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9.6.3 Standard Conversion Methods

9.6.3 Standard Conversion Methods


§3.8.3 and §6.10 described important methods used for type conversion of objects, some of which are invoked automatically by the JavaScript interpreter when conversion is necessary. You do not need to implement these methods for every class you write, but they are important methods, and if you do not implement them for your classes, it should be a conscious choice not to implement them rather than mere oversight.

The first, and most important, method is toString(). The purpose of this method is to return a string representation of an object. JavaScript automatically invokes this method if you use an object where a string is expected—as a property name, for example,

9.6 Object-Oriented Techniques in JavaScript | 219

or with the + operator to perform string concatenation. If you don’t implement this method, your class will inherit the default implementation from Object.prototype and will convert to the useless string “[object Object]”. A toString() method might return a human-readable string suitable for display to end users of your program. Even if this is not necessary, however, it is often useful to define toString() for ease of debugging. The Range and Complex classes in Examples 9-2 and 9-3 have toString() methods, as do the enumerated types of Example 9-7 . We’ll define a toString() method for the Set class of Example 9-6 below.

The toLocaleString() is closely related to toString(): it should convert an object to a string in a locale-sensitive way. By default, objects inherit a toLocaleString() method that simply calls their toString() method. Some built-in types have useful toLocale String() methods that actually return locale-dependent strings. If you find yourself writing a toString() method that converts other objects to strings, you should also define a toLocaleString() method that performs those conversions by invoking the toLocaleString() method on the objects. We’ll do this for the Set class below.

The third method is valueOf(). Its job is to convert an object to a primitive value. The valueOf()method is invoked automatically when an object is used in a numeric context, with arithmetic operators (other than +) and with the relational operators, for example. Most objects do not have a reasonable primitive representation and do not define this method. The enumerated types in Example 9-7 demonstrate a case in which the valueOf() method is important, however.

The fourth method is toJSON(), which is invoked automatically by JSON.stringify(). The JSON format is intended for serialization of data structures and can handle Java-Script primitive values, arrays, and plain objects. It does not know about classes, and when serializing an object, it ignores the object’s prototype and constructor. If you call JSON.stringify() on a Range or Complex object, for example, it returns a string like {"from":1, "to":3}or {"r":1, "i":-1}. If you pass these strings to JSON.parse(), you’ll obtain a plain object with properties appropriate for Range and Complex objects, but which do not inherit the Range and Complex methods.

This kind of serialization is appropriate for classes like Range and Complex, but for other classes you may want to write a toJSON() method to define some other serialization format. If an object has a toJSON() method, JSON.stringify() does not serialize the object but instead calls toJSON() and serializes the value (either primitive or object) that it returns. Date objects, for example, have a toJSON() method that returns a string representation of the date. The enumerated types of Example 9-7 do the same: their toJSON() method is the same as their toString() method. The closest JSON analog to a set is an array, so we’ll define a toJSON() method below that converts a Set object to an array of values.

The Set class of Example 9-6 does not define any of these methods. A set has no primitive representation, so it doesn’t make sense to define a valueOf() method, but the class should probably have toString(), toLocaleString(), and toJSON() methods. We can do that with code like the following. Note the use of the extend() function ( Ex ample 6-2 ) to add methods to Set.prototype:

// Add these methods to the Set prototype object.

extend(Set.prototype, { // Convert a set to a string toString: function() {

var s = "{", i = 0; this.foreach(function(v) { s += ((i++ > 0)?", ":"") + v; }); return s + "}";

}, // Like toString, but call toLocaleString on all values toLocaleString : function() {

var s = "{", i = 0;

this.foreach(function(v) { if (i++ > 0) s += ", "; if (v == null) s += v; // null & undefined else s += v.toLocaleString(); // all others

});

return s + "}"; }, // Convert a set to an array of values toArray: function() {

var a = []; this.foreach(function(v) { a.push(v); }); return a;

} });

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