A function definition expression defines a JavaScript function,
and the value of such an expression is the newly defined function. In
a sense, a function definition expression is a “function literal” in
the same way that an object initializer is an “object literal.” A
function definition expression typically consists of the keyword
function followed by a
comma-separated list of zero or more identifiers (the parameter names)
in parentheses and a block of JavaScript code (the function body) in
curly braces. For example:

// This function returns the square of the value passed to it.
var square = function(x) { return x * x; }

A function definition expression can also include a name for the
function. Functions can also be defined using a function statement
rather than a function expression. Complete details on function
definition are in Chapter 8.