JavaScript let and const

let Declaration

Scope:

• let allows block-scoped variable declarations.
• Variables declared with let are limited in scope to the block, statement, or expression in which they are defined.

Reassignment:

• Variables declared with let can be reassigned a new value.

Example: Block Scope

let x = 10;
if (true) {
  let x = 20;
  console.log(x); // Output: 20
}
console.log(x); // Output: 10

Example: Loop Iteration

for (let i = 0; i < 5; i++) {
  setTimeout(() => {
    console.log(i); // Outputs 0, 1, 2, 3, 4 (each on a separate line)
  }, 1000);
}

 

const Declaration

Immutability:

• const declares constants with immutable values.
• Once assigned, the value of a const variable cannot be changed.

Scope:

• Similar to let, const is also block-scoped.

Initialization:

• A const variable must be initialized during declaration.

Example: Immutable Value

const PI = 3.14;
// PI = 3.14159; // Error: Assignment to constant variable

Example: Object Constants

const person = {
  name: 'John',
  age: 30
};
person.age = 31; // Allowed, the object's properties can still be modified

Example: Array Constants

const colors = ['red', 'green', 'blue'];
colors.push('yellow'); // Allowed, array methods that do not change the structure are permitted

 

Key Points

• let variables have block scope and can be reassigned.
• const variables are immutable but allow modification of object and array properties.
• Use let for variables that may change, and const for constants that should not be reassigned.