What is the difference between 'let' and 'const' ECMAScript 2015 (ES6)?

Question

I'm wondering what is the difference between let and const in ES6. Both of them are block scoped, as the example in the following code:

const PI = 3.14;
console.log(PI);

PI = 3;
console.log(PI);

const PI = 4;
console.log(PI);

var PI = 5;
console.log(PI);

In ES5 the output will be:

3.14
3.14
3.14
3.14

But in ES6 it will be:

3.14
3
4
5

I'm wondering why ES6 allows the change of const value, the question is why should we use 'const' now? we can use 'let' instead?

Note: jsbin can be used for testing, choose JavaScript to run ES5 code and Traceur to run it with ES6 capabilities.

Answer 1

The difference between let and const is that once you bind a value/object to a variable using const, you can't reassign to that variable. In other words Example:

const something = {};
something = 10; // Error.

let somethingElse = {};
somethingElse = 1000; // This is fine.

The question details claim that this is a change from ES5 — this is actually a misunderstanding. Using const in a browser that only supports ECMAScript 5 will always throw an error. The const statement did not exist in ECMAScript 5. The behaviour in is either JS Bin being misleading as to what type of JavaScript is being run, or it’s a browser bug.

In practice, browsers didn't just go from 0% support for ECMAScript 2015 (ECMAScript 6) to 100% in one go — features are added bit-by-bit until the browser is fully compliant. What JS Bin calls ‘JavaScript’ just means whatever ECMAScript features your browser currently supports — it doesn’t mean ‘ES5’ or ‘ES6’ or anything else. Many browsers supported const and let before they fully supported ES6, but some (like Firefox) treated const like let for some time. It is likely that the question asker’s browser supported let and const but did not implement them correctly.

Secondly, tools like Babel and Traceur do not make ES6 ‘run’ in an older browser — they instead turn ES6 code into ES5 that does approximately the same thing. Traceur is likely turning const statements into var statements, but I doubt it is always enforcing that the semantics of a const statement are exactly replicated in ES5. Using JS Bin to run ES6 using Traceur is not going to give exactly the same results as running ES6 in a fully ES6 specification-compliant browser.

---

It is important to note that const does not make a value or object immutable.

const myArray = [];
myArray.push(1); // Works fine.
myArray[1] = 2; // Also works fine.
console.log(myArray); // [1, 2]
myArray = [1, 2, 3] // This will throw.

Probably the best way to make an object (shallowly) immutable at the moment is to use Object.freeze() on it. However, this only makes the object itself read-only; the values of the object’s properties can still be mutated.

Answer 2

What you're seeing is just an implementation mistake. According to the ES6 spec wiki on const, const is:

A initialize-once, read-only thereafter binding form is useful and has precedent in existing implementations, in the form of const declarations.

It's meant to be read-only, just like it currently is. The ES6 implementation of const in Traceur and Continuum are buggy (they probably just overlooked it)

Here's a Github issue regarding Traceur not implementing const

Answer 3

The let and const

ES6 let allows you to declare a variable that is limited in scope to the block (Local variable). The main difference is that the scope of a var variable is the entire enclosing function:

if (true) {
  var foo = 42; // scope globally
}

console.log(foo); // 42

The let Scope

if (true) {
  let foo = 42; // scoped in block
}

console.log(foo); // ReferenceError: foo is not defined

Using var in function scope is the same as using let:

function bar() {
  var foo = 42; // scoped in function
}

console.log(foo); // ReferenceError: foo is not defined

The let keyword attaches the variable declaration to the scope of whatever block it is contained in.

