var
statement
declares a function-scoped or globally-scoped variable, optionally initializing it to a value.
var varname1 [= value1] [, varname2 [= value2] ... [, varnameN [= valueN]]];
varname
N
Variable name. It can be any legal identifier.
value
N
可选
undefined
.
var
declarations, wherever they occur, are processed before any code is executed. This is called
hoisting
, and is discussed further below.
The scope of a variable declared with
var
is its current
execution context and closures thereof
, which is either the enclosing function and functions declared within it, or, for variables declared outside any function, global. Duplicate variable declarations using
var
will not trigger an error, even in strict mode, and the variable will not lose its value, unless another assignment is performed.
'use strict';
function foo() {
var x = 1;
function bar() {
var y = 2;
console.log(x); // 1 (function `bar` closes over `x`)
console.log(y); // 2 (`y` is in scope)
}
bar();
console.log(x); // 1 (`x` is in scope)
console.log(y); // ReferenceError in strict mode, `y` is scoped to `bar`
}
foo();
Variables declared using
var
are created before any code is executed in a process known as hoisting. Their initial value is
undefined
.
'use strict';
console.log(x); // undefined (note: not ReferenceError)
console.log('still going...'); // still going...
var x = 1;
console.log(x); // 1
console.log('still going...'); // still going...
In the global context, a variable declared using
var
is added as a non-configurable property of the global object. This means its property descriptor cannot be changed and it cannot be deleted using
delete
. The corresponding name is also added to a list on the internal
[[VarNames]]
slot on the
global environment record
(which forms part of the global lexical environment). The list of names in
[[VarNames]]
enables the runtime to distinguish between global variables and straightforward properties on the global object.
The property created on the global object for global variables, is set to be non-configurable because the identifier is to be treated as a variable, rather than a straightforward property of the global object. JavaScript has automatic memory management, and it would make no sense to be able to use the
delete
operator on a global variable.
'use strict';
var x = 1;
globalThis.hasOwnProperty('x'); // true
delete globalThis.x; // TypeError in strict mode. Fails silently otherwise.
delete x; // SyntaxError in strict mode. Fails silently otherwise.
Note that in both NodeJS CommonJS modules and native ECMAScript modules , top-level variable declarations are scoped to the module, and are not, therefore added as properties to the global object.
The global object sits at the top of the scope chain. When attempting to resolve a name to a value, the scope chain is searched. This means that properties on the global object are conveniently visible from every scope, without having to qualify the names with
globalThis.
or
窗口。
or
global.
.
So you can just type:
function foo() {
String('s') // Note the function `String` is implicitly visible
}
...because
globalThis.hasOwnProperty('String') // true
So the global object will ultimately be searched for unqualified identifiers. You don't have to type
globalThis.String
, you can just type the unqualified
String
. The corollary, in non-strict mode, is that assignment to unqualified identifiers will, if there is no variable of the same name declared in the scope chain, assume you want to create a property with that name on the global object.
foo = 'f' // In non-strict mode, assumes you want to create a property named `foo` on the global object
globalThis.hasOwnProperty('foo') // true
In ECMAScript 5, this behavior was changed for
严格模式
. Assignment to an unqualified identifier in strict mode will result in a
ReferenceError
, to avoid the accidental creation of properties on the global object.
Note that the implication of the above, is that, contrary to popular misinformation, JavaScript does not have implicit or undeclared variables, it merely has a syntax that looks like it does.
Because variable declarations (and declarations in general) are processed before any code is executed, declaring a variable anywhere in the code is equivalent to declaring it at the top. This also means that a variable can appear to be used before it's declared. This behavior is called " hoisting ", as it appears that the variable declaration is moved to the top of the function or global code.
bla = 2; var bla; // ...is implicitly understood as: var bla; bla = 2;
For that reason, it is recommended to always declare variables at the top of their scope (the top of global code and the top of function code) so it's clear which variables are function scoped (local) and which are resolved on the scope chain.
It's important to point out that the hoisting will affect the variable declaration, but not its value's initialization. The value will be indeed assigned when the assignment statement is reached:
function do_something() {
console.log(bar); // undefined
var bar = 111;
console.log(bar); // 111
}
// ...is implicitly understood as:
function do_something() {
var bar;
console.log(bar); // undefined
bar = 111;
console.log(bar); // 111
}
var a = 0, b = 0;
var a = 'A'; var b = a; // ...is equivalent to: var a, b = a = 'A';
Be mindful of the order:
var x = y, y = 'A'; console.log(x + y); // undefinedA
Here,
x
and
y
are declared before any code is executed, but the assignments occur later. At the time "
x = y
" is evaluated,
y
exists so no
ReferenceError
is thrown and its value is
undefined
. So,
x
is assigned the undefined value. Then,
y
is assigned the value
'A'
. Consequently, after the first line,
x === undefined && y === 'A'
, hence the result.
var x = 0;
function f() {
var x = y = 1; // Declares x locally; declares y globally.
}
f();
console.log(x, y); // 0 1
// In non-strict mode:
// x is the global one as expected;
// y is leaked outside of the function, though!
The same example as above but with a strict mode:
'use strict';
var x = 0;
function f() {
var x = y = 1; // Throws a ReferenceError in strict mode.
}
f();
console.log(x, y);
Variables that appear to be implicit globals may be references to variables in an outer function scope:
var x = 0; // Declares x within file scope, then assigns it a value of 0.
console.log(typeof z); // "undefined", since z doesn't exist yet
function a() {
var y = 2; // Declares y within scope of function a, then assigns it a value of 2.
console.log(x, y); // 0 2
function b() {
x = 3; // Assigns 3 to existing file scoped x.
y = 4; // Assigns 4 to existing outer y.
z = 5; // Creates a new global variable z, and assigns it a value of 5.
// (Throws a ReferenceError in strict mode.)
}
b(); // Creates z as a global variable.
console.log(x, y, z); // 3 4 5
}
a(); // Also calls b.
console.log(x, z); // 3 5
console.log(typeof y); // "undefined", as y is local to function a
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'variable statement' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
var
|
Chrome 1 | Edge 12 | Firefox 1 | IE 3 | Opera 3 | 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