Object Methods

As discussed earlier, all JavaScript objects (except those explicitly created without a prototype) inherit properties from Object.prototype. These inherited properties are primarily methods, and because they are universally available, they are of particular interest to JavaScript programmers. We’ve already seen the hasOwnProperty(), propertyIsEnumerable(), and isPrototypeOf() methods. (And we’ve also already cov-ered quite a few static functions defined on the Object constructor, such as Object.create() and Object.getPrototypeOf().) This section explains a handful of uni-versal object methods that are defined on Object.prototype, but which are intended to be overridden by other, more specialized classes.

6.10.1 The toString() Method

The toString() method takes no arguments; it returns a string that somehow represents the value of the object on which it is invoked. JavaScript invokes this method of an object whenever it needs to convert the object to a string. This occurs, for example, when you use the + operator to concatenate a string with an object or when you pass an object to a method that expects a string.

The default toString() method is not very informative (though it is useful for deter-mining the class of an object, as we saw in §6.8.2). For example, the following line of code simply evaluates to the string “[object Object]”:

var s = { x:1, y:1 }.toString();

Because this default method does not display much useful information, many classes define their own versions of toString(). For example, when an array is converted to a string, you obtain a list of the array elements, themselves each converted to a string, and when a function is converted to a string, you obtain the source code for the function.

These customized versions of the toString() method are documented in the reference section. See Array.toString(), Date.toString(), and Function.toString(), for example.

§9.6.3 describes how to define a custom toString() method for your own classes.

6.10.2 The toLocaleString() Method

In addition to the basic toString() method, objects all have a toLocaleString(). The purpose of this method is to return a localized string representation of the object. The default toLocaleString() method defined by Object doesn’t do any localization itself:

it simply calls toString() and returns that value. The Date and Number classes define customized versions of toLocaleString() that attempt to format numbers, dates, and times according to local conventions. Array defines a toLocaleString() method that works like toString() except that it formats array elements by calling their toLocale String() methods instead of their toString() methods.

6.10.3 The toJSON() Method

Object.prototype does not actually define a toJSON() method, but the JSON.stringify() method (see §6.9) looks for a toJSON() method on any object it is asked to serialize. If this method exists on the object to be serialized, it is invoked, and the return value is serialized, instead of the original object. See Date.toJSON() for an example.

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Core JavaScript

6.10.4 The valueOf() Method

The valueOf() method is much like the toString() method, but it is called when Java-Script needs to convert an object to some primitive type other than a string—typically, a number. JavaScript calls this method automatically if an object is used in a context where a primitive value is required. The default valueOf() method does nothing inter-esting, but some of the built-in classes define their own valueOf() method (see Date.valueOf(), for example). §9.6.3 explains how to define a valueOf() method for custom object types you define.

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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 ma-nipulation 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.