Understand this

Credits

https://yehudakatz.com/2011/08/11/understanding-javascript-function-invocation-and-this/

Core Primitives

Core primitive is a functions call method.
The first argument is the thisValue of the method,

function hello(thing) {
    console.log(this + "says" + thing);
}

hello.call("Sean", "How cool is this not")

The short version is: a function invocation like fn(...args) is the same as fn.call(window [ES5-strict: undefined], ...args).

** A function does not necessarily have a consistent notion of this. **

var bind = function(func, thisValue) {
  return function() {
    return func.apply(thisValue, arguments);
  }
}