JavaScript中with/this用法

1.with用法

为一个或一组语句指定默认对象。不过，with用法并不推荐！

用法：with (<对象>) <语句>;

with 语句通常用来缩短特定情形下必须写的代码量。在下面的例子中，请注意 Math 的重复使用：

x = Math.cos(3 * Math.PI) + Math.sin(Math.LN10);

y = Math.tan(14 * Math.E);

当使用 with 语句时，代码变得更短且更易读：

with (Math) {

   x = cos(3 * PI) + sin(LN10);

   y = tan(14 * E);
}

HISTORY 

EDIT

TABLE OF CONTENTS

Summary
Syntax

Parameters

Description

Examples

Example: Using 

with

TAGS 

FILES

Use of the 

with

 statement is not recommended, as it may be the source of confusing bugs and compatibility issues. See the "Ambiguity Con" paragraph
in the "Description" section below for details.

Summary

Extends the scope chain for a statement.

Statement
Implemented in	JavaScript 1.0
ECMAScript Edition	ECMA-262

Syntax

with (object) {
statement
}

Parameters

object

Adds the given obj
4000
ect to the scope chain used when evaluating the statement. The parentheses around object are required.

statement

Any statement. To execute multiple statements, use a block statement
({ ... }) to group those statements.

Description

JavaScript looks up an unqualified name by searching a scope chain associated with the execution context of the script or function containing that unqualified name. The 'with' statement adds the given object to the head of this scope chain during the evaluation
of its statement body. If an unqualified name used in the body matches a property in the scope chain, then the name is bound to the property and the object containing the property. Otherwise a 'ReferenceError' is thrown.

Using 

with

 is not recommended, and is forbidden in ECMAScript 5 strict
mode. The recommended alternative is to assign the object whose properties you want to access to a temporary variable.

(On a historical note, Firefox 1.5 used to generate a warning when the 'with' statement was used: "deprecated with statement usage". This has been removed in Firefox 1.5.0.1 (bug 322430).)

Performance Pro & Con
Pro: 'with' can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. The scope chain change required by 'with' is not computationally expensive. Use of 'with' will relieve
the interpreter of parsing repeated object references. Note, however, that in many cases this benefit can be achieved by using a temporary variable to store a reference to the desired object.
Con: 'with' forces the specified object to be searched first for all name lookups. Therefore all identifiers that aren't members of the specified object will be found more slowly in a 'with' block. Where performance is important,
'with' should only be used to encompass code blocks that access members of the specified object.

Ambiguity Con
Con: 'with' makes it hard for a human reader or JavaScript compiler to decide whether an unqualified name will be found along the scope chain, and if so, in which object. So given this example:

function f(x, o) {
with (o)
print(x);
}

only when 

f

 is called is 

x

 either found or not, and if found, either in 

o

 or
(if no such property exists) in 

f

's activation object, where 

x

 names the first
formal argument. If you forget to define 

x

 in the object you pass as the second argument, or if there's some similar bug or confusion, you won't get an error -- just
unexpected results.

Con: Code using 

with

 may not be forward compatible, especially when used with something else than a plain object. Consider this example:

[code]console[code]o[code]