JavaScript 'this' Interview Questions
The `this` keyword confuses more candidates than almost anything else. Here are the questions and clear rules.
How is the value of `this` determined?
By HOW a function is called, checked in order: new binding (the new object), explicit binding (call/apply/bind), implicit binding (obj.method() → obj), default binding (plain fn() → undefined in strict mode, else global).
What's the difference between call, apply and bind?
All set this. call invokes now with args listed; apply invokes now with an args array; bind returns a NEW function with this fixed for later.
How do arrow functions treat `this`?
They have no own this — they capture it lexically from the surrounding scope and can't be rebound. Perfect for callbacks inside methods, wrong as object methods.
What does it mean to 'lose this'?
Passing a method as a bare callback (setTimeout(obj.method)) drops the implicit binding, so this becomes undefined/global. Fix with .bind(obj) or an arrow wrapper.
What is `this` at the top level?
In a module or strict mode, this is undefined in a plain function call. In non-strict scripts, it defaults to the global object (window).
Master this and binding
The four this rules, call/apply/bind, and the closures behind them — with clear answers in the JavaScript Interview Kit.
⚡ Get the JavaScript Interview Kit → ₹299Frequently asked questions
- Why is `this` inside setTimeout undefined?
- The callback is called without the dot, so the implicit binding is lost. Use an arrow function or bind to preserve this.
- Can you change an arrow function's this?
- No — call/apply/bind cannot change an arrow's this. It's fixed at definition time.