The Difference Between let and const in ES6

In ES6, let and const were introduced as new ways to declare variables. Here are the main differences between them:

Scope

Both let and const declare variables with block scope rather than function scope. This means that the variable is only available within the block { } it is defined in:

{
  let x = 1; 
  const y = 2;
}
console.log(x); // ReferenceError
console.log(y); // ReferenceError

Compare this to var which has function scope:

{
  var x = 1;
}
console.log(x); // 1

Reassignment

Variables declared with let can be reassigned:

let x = 1;
x = 2; // Okay

While variables declared with const cannot be reassigned:

const y = 1;
y = 2; // TypeError: invalid assignment to const `y`

Initialization

let variables do not have to be initialized when declared:

let x;
x = 1;

While const variables must be initialized when declared:

const y; // SyntaxError

Mutation

While const variables cannot be reassigned, they can be mutated:

const array = [1, 2, 3];
array.push(4);
console.log(array); // [1, 2, 3, 4]

This is because const only prevents reassigning the variable identifier to a completely new value - it does not make the value itself immutable.

Usage

In general, you should use:

• let when you know the variable will need to change

• const by default, then only change to let if you find you do need to reassign the variable

Using const by default makes your intent clearer - that the variable is meant to be constant. Only switching to let if truly needed.

Hope this explanation helped! Let me know if you have any other questions.