with - JavaScript 编辑

Use of the 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.

The with statement extends the scope chain for a statement.

Syntax

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

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.

Performance pro & contra

Pro: The 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: The 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.

Ambiguity contra

Contra: The 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 values reference inside the with statement will resolve to obj. However, ECMAScript 2015 introduces a values property on Array.prototype (so that it will be available on every array). So, in a JavaScript environment that supports ECMAScript 2015, the values reference inside the with statement could resolve to [1,2,3].values. However, in this particular example, Array.prototype has been defined with values in its Symbol.unscopables object. If it were not, one can see how this would be a difficult issue to debug.

Examples

Using with

The following 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);
}

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'with statement' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• block
• Strict mode
• Symbol.unscopables
• Array.prototype[@@unscopables]