JavaScript: The Definitive Guide, 6th Edition

Reading and Writing Array Elements

You access an element of an array using the [] operator. A reference to the array should appear to the left of the brackets. An arbitrary expression that has a non-negative integer value should be inside the brackets. You can use this syntax to both read and write the value of an element of an array. Thus, the following are all legal JavaScript statements:

var a = ["world"];     // Start with a one-element array
var value = a[0];      // Read element 0
a[1] = 3.14;           // Write element 1
i = 2;         
a[i] = 3;              // Write element 2
a[i + 1] = "hello";    // Write element 3
a[a[i]] = a[0];        // Read elements 0 and 2, write element 3

Remember that arrays are a specialized kind of object. The square brackets used to access array elements work just like the square brackets used to access object properties. JavaScript converts the numeric array index you specify to a string—the index 1 becomes the string "1"—then uses that string as a property name. There is nothing special about the conversion of the index from a number to a string: you can do that with regular objects, too:

o = {};        // Create a plain object
o[1] = "one";  // Index it with an integer

What is special about arrays is that when you use property names that are non-negative integers less than 2³², the array automatically maintains the value of the length property for you. Above, for example, we created an array a with a single element. We then assigned values at indexes 1, 2, and 3. The length property of the array changed ...