with
Warning: 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.
Note: 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.
let a, x, y;
const r = 10;
with (Math) {
a = PI * r * r;
x = r * cos(PI);
y = r * sin(PI / 2);
}
Specifications
Specification |
---|
ECMAScript Language Specification # sec-with-statement |
Browser compatibility
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