Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Obsolete since Gecko 60 (Firefox 60 / Thunderbird 60 / SeaMonkey 2.57)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
The expression closure syntax is a deprecated Firefox-specific feature and has been removed starting with Firefox 60. For future-facing usages, consider using arrow functions.
Expression closures are a shorthand function syntax for writing simple functions.
Syntax
function [name]([param1[, param2[, ..., paramN]]]) expression
Parameters
name- The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
paramN- The name of an argument to be passed to the function. A function can have up to 255 arguments.
expression- The expression which comprise the body of the function.
Description
This addition is nothing more than a shorthand for writing simple functions, giving the language something similar to a typical Lambda notation.
JavaScript 1.7 and older:
function(x) { return x * x; }
JavaScript 1.8:
function(x) x * x
This syntax allows you to leave off the braces and 'return' statement - making them implicit. There is no added benefit to writing code in this manner, other than having it be syntactically shorter.
Examples
A shorthand for binding event listeners:
document.addEventListener('click', function() false, true);
Using this notation with some of the array functions from JavaScript 1.6:
elems.some(function(elem) elem.type == 'text');
Browser compatibility
Supported nowhere. Historically supported in Firefox 3 till 60.