Basic Javascript Knowledge: When to use var, let or const?

In JavaScript, variables are essential for storing data. The keywords var, let, and const provide different ways to declare these variables.

var

• Scope: Function scope.
• Hoisting: Variables are hoisted.
• Re-assignment: Allowed.
• Re-declaration: Allowed.

  function testVar() {
    var x = 1;
    if (true) {
        var x = 2; // same variable!
        console.log(x); // 2
    }
    console.log(x); // 2
  }
  testVar();

let

• Scope: Block scope.
• Hoisting: Hoisted but not initialized.
• Re-assignment: Allowed.
• Re-declaration: Not allowed.

  function testLet() {
    let y = 1;
    console.log(y); // 1
    if (true) {
        let y = 2; // different variable
        console.log(y); // 2
    }
    y = 5
    console.log(y); // 5
  }
  testLet();

const

• Scope: Block scope.
• Hoisting: Hoisted but not initialized.
• Re-assignment: Not allowed.
• Re-declaration: Not allowed.
• Initialization: Required during declaration.

  function testConst() {
    const z = 1;
    // z = 2; // Error: Uncomment this see error
    console.log(z);
  }
  testConst();

Exercise: Try This To Practise Using Variables

1. Var Scope: Use var in a loop, then access it outside.
2. Let Block Scope: Try let in a loop and access it outside the block.
3. Const Immutability: Attempt re-assigning a const variable.

Good luck!