3.2 Anonymous Functions

>>>

var f = function (value) {
    return 2 * value;
}

The function name can be omitted, creating an anonymous function. This is an example of a function expression, which can be handled just as other expression values. If stored in a variable...

>>>

f(1)

...the function call works just as when using a named function.

>>>

var g = f;

Since functions are first-class citizens, we can pass the function reference between variables. Or use it in any other situation where a normal value would be used.

>>>

g(2)

It doesn't matter which name is used when calling the function. This is sometimes used to abbreviate long function names.

>>>

var g = (value) => 2 * value;

Using the => arrow function syntax (since ES6), anonymous functions can be created with less code (i.e. without using the function or return keywords, etc). Other differences will be explained later.

>>>

var h = function () { return 1; };
function h() { return 2; };

>>>

h()

The above illustrates an important difference between function declarations and function expressions. The named function variable is bound only once, at parse-time. But normal variables are statements, which are evaluated and bound at run-time (when the code is executed). Hence, a variable name can always shadow a named function.

>>>

(function () { return 4711; })()

With the proper parentheses to avoid syntactic confusion, we can call an anonymous function immediately. This has been a common pattern for doing one-off initializer functions.

>>>

Console Learner — An interactive JavaScript course.