with
statement is not recommended, as it may be the source of confusing bugs and compatibility issues. See the "Ambiguity Contra" paragraph in the "Description" section below for details.
with statement extends the scope chain for a statement.
with (expression) statement
expression
Adds the given expression to the scope chain used when evaluating the statement. The parentheses around the expression are required.
statement
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.
with
is not recommended, and is forbidden in ECMAScript 5
严格模式
. The recommended alternative is to assign the object whose properties you want to access to a temporary variable.
Pro:
with
statement 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.
Contra:
with
statement 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.
Contra:
with
statement 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) {
console.log(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.
Contra:
Code using
with
may not be forward compatible, especially when used with something other than a plain object. Consider this example:
function f(foo, values) {
with (foo) {
console.log(values);
}
}
If you call
f([1,2,3], obj)
in an ECMAScript 5 environment, then the
值
reference inside the
with
statement will resolve to
obj
. However, ECMAScript 2015 introduces a
值
property on
Array.prototype
(so that it will be available on every array). So, in a JavaScript environment that supports ECMAScript 2015, the
值
reference inside the
with
statement could resolve to
[1,2,3].values
. However, in this particular example,
Array.prototype
has been defined with
值
in its
Symbol.unscopables
object. If it were not, one can see how this would be a difficult issue to debug.
with
下列
with
statement specifies that the
Math
object is the default object. The statements following the
with
statement refer to the
PI
property and the
cos
and
sin
methods, without specifying an object. JavaScript assumes the
Math
object for these references.
var a, x, y;
var r = 10;
with (Math) {
a = PI * r * r;
x = r * cos(PI);
y = r * sin(PI / 2);
}
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'with statement' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
with
弃用
|
Chrome 1 | Edge 12 | Firefox 1 | IE 3 | Opera 4 | Safari 1 | WebView Android 1 | Chrome Android 18 | Firefox Android 4 | Opera Android 10.1 | Safari iOS 1 | Samsung Internet Android 1.0 | nodejs 0.1.100 |
完整支持
弃用。不要用于新网站。