this, call, apply, bind Interview Questions

this depends on how the function is called, not where it was written. A method call uses the object. A plain function is undefined in strict mode. new gives a new instance. Arrow functions take this from the outer scope. call and apply run the function now with a this I choose. apply takes arguments as an array. bind returns a new function with this fixed, which is why old React class handlers used bind. If they mix arrows and regular functions in a snippet, I walk through each call site. That is the whole question.

Lexical this from the enclosing method. That is why arrows keep the instance in callbacks. A regular function callback is undefined this in strict mode. I would not put an arrow on a prototype if I need dynamic this.

bind fixes this and can prepend args. Passing obj.method as a callback loses this. I would not bind in a render loop. Arrows are often clearer in modern code.

Creates object, sets this, sets prototype, returns this unless the constructor returns an object. Arrows cannot be newed. class is still this-based sugar.

apply takes an array. Spread is the modern version. I recognize apply in old code. Math.max.apply is now Math.max(...arr) with spread limits.