Differences Between var, let and const

In JavaScript there are three keywords for declaring variables: var, let and const. They differ in scope, ability to redefine values and hoisting mechanism.

var

• Scope: Function scope. Variables declared with var are accessible throughout the function, even if declared inside a block.
• Hoisting: var variables are hoisted — their declaration is moved to top of function, but initialization remains in place.
• Redefinition: Can redefine and redeclare the same variable in same scope.

Example with var:

function example() {
  console.log(a); // undefined, variable hoisted but value not assigned yet
  var a = 10;
  if (true) {
    var a = 20; // redefining variable in same function
    console.log(a); // 20
  }
  console.log(a); // 20
}

example();

let

• Scope: Block scope. Variables declared with let are only accessible inside the block where they're declared (e.g., inside curly braces {}).

• Hoisting: let is also hoisted but stays in "Temporal Dead Zone" until initialization.

• Redefinition: Cannot redeclare variable with same name in same scope.

Example with let:

function example() {
  // console.log(b); // Error: cannot access before initialization (ReferenceError)
  let b = 10;
  if (true) {
    let b = 20; // new variable, local to if block
    console.log(b); // 20
  }
  console.log(b); // 10
}

example();

const

• Scope: Block scope, like let.
• Hoisting: const is also subject to hoisting and temporal dead zone until initialization.
• Redefinition: Variables declared with const cannot be redefined or redeclared. However, an object declared with const can be mutated if its structure allows changes.

Example with const:

function example() {
  const c = 10;
  // c = 20; // Error: Assignment to constant variable.
  
  if (true) {
    const c = 30; // new variable in local scope
    console.log(c); // 30
  }
  console.log(c); // 10

  const obj = { name: "Alice" };
  obj.name = "Bob"; // Allowed: object property changes
  console.log(obj.name); // "Bob"
  // obj = {}; // Error: cannot reassign variable
}

example();

Main Differences