What Is the Scope of Variables in JavaScript

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, let and const
Function parameters
Catch block parameter
Function declarations
Named function expressions
Implicitly defined properties on the global object (i.e., missing out var in non-strict mode)
import statements
eval

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`!

What is scope default of a variable defined by JavaScript var?

Based on this source

Scope of the variables declared without var keyword become global irrespective of where it is declared. Global variables can be accessed from anywhere in the web page. Visit Scope for more information.

Javascript for-loop variable scope?

Before ECMAScript 6, javascript only supports function scoping. A variable declared inside a function is visible anywhere inside that function. Even this:

function foo() {  if(true) {    if(true) {      var v = 7;    }  }
  console.log(v); // 7}foo();

Variables with the same name, but the local scope variable isn't being used, why?

You have declared a variable named a three times.

Once as a global on line 1: var a = 15;
Twice inside the function:
- Once with a var statement on line 4: var a;
- Once with an argument definition on line 3: function checkScope(a) {

The line 4 var has no effect. The variable is already declared.

Then you call checkScope(a); which passes the value of the global a (15) to the local a argument of checkScope. This is why it is 15 and not undefined.

Since they're re-declared, If I'm interpreting MDN correctly, does that mean line 3 is basically ignored?

var a is basically ignored.

But I don't think so because both variable are declared in their own, separate execution contexts

This is where you are being tripped up by having three and not two declarations of a. It declares the variable defined by the function argument name.

Javascript: scope of variable

The let keyword gives a variable block-level scope, meaning the v1 variable in b() is only usable inside b(). a() does not know what v1 is.

//// does not work
function a() {
    console.log(v1);
}

function b() {
    let v1 = 1;
    a();
}

b();

In this example, v1 is being set in the global scope, so all your functions will recognize it.

//// does work

function a() {
    console.log(v1);
}

function b() {
    a();
}

let v1 = 1;
b();

What is file scope in javascript

ES6 modules form their own file scope (as if the entire contents of the file were wrapped in a function).

Variables declared in a module are completely inaccessible from outside that module (unless they're exported).

What Is the Scope of Variables in JavaScript