JavaScript: The Definitive Guide, Sixth Editio javaScript权威指南(第6版) pdf 文字版-文字版, javascript电子书, 和javascript 有关的电子书:

3.6 Wrapper Objects

JavaScript objects are composite values: they are a collection of properties or named
values. We refer to the value of a property using the . notation. When the value of a
property is a function, we call it a method. To invoke the method m of an object o, we
write o.m().
We’ve also seen that strings have properties and methods:
var s = "hello world!";                             // A string
var word = s.substring(s.indexOf(" ")+1, s.length); // Use string properties
Strings are not objects, though, so why do they have properties? Whenever you try to
refer to a property of a string s, JavaScript converts the string value to an object as if by
calling new String(s). This object inherits (see §6.2.2) string methods and is used to
resolve the property reference. Once the property has been resolved, the newly created
object is discarded. (Implementations are not required to actually create and discard
this transient object: they must behave as if they do, however.)
Numbers and booleans have methods for the same reason that strings do: a temporary
object is created using the Number() or Boolean() constructor, and the method is re-
solved using that temporary object. There are not wrapper objects for the null and
undefined values: any attempt to access a property of one of these values causes a
TypeError.
Consider the following code and think about what happens when it is executed:
var s = "test";         // Start with a string value.
s.len = 4;              // Set a property on it.
var t = s.len;          // Now query the property.
When you run this code, the value of t is undefined. The second line of code creates a
temporary String object, sets its len property to 4, and then discards that object. The
third line creates a new String object from the original (unmodified) string value and
then tries to read the len property. This property does not exist, and the expression
evaluates to undefined. This code demonstrates that strings, numbers, and boolean
values behave like objects when you try to read the value of a property (or method)
from them. But if you attempt to set the value of a property, that attempt is silently
ignored: the change is made on a temporary object and does not persist.
The temporary objects created when you access a property of a string, number, or
boolean are known as wrapper objects, and it may occasionally be necessary to distin-
guish a string value from a String object or a number or boolean value from a Number

or Boolean object. Usually, however, wrapper objects can be considered an

implementation detail and you don’t have to think about them. You just need to know
that string, number, and boolean values differ from objects in that their properties are
read-only and that you can’t define new properties on them.
Note that it is possible (but almost never necessary or useful) to explicitly create wrap-
per objects, by invoking the String(), Number(), or Boolean() constructors:
var s = "test", n = 1, b = true;  // A string, number, and boolean value.
var S = new String(s);            // A String object
var N = new Number(n);            // A Number object
var B = new Boolean(b);           // A Boolean object
JavaScript converts wrapper objects into the wrapped primitive value as necessary, so
the objects S, N, and B above usually, but not always, behave just like the values s, n,
and b. The == equality operator treats a value and its wrapper object as equal, but you
can distinguish them with the === strict equality operator. The typeof operator will also
show you the difference between a primitive value and its wrapper object.

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