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() {
this.foo = "bar";
}
Operators
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"
.
Literals
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.
Prefixes
k=constant (e.g.
kNC_child
). Not all code uses this style; some usesALL_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
orx == false
. Use(x)
or(!x)
instead.x == true
, is certainly different from if(x)
! Compare objects tonull
, numbers to0
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;
withreturn 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"
inaObject
, or if you are expecting a particular type you may usetypeof(aObject.name) == "function"
(or whichever type you are expecting).
Use
['value1, value2']
to create a JavaScript array in preference to usingnew {{JSxRef("Array", "Array", "Syntax", 1)}}(value1, value2)
which can be confusing, asnew 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 overnew {{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(“String.search”, “aString.search()”)}} 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 inaString
, or the empty string ifaString
is empty.