.match()”)}} if you need to collect matching substrings (delimited by parentheses in the regular expression). Regular expressions are less useful if the match is unknown in advance, or to extract substrings in known positions in the string. For instance, {{JSxRef(“String.slice”, “aString.slice(-1)”)}} returns the last letter in aString, or the empty string if aString is empty.