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Valentino Gagliardi
Valentino Gagliardi

Posted on • Originally published at valentinog.com

var, let, and const in JavaScript: a cheatsheet

var, let, and const in JavaScript: a cheatsheet

var

The var statement declares a variable in JavaScript which abides to the following rules:

  • is function-scoped or globally-scoped.
  • is not subject to the temporal dead zone.
  • it creates a global property on window with the same name.
  • is reassignable.
  • is redeclarable.

Function-scoped or globally-scoped

var creates a global variable when appears in the global scope. In addition, it creates a global property on window with the same name:

var city = "Florence";

console.log(window.city); // "Florence"
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When declared inside a function, the variable is scoped to that function:

var city = "Florence";

function bubble() {
  var city = "Siena";
  console.log(city);
}

bubble(); // "Siena"

console.log(city); // "Florence"
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var declarations are subject to hoisting:

function bubble() {
  city = "Siena";
  console.log(city);
  var city; // hoists
}

bubble(); // "Siena"
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Accidental global variables

Variables assigned without any statement, be it var, let, or const, become global variables by default:

function bubble() {
  city = "Siena";
  console.log(window.city);
}

bubble(); // "Siena"
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To neutralize this behaviour we use strict mode:

"use strict";

function bubble() {
  city = "Siena";
  console.log(window.city);
}

bubble(); // ReferenceError: assignment to undeclared variable city
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reassignable and redeclarable

Any variable declared with var can be later reassigned, or redeclared. An example of redeclaration:

function bubble() {
  var city = "Florence";
  var city = "Siena";
  console.log(city);
}

bubble(); // "Siena"
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An example of reassignation:

function bubble() {
  var city = "Siena";
  city = "Florence";
  console.log(city);
}

bubble(); // "Florence"
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let

The let statement declares a variable in JavaScript which abides to the following rules:

  • is block scoped.
  • is subject to the temporal dead zone.
  • it does not create any global property on window.
  • is reassignable.
  • is not redeclarable.

Block scoped

A variable declared with let does not create any global property on window:

let city = "Florence";

console.log(window.city); // undefined
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When declared inside a function, the variable is scoped to that function:

let city = "Florence";

function bubble() {
  let city = "Siena";
  console.log(city);
}

bubble(); // "Siena"

console.log(city); // "Florence"
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When declared inside a block, the variable is scoped to that block. An example with the block statement:

let city = "Florence";

{
  let city = "Siena";
  console.log(city); // "Siena";
}

console.log(city); // "Florence"
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An example with an if block:

let city = "Florence";

if (true) {
  let city = "Siena";
  console.log(city); // "Siena";
}

console.log(city); // "Florence"
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var instead, doesn't care about blocks:

var city = "Florence";

{
  var city = "Siena";
  console.log(city); // "Siena";
}

console.log(window.city); // "Siena"
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Temporal dead zone

let declarations are subject to hoisting, but temporal dead zone kicks in:

function bubble() {
  city = "Siena";
  console.log(city); // TDZ
  let city;
}

bubble();

// ReferenceError: can't access lexical declaration 'city' before initialization
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Temporal dead zone prevents access to let declarations before their initialization. Another example:

function bubble() {
  console.log(city); // TDZ
  let city = "Siena";
}

bubble();

// ReferenceError: can't access lexical declaration 'city' before initialization
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We can see that the exceptions are the same in both examples: the proof that temporal dead zone kicked in.

Further resources on the topic: Temporal dead zone demystified.

Reassignable, not redeclarable

Any variable declared with let can't be redeclared. An example of redeclaration which throws:

function bubble() {
  let city = "Siena";
  let city = "Florence";
  console.log(city);
}

bubble(); // SyntaxError: redeclaration of let city
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An example of valid reassignation:

function bubble() {
  let city = "Siena";
  city = "Florence";
  console.log(city);
}

bubble(); // "Florence"
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const

The const statement declares a variable in JavaScript which abides to the following rules:

  • is block scoped.
  • is subject to the temporal dead zone.
  • it does not create any global property on window.
  • is not reassignable.
  • is not redeclarable.

Block scoped

A variable declared with const does not create any global property on window:

const city = "Florence";

console.log(window.city); // undefined
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When declared inside a function, the variable is scoped to that function:

const city = "Florence";

function bubble() {
  const city = "Siena";
  console.log(city);
}

bubble(); // "Siena"

console.log(city); // "Florence"
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When declared inside a block, the variable is scoped to that block. An example with the block statement {}:

const city = "Florence";

{
  const city = "Siena";
  console.log(city); // "Siena";
}

console.log(city); // "Florence"
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An example with an if block:

const city = "Florence";

if (true) {
  const city = "Siena";
  console.log(city); // "Siena";
}

console.log(city); // "Florence"
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Temporal dead zone

const declarations are subject to hoisting, but temporal dead zone kicks in:

function bubble() {
  console.log(city);
  const city = "Siena";
}

bubble();

// ReferenceError: can't access lexical declaration 'city' before initialization
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Not reassignable, not redeclarable

Any variable declared with const can't be redeclared, nor reassigned. An example of redeclaration which throws:

function bubble() {
  const city = "Siena";
  const city = "Florence";
  console.log(city);
}

bubble(); // SyntaxError: redeclaration of const city
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An example of reassignation which trows as well:

function bubble() {
  const city = "Siena";
  city = "Florence";
  console.log(city);
}

bubble(); // TypeError: invalid assignment to const 'city'
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Top comments (2)

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qq449245884 profile image
qq449245884

Hello, may I translate your article into Chinese?I would like to share it with more developers in China. I will give the original author and original source.

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amt8u profile image
amt8u

Covered most of the scenarios and the top diagram would be quite effective to start. But I can say from my experience, Its really tricky to understand TDZ.

🀯

"I made 10x faster JSON.stringify() functions, even type safe"

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