Common this Keyword Interview Questions in JavaScript

Common this Keyword Interview Questions in JavaScript

this is one of the most asked interview topics. Here are the common questions, with answers and code you can practice.

Q1: What is the output?

function foo() {
  console.log(this);
}
foo();

Answer: window (non-strict) or undefined (strict mode). Plain call uses default binding.

Q2: What is the output?

const obj = {
  x: 10,
  foo() {
    console.log(this.x);
  },
};
obj.foo();

Answer: 10. Method call: this is obj.

Q3: What is the output?

const obj = {
  x: 10,
  foo() {
    console.log(this.x);
  },
};
const fn = obj.foo;
fn();

Answer: undefined (strict) or window.x (non-strict). Detaching the method loses this.

Q4: What is the output?

const obj = {
  x: 10,
  foo: () => console.log(this.x),
};
obj.foo();

Answer: undefined. Arrow inherits this from the enclosing scope, not obj.

Q5: What is the output?

function Person(name) {
  this.name = name;
  setTimeout(function () {
    console.log(this.name);
  }, 0);
}
new Person("Kunal");

Answer: undefined (or window.name). The callback is a plain call, so this is not the instance.

Q6: What is the output?

function Person(name) {
  this.name = name;
  setTimeout(() => {
    console.log(this.name);
  }, 0);
}
new Person("Kunal");

Answer: "Kunal". The arrow inherits this from Person's scope, where this is the instance.

Q7: What is the output?

const obj = { x: 10 };
function foo() { return this.x; }
console.log(foo.call(obj));

Answer: 10. call sets this to obj.

Q8: What is the output?

const obj = { x: 10 };
function foo(y) { return this.x + y; }
const bound = foo.bind(obj, 5);
console.log(bound());

Answer: 15. bind presets this and the first argument.

Tips for Interviews

• Identify the call type: plain, method, call/apply/bind, new, or arrow.
• Apply precedence: new > bind > call/apply > method > default.
• For arrows, ignore call type; this is lexical.
• Mention strict mode: plain calls give undefined, not window.

The Takeaway

this interview questions test the five rules: default, implicit, explicit, new, and arrow (lexical). Identify the call type, apply precedence, account for strict mode, and remember arrows inherit this.