Declaration Order

Another difference between let and var is the declaration/initialization order. Accessing a variable declared by let earlier than its declaration causes a ReferenceError.

console.log(a); // undefined
console.log(b); // ReferenceError: b is not defined
  
var a = 1;
let b = 2;

Using const

On the other hand, using ES6 const is much like using the let, but once a value is assigned, it cannot be changed. Use const as an immutable value to prevent the variable from accidentally re-assigned:

const num = 42;

try {
  num = 99;
} catch(err) {
  console.log(err);
  // TypeError: invalid assignment to const `number'

}

num; // 42

Use const to assign variables that are constant in real life (e.g. freezing temperature). JavaScript const is not about making unchangeable values, it has nothing to do with the value, const is to prevent re-assigning another value to the variable and make the variable as read-only. However, values can be always changed:

const arr = [0, 1, 2];
arr[3] = 3; // [0, 1, 2, 3]

To prevent value change, use Object.freeze():

let arr = Object.freeze([0, 1, 2]);
arr[0] = 5;

arr; // [0, 1, 2]

Using let With For Loop

A particular case where let is really shines, is in the header of for loop:

for (let i = 0; i <= 5; i++) {
  console.log(i);
}

// 0 1 2 3 4 5

console.log(i); // ReferenceError, great! i is not global

Answer 4

let

• Use block scope in programming.
• for every block let create its own new scope which you cannot access in outside of that block.
• value can be changed as many times as you want.

• let is extremely useful to have for the vast majority of code. It can greatly enhance your code readability and decrease the chance of a programming error.

  let abc = 0;
  
  if(true)
   abc = 5 //fine
  
  if(true){
    let def = 5
  }
  console.log(def)
  

const

• It allows you to be immutable with variables.

• const is a good practice for both readability and maintainability and avoids using magic literals e.g.

  // Low readability
  if (x > 10) {
  }
  
  //Better!
  const maxRows = 10;
  if (x > maxRows) {
   }
  

• const declarations must be initialized

   const foo; // ERROR: const declarations must be initialized
  

• A const is block scoped like we saw with let:+

const foo = 123;
if (true) {
    const foo = 456; // Allowed as its a new variable limited to this `if` block
}

Answer 5

Summary:

Both the let and the const keyword are ways to declare block scoped variables. There is one big difference though:

• Variables declared with let can be reassigned.
• Variables declared with const have to be initialized when declared and can't be reassigned.

If you try to reassign variables with declared with the const keyword you will get the following error (chrome devtools):

Why should we use this?

If we know that we want to assign a variable once and that we don't want to reassign the variable, using the const keywords offers the following advantages:

• We communicate in our code that we don't want to reassign the variable. This way if other programmmers look to your code (or even you to your own code you wrote a while ago) you know that the variables which are declared with const should not be reassigned. This way our code becomes more declarative and easier to work with.
• We force the principle of not being able to reassign a variable (JS engine throws error). This way if you accidentally try to reassign a variable which is not meant to be reassigned you can detect this at an earlier stage (because it's logged to the console).

Caveat:

Although a variable declared with const can't be reassigned this doesn't mean that an assigned object isn't mutable. For example:

const obj = {prop1: 1}

// we can still mutate the object assigned to the 
// variable declared with the const keyword
obj.prop1 = 10;
obj.prop2 = 2;

console.log(obj);

If you also want your object to be non mutable you can use Object.freeze() in order to achieve this.

Answer 6

let and const

Variables declared with let and const eliminate specific issue of hoisting because they’re scoped to the block, not to the function.

If a variable is declared using let or const inside a block of code (denoted by curly braces { }), then the variable is stuck in what is known as the temporal dead zone until the variable’s declaration is processed. This behavior prevents variables from being accessed only until after they’ve been declared.

Rules for using let and const

let and const also have some other interesting properties.

• Variables declared with let can be reassigned, but can’t be redeclared in the same scope.
• Variables declared with const must be assigned an initial value, but can’t be redeclared in the same scope, and can’t be reassigned.

Use cases

The big question is when should you use let and const? The general rule of thumb is as follows:

• use let when you plan to reassign new values to a variable, and
• use const when you don’t plan on reassigning new values to a variable.

Since const is the strictest way to declare a variable, it is suggest that you always declare variables with const because it'll make your code easier to reason about since you know the identifiers won't change throughout the lifetime of your program. If you find that you need to update a variable or change it, then go back and switch it from const to let.

Answer 7

• var declarations are globally scoped or function scoped while let and const are block scoped.

• var variables can be updated and re-declared within its scope; let variables can be updated but not re-declared;const variables can neither be updated nor re-declared.

• They are all hoisted to the top of their scope. But while var variables are initialized with undefined, let and const variables are not initialized.

• While var and let can be declared without being initialized, const must be initialized during declaration.

