What is the scope of variables in JavaScript?
TLDR
JavaScript has lexical (also called static) scoping and closures. This means you can tell the scope of an identifier by looking at the source code.
The four scopes are:
- Global - visible by everything
- Function - visible within a function (and its sub-functions and blocks)
- Block - visible within a block (and its sub-blocks)
- Module - visible within a module
Outside of the special cases of global and module scope, variables are declared using var (function scope), let (block scope), and const (block scope). Most other forms of identifier declaration have block scope in strict mode.
Overview
Scope is the region of the codebase over which an identifier is valid.
A lexical environment is a mapping between identifier names and the values associated with them.
Scope is formed of a linked nesting of lexical environments, with each level in the nesting corresponding to a lexical environment of an ancestor execution context.
These linked lexical environments form a scope "chain". Identifier resolution is the process of searching along this chain for a matching identifier.
Identifier resolution only occurs in one direction: outwards. In this way, outer lexical environments cannot "see" into inner lexical environments.
There are three pertinent factors in deciding the scope of an identifier in JavaScript:
- How an identifier was declared
- Where an identifier was declared
- Whether you are in strict mode or non-strict mode
Some of the ways identifiers can be declared:
var,letandconst- Function parameters
- Catch block parameter
- Function declarations
- Named function expressions
- Implicitly defined properties on the global object (i.e., missing out
varin non-strict mode) importstatementseval
Some of the locations identifiers can be declared:
- Global context
- Function body
- Ordinary block
- The top of a control structure (e.g., loop, if, while, etc.)
- Control structure body
- Modules
Declaration Styles
var
Identifiers declared using var have function scope, apart from when they are declared directly in the global context, in which case they are added as properties on the global object and have global scope. There are separate rules for their use in eval functions.
let and const
Identifiers declared using let and const have block scope, apart from when they are declared directly in the global context, in which case they have global scope.
Note: let, const and var are all hoisted. This means that their logical position of definition is the top of their enclosing scope (block or function). However, variables declared using let and const cannot be read or assigned to until control has passed the point of declaration in the source code. The interim period is known as the temporal dead zone.
function f() { function g() { console.log(x) } let x = 1 g()}f() // 1 because x is hoisted even though declared with `let`!Function parameter names
Function parameter names are scoped to the function body. Note that there is a slight complexity to this. Functions declared as default arguments close over the parameter list, and not the body of the function.
Function declarations
Function declarations have block scope in strict mode and function scope in non-strict mode. Note: non-strict mode is a complicated set of emergent rules based on the quirky historical implementations of different browsers.
Named function expressions
Named function expressions are scoped to themselves (e.g., for the purpose of recursion).
Implicitly defined properties on the global object
In non-strict mode, implicitly defined properties on the global object have global scope, because the global object sits at the top of the scope chain. In strict mode, these are not permitted.
eval
In eval strings, variables declared using var will be placed in the current scope, or, if eval is used indirectly, as properties on the global object.
Examples
The following will throw a ReferenceError because the namesx, y, and z have no meaning outside of the function f.
function f() { var x = 1 let y = 1 const z = 1}console.log(typeof x) // undefined (because var has function scope!)console.log(typeof y) // undefined (because the body of the function is a block)console.log(typeof z) // undefined (because the body of the function is a block)The following will throw a ReferenceError for y and z, but not for x, because the visibility of x is not constrained by the block. Blocks that define the bodies of control structures like if, for, and while, behave similarly.
{ var x = 1 let y = 1 const z = 1}console.log(x) // 1console.log(typeof y) // undefined because `y` has block scopeconsole.log(typeof z) // undefined because `z` has block scopeIn the following, x is visible outside of the loop because var has function scope:
for(var x = 0; x < 5; ++x) {}console.log(x) // 5 (note this is outside the loop!)...because of this behavior, you need to be careful about closing over variables declared using var in loops. There is only one instance of variable x declared here, and it sits logically outside of the loop.
