JavaScript: The Definitive Guide, 6th Edition

Function Properties, Methods, and Constructor

We’ve seen that functions are values in JavaScript programs. The typeof operator returns the string “function” when applied to a function, but functions are really a specialized kind of JavaScript object. Since functions are objects, they can have properties and methods, just like any other object. There is even a Function() constructor to create new function objects. The subsections that follow document function properties and methods and the Function() constructor. You can also read about these in the reference section.

The length Property

Within the body of a function, arguments.length specifies the number of arguments that were passed to the function. The length property of a function itself, however, has a different meaning. This read-only property returns the arity of the function—the number of parameters it declares in its parameter list, which is usually the number of arguments that the function expects.

The following code defines a function named check() that is passed the arguments array from another function. It compares arguments.length (the number of arguments actually passed) to arguments.callee.length (the number expected) to determine whether the function was passed the right number of arguments. If not, it throws an exception. The check() function is followed by a test function f() that demonstrates how check() can be used:

// This function uses arguments.callee, so it won't work in strict mode.
function check(args) {