JavaScript: The Definitive Guide, 6th Edition

Functions As Values

The most important features of functions are that they can be defined and invoked. Function definition and invocation are syntactic features of JavaScript and of most other programming languages. In JavaScript, however, functions are not only syntax but also values, which means they can be assigned to variables, stored in the properties of objects or the elements of arrays, passed as arguments to functions, and so on.^[12]

To understand how functions can be JavaScript data as well as JavaScript syntax, consider this function definition:

function square(x) { return x*x; }

This definition creates a new function object and assigns it to the variable square. The name of a function is really immaterial; it is simply the name of a variable that refers to the function object. The function can be assigned to another variable and still work the same way:

var s = square;     // Now s refers to the same function that square does
square(4);          // => 16
s(4);               // => 16

Functions can also be assigned to object properties rather than variables. When you do this, they’re called methods:

var o = {square: function(x) { return x*x; }}; // An object literal
var y = o.square(16);                          // y equals 256

Functions don’t even require names at all, as when they’re assigned to array elements:

var a = [function(x) { return x*x; }, 20];   // An array literal
a[0](a[1]);                                  // => 400

The syntax of this last example looks strange, but it is still a legal function invocation expression!

Example 8-2 demonstrates the kinds ...