Answer 8

Here are some notes that I took that helped me on this subject. Also comparing const and let to var.

Here's about var:

// Var
// 1. var is hoisted to the top of the function, regardless of block
// 2. var can be defined as last line and will be hoisted to top of code block
// 3. for undefined var //output error is 'undefined' and code continues executing
// 4. trying to execute function with undefined variable
// Example: // log(myName); // output: ReferenceError: myName is not defined and code stops executing 

Here's about let and const:

// Let and Const
// 1. use `const` to declare variables which won't change
// 2. `const` is used to initialize-once, read-only thereafter
// 3. use `let` to declare variables which will change
// 4. `let` or `const` are scoped to the "block", not the function
// 5. trying to change value of const and then console.logging result will give error
// const ANSWER = 42;
// ANSWER = 3.14159;
// console.log(ANSWER);
// Error statement will be "TypeError: Assignment to constant variable." and code will stop executing
// 6. `let` won't allow reference before definition
// function letTest2 () {
//   log(b);
//   let b = 3;}
// Error statement will be "ReferenceError: b is not defined." and code will stop executing

Answer 9

Var

The var keyword was introduced with JavaScript. 
It has global scope.    
It can be declared globally and can be accessed globally.   
Variable declared with var keyword can be re-declared and updated in the same scope.
Example:
function varGreeter(){
  var a = 10;        
  var a = 20; //a is replaced
  console.log(a);
}
varGreeter();

It is hoisted.

Example:
{
  console.log(c); // undefined. 
  //Due to hoisting
  var c = 2;
}

Let

The let keyword was added in ES6 (ES 2015) version of JavaScript.
It is limited to block scope.
It can be declared globally but cannot be accessed globally.
Variable declared with let keyword can be updated but not re-declared.
Example:
function varGreeter(){
  let a = 10;        
 let a = 20; //SyntaxError: 
 //Identifier 'a' has already been declared
  console.log(a);
}
varGreeter();

It is not hoisted.

Example:
{
  console.log(b); // ReferenceError: 
  //b is not defined
  let b = 3;
}

Global object property

  var no1 = "123";  // globally scoped
  let no2 = "789"; // globally scoped

  console.log(window.no1); // 123
  console.log(window.no2); // undefined

Redeclaration:

'use strict';

  var name= "Keshav";
  var name= "Keshav Gera"; // No problem, 'name' is replaced.

  let surname= "Rahul Kumar";
  let surname= "Rahul Khan "; // SyntaxError: Identifier 'surname' has already been declared

Hoisting

function run() {
    console.log(name); // undefined
    var name= "Keshav";
    console.log(name); // Keshav
}

 run();

 function checkHoisting() {
     console.log(name); // ReferenceError
     let name= "Keshav";
     console.log(name); // Keshav
 }
 checkHoisting();

Note: in case of var, you will get undefined, in case of let you will get reference error

Const

It allows you to be immutable with variables.

Const declarations must be initialized

const name; // ERROR: const declarations must be initialized

A const is block scoped like we saw with let:+

const num = 10;
if (true) {
 const num = 20; // Allowed as its a new variable limited to this `if` block
}

Answer 10

/*
// declaration of const in same block scope is not allowed
const a = 10;
const a = 15; //Redeclaration of const a Error
console.log(`const outer value `+a);
*/

/*
//declaration of const in different block scope is allowed
const a = 10;
console.log(`outer value of a `+a)
{
    const a = 15; //Redeclaration of const allowed in different block scope
    console.log(`ineer value of a `+a);
}
*/

/*
// re assigning const variable in any block scope is not allowed
const a = 10;
a = 15; //invalid assignment to const 'a'
{
    a = 15;  //invalid assignment to const 'a'
}
*/

/*
// let also can not be re declared in the same block scope
let a = 10;
let a = 15; //SyntaxError: redeclaration of let a
*/

/*
// let can be redeclared in different block scope
let a = 10;
{
    let a = 15; //allowed.
}
*/

/*
// let can be re assigned in same block or different block
let a = 10;
a = 15; //allowed for let but for const its not allowed.
*/

/*
let a = 10;
{
    a = 15; //allowed
}
*/