Implement Function.prototype.call yourself

this in JavaScript is not a property of a function, it is a property of the *call*: whatever sits to the left of the dot becomes the receiver. Once that clicks, invoking a function with an arbitrary receiver needs no special machinery at all — you can put the function on the object temporarily and call it through the object.

Implement myCall(fn, thisArg, ...args). It invokes fn with this set to thisArg and with args as its arguments, and returns whatever fn returns. You may assume thisArg is always an object. Your implementation must not use Function.prototype.call, apply or