JavaScript Interview Questions

Answer:

• var — Function-scoped, hoisted, can be redeclared. Avoid in modern code.
• let — Block-scoped, hoisted but in Temporal Dead Zone (TDZ), cannot be redeclared in same scope.
• const — Block-scoped, hoisted but in TDZ, cannot be reassigned. Use by default.

var x = 1;     // function-scoped
let y = 2;     // block-scoped
const z = 3;   // block-scoped, cannot reassign

Answer:

• == — Loose equality. Performs type coercion before comparing.
• === — Strict equality. Compares both value and type. Always use this.

5 == '5'   // true (type coercion)
5 === '5'  // false (different types)
0 == false // true
0 === false // false

Answer:

A closure is a function that remembers its lexical scope even when executed outside that scope. Every function in JavaScript is a closure.

function outer() {
  let message = "Hello";
  function inner() {
    console.log(message); // remembers 'message'
  }
  return inner;
}
const fn = outer();
fn(); // "Hello"

Answer:

The event loop is a mechanism that handles asynchronous operations in JavaScript. It continuously checks if the call stack is empty and moves tasks from the task queue and microtask queue to the call stack.

• Microtasks (Promises) have higher priority
• Macrotasks (setTimeout, events) run after microtasks

Answer:

• undefined — A variable that has been declared but not assigned a value. Type: "undefined"
• null — An intentional absence of value. Type: "object" (historical bug)

let x;        // undefined
let y = null; // null
console.log(typeof null); // "object"