Category: Interview Questions

• Refers to the current execution context.
• In global → this is window (in browsers).
• Inside objects → refers to that object.

Example:

let person = {
  name: "Ali",
  greet: function() {
console.log("Hello " + this.name);
  }
};
person.greet(); //

Answer:

• Refers to the current execution context.
• In global → this is window (in browsers).
• Inside objects → refers to that object.

Example:

let person = {
  name: "Ali",
  greet: function() {
console.log("Hello " + this.name);
  }
};
person.greet(); //

• forEach() → Executes a function on each item, but returns undefined.
• map() → Transforms elements and returns a new array.

Example:

let arr = [1, 2, 3];
arr.forEach(x => console.log(x * 2)); // prints 2, 4, 6
console.log(arr.map(x => x * 2)); // [2, 4, 6]

• JavaScript is single-threaded.
• The event loop handles asynchronous tasks (like promises, timers) by moving them from the callback queue to the main execution stack when it’s free.

Example:

console.log("Start");
setTimeout(() => console.log("Callback"), 0);
console.log("End");
// Output: Start → End → Callback

Even with 0ms, the async callback runs after synchronous code.

• var → Function scoped.
• let & const → Block scoped {}.

Example:

if (true) {
  var x = 1;   // accessible outside
  let y = 2;   // block scope
  const z = 3; // block scope
}
console.log(x); // 1
// console.log(y); // error

• Shorter way to write functions.
• Do not have their own this.

let add = (a, b) => a + b;
console.log(add(5, 3)); // 8

• Synchronous → Code executes line by line (blocking).
• Asynchronous → Tasks (like API calls, setTimeout) run in the background without blocking code.

Example:

console.log("Start");
setTimeout(() => console.log("Async Task"), 2000);
console.log("End");
// Output: Start → End → (after 2 sec) Async Task

• undefined → variable declared but not assigned.
• null → assigned value, meaning “nothing”.

Example:

let a;      // undefined
let b = null; // null

• Function Declaration → Hoisted, can be called before definition.
• Function Expression → Not hoisted, assigned to a variable.

Example:

// Declaration
function greet() { return "Hello"; }

// Expression
let sayHi = function() { return "Hi"; };

• Hoisting means variable and function declarations are moved to the top of their scope before code execution.

Example:

console.log(a); // undefined
var a = 5;

• var is hoisted (declared but not initialized).
• With let and const, hoisting happens too, but they remain in a temporal dead zone until declared.