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CHAPTER 7 Arrays



An array is an ordered collection of values. Each value is called an element, and each element has a numeric position in the array, known as its index. JavaScript arrays are untyped: an array element may be of any type, and different elements of the same array may be of different types. Array elements may even be objects or other arrays, which allows you to create complex data structures, such as arrays of objects and arrays of arrays. JavaScript arrays are zero-based and use 32-bit indexes: the index of the first element is 0, and the highest possible index is 4294967294 (232−2), for a maximum array size of 4,294,967,295 elements. JavaScript arrays are dynamic: they grow or shrink as needed and there is no need to declare a fixed size for the array when you create it or to reallocate it when the size changes. JavaScript arrays may be sparse: the elements need not have contiguous indexes and there may be gaps. Every JavaScript array has a length property. For nonsparse arrays, this property specifies the number of elements in the array. For sparse arrays, length is larger than the index of all elements.

JavaScript arrays are a specialized form of JavaScript object, and array indexes are really little more than property names that happen to be integers. We’ll talk more about the specializations of arrays elsewhere in this chapter. Implementations typically optimize arrays so that access to numerically indexed array elements is generally significantly faster than access to regular object properties.

Arrays inherit properties from Array.prototype, which defines a rich set of array manipulation methods, covered in §7.8 and §7.9 . Most of these methods are generic, which means that they work correctly not only for true arrays, but for any “array-like object.” We’ll discuss array-like objects in §7.11 . In ECMAScript 5, strings behave like arrays of characters, and we’ll discuss this in §7.12 .

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