What are the differences between variables created using `let`, `var` or `const`?

Variables declared using the var keyword are scoped to the function in which they are created, or if created outside of any function, to the global object. let and const are block scoped, meaning they are only accessible within the nearest set of curly braces (function, if-else block, or for-loop).

function foo() {
  // All variables are accessible within functions.
  var bar = 'bar';
  let baz = 'baz';
  const qux = 'qux';

  console.log(bar); // bar
  console.log(baz); // baz
  console.log(qux); // qux
}

console.log(bar); // ReferenceError: bar is not defined
console.log(baz); // ReferenceError: baz is not defined
console.log(qux); // ReferenceError: qux is not defined

if (true) {
  var bar = 'bar';
  let baz = 'baz';
  const qux = 'qux';
}

// var declared variables are accessible anywhere in the function scope.
console.log(bar); // bar
// let and const defined variables are not accessible outside of the block they were defined in.
console.log(baz); // ReferenceError: baz is not defined
console.log(qux); // ReferenceError: qux is not defined

var ,let and const declared variables are all hoisted. var declared variables are auto-initialised with an undefined value. However, let and const variables are not initialised and accessing them before the declaration will result in a ReferenceError exception because they are in a "temporal dead zone" from the start of the block until the declaration is processed.

console.log(foo); // undefined

var foo = 'foo';

console.log(baz); // ReferenceError: can't access lexical declaration 'baz' before initialization

let baz = 'baz';

console.log(bar); // ReferenceError: can't access lexical declaration 'bar' before initialization

const bar = 'bar';

Redeclaring a variable with var will not throw an error, but let and const will.

var foo = 'foo';
var foo = 'bar';
console.log(foo); // "bar"

let baz = 'baz';
let baz = 'qux'; // Uncaught SyntaxError: Identifier 'baz' has already been declared

let and const differ in that let allows reassigning the variable's value while const does not.

// This is fine.
let foo = 'foo';
foo = 'bar';

// This causes an exception.
const baz = 'baz';
baz = 'qux';

Notes

Since most browsers support let and const these days, using var is no longer recommended. If you need to target older browsers, write your code using let, and use a transpiler like Babel to compile your code to older syntax.

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