passing thisArg to a map function [duplicate]

Question

This question might already be asked. From the documentation, we can pass a thisVariable to a map function.

var foo=[1,2].map(function(){return this.length},[2,4]); // [2,2]

This syntax in ES6, however, returns something else

var foo=[1,2].map(_ => this.length ,[2,4]); // [0,0]

What is the [0,0] returned?

Answer 1

An arrow function retains the thisValue that was in scope in the calling function, it can't be reset. So the thisValue argument passed to map() is ignored.

So when your uses this.length, it's using the value of this from the calling context. If you're executing the code at top level (e.g. in the Javascript console), this == window, so you're getting the result of window.length, which is 0.