JavaScript: The Definitive Guide, 6th Edition

ECMAScript 5 Array Methods

ECMAScript 5 defines nine new array methods for iterating, mapping, filtering, testing, reducing, and searching arrays. The subsections below describe these methods.

Before we cover the details, however, it is worth making some generalizations about these ECMAScript 5 array methods. First, most of the methods accept a function as their first argument and invoke that function once for each element (or some elements) of the array. If the array is sparse, the function you pass is not invoked for nonexistent elements. In most cases, the function you supply is invoked with three arguments: the value of the array element, the index of the array element, and the array itself. Often, you only need the first of these argument values and can ignore the second and third values. Most of the ECMAScript 5 array methods that accept a function as their first argument accept an optional second argument. If specified, the function is invoked as if it is a method of this second argument. That is, the second argument you pass becomes the value of the this keyword inside of the function you pass. The return value of the function you pass is important, but different methods handle the return value in different ways. None of the ECMAScript 5 array methods modify the array on which they are invoked. If you pass a function to these methods, that function may modify the array, of course.

forEach()

The forEach() method iterates through an array, invoking a function you specify for ...