JavaScript: The Definitive Guide, Fourth Edition

Functions as Data

The most important features of functions are that they can be defined and invoked, as shown in the previous section. 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 data, which means that 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.^[24]

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 holds the function. The function can be assigned to another variable and still work the same way:

var a = square(4);  // a contains the number 16
var b = square;     // Now b refers to the same function that square does
var c = b(5);       // c contains the number 25

Functions can also be assigned to object properties rather than global variables. When we do this, we call them methods:

var o = new Object;
o.square = new Function("x", "return x*x");  // Note Function(  ) constructor
y = o.square(16);                            // y equals 256

Functions don’t even require names at all, as when we assign them to array elements:

var a = new Array(3); a[0] = function(x) { return x*x; } // Note function literal a[1] = 20; a[2] = a[0](a[1]); ...