The following prints 5, five times, and then prints 5 a sixth time for the console.log outside the loop:
for(var x = 0; x < 5; ++x) { setTimeout(() => console.log(x)) // closes over the `x` which is logically positioned at the top of the enclosing scope, above the loop}console.log(x) // note: visible outside the loopThe following prints undefined because x is block-scoped. The callbacks are run one by one asynchronously. New behavior for let variables means that each anonymous function closed over a different variable named x (unlike it would have done with var), and so integers 0 through 4 are printed.:
for(let x = 0; x < 5; ++x) { setTimeout(() => console.log(x)) // `let` declarations are re-declared on a per-iteration basis, so the closures capture different variables}console.log(typeof x) // undefinedThe following will NOT throw a ReferenceError because the visibility of x is not constrained by the block; it will, however, print undefined because the variable has not been initialised (because of the if statement).
if(false) { var x = 1}console.log(x) // here, `x` has been declared, but not initialisedA variable declared at the top of a for loop using let is scoped to the body of the loop:
for(let x = 0; x < 10; ++x) {} console.log(typeof x) // undefined, because `x` is block-scopedThe following will throw a ReferenceError because the visibility of x is constrained by the block:
if(false) { let x = 1}console.log(typeof x) // undefined, because `x` is block-scopedVariables declared using var, let or const are all scoped to modules:
// module1.jsvar x = 0export function f() {}//module2.jsimport f from 'module1.js'console.log(x) // throws ReferenceErrorThe following will declare a property on the global object because variables declared using var within the global context are added as properties to the global object:
var x = 1console.log(window.hasOwnProperty('x')) // truelet and const in the global context do not add properties to the global object, but still have global scope:
let x = 1console.log(window.hasOwnProperty('x')) // falseFunction parameters can be considered to be declared in the function body:
function f(x) {}console.log(typeof x) // undefined, because `x` is scoped to the functionCatch block parameters are scoped to the catch-block body:
try {} catch(e) {}console.log(typeof e) // undefined, because `e` is scoped to the catch blockNamed function expressions are scoped only to the expression itself:
(function foo() { console.log(foo) })()console.log(typeof foo) // undefined, because `foo` is scoped to its own expressionIn non-strict mode, implicitly defined properties on the global object are globally scoped. In strict mode, you get an error.
x = 1 // implicitly defined property on the global object (no "var"!)console.log(x) // 1console.log(window.hasOwnProperty('x')) // trueIn non-strict mode, function declarations have function scope. In strict mode, they have block scope.
'use strict'{ function foo() {}}console.log(typeof foo) // undefined, because `foo` is block-scopedHow it works under the hood
Scope is defined as the lexical region of code over which an identifier is valid.
In JavaScript, every function-object has a hidden [[Environment]] reference that is a reference to the lexical environment of the execution context (stack frame) within which it was created.
When you invoke a function, the hidden [[Call]] method is called. This method creates a new execution context and establishes a link between the new execution context and the lexical environment of the function-object. It does this by copying the [[Environment]] value on the function-object, into an outer reference field on the lexical environment of the new execution context.
Note that this link between the new execution context and the lexical environment of the function object is called a closure.
Thus, in JavaScript, scope is implemented via lexical environments linked together in a "chain" by outer references. This chain of lexical environments is called the scope chain, and identifier resolution occurs by searching up the chain for a matching identifier.
Find out more.
Javascript uses scope chains to establish the scope for a given function. There is typically one global scope, and each function defined has its own nested scope. Any function defined within another function has a local scope which is linked to the outer function. It's always the position in the source that defines the scope.
An element in the scope chain is basically a Map with a pointer to its parent scope.
When resolving a variable, javascript starts at the innermost scope and searches outwards.
Variables declared globally have a global scope. Variables declared within a function are scoped to that function, and shadow global variables of the same name.
(I'm sure there are many subtleties that real JavaScript programmers will be able to point out in other answers. In particular I came across this page about what exactly this means at any time. Hopefully this more introductory link is enough to get you started though.)