var
The var
statement declares a function-scoped or
globally-scoped variable, optionally initializing it to a value.
Try it
Syntax
js
var name1;
var name1 = value1;
var name1 = value1, name2 = value2;
var name1, name2 = value2;
var name1 = value1, name2, /* …, */ nameN = valueN;
nameN
-
Variable name. It can be any legal identifier.
valueN
Optional-
Initial value of the variable. It can be any legal expression. Default value is
undefined
.
Alternatively, the Destructuring Assignment syntax can also be used to declare variables.
js
var { bar } = foo; // where foo = { bar:10, baz:12 };
/* This creates a variable with the name 'bar', which has a value of 10 */
Description
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.
js
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, `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
.
js
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.
js
"use strict";
var x = 1;
Object.hasOwn(globalThis, "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.
Unqualified identifier assignments
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 window.
or global.
.
Because the global object has a String
property (Object.hasOwn(globalThis, 'String')
), you can use the following code:
js
function foo() {
String("s"); // Note the function `String` is implicitly visible
}
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.
js
foo = "f"; // In non-strict mode, assumes you want to create a property named `foo` on the global object
Object.hasOwn(globalThis, "foo"); // true
In strict mode, 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.
var hoisting
Because var
declarations 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.
js
bla = 2;
var bla;
This is implicitly understood as:
js
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 only a variable's declaration is hoisted,
not its initialization. The initialization happens only when the assignment
statement is reached. Until then the variable remains undefined
(but declared):
js
function do_something() {
console.log(bar); // undefined
var bar = 111;
console.log(bar); // 111
}
This is implicitly understood as:
js
function do_something() {
var bar;
console.log(bar); // undefined
bar = 111;
console.log(bar); // 111
}
Examples
Declaring and initializing two variables
js
var a = 0,
b = 0;
Assigning two variables with single string value
js
var a = "A";
var b = a;
This is equivalent to:
js
var a, b = a = 'A';
Be mindful of the order:
js
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.
Initialization of several variables
js
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:
js
"use strict";
var x = 0;
function f() {
var x = y = 1; // Throws a ReferenceError in strict mode.
}
f();
console.log(x, y);
Implicit globals and outer function scope
Variables that appear to be implicit globals may be references to variables in an outer function scope:
js
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
Specifications
Specification |
---|
ECMAScript Language Specification # sec-variable-statement |
Browser compatibility
BCD tables only load in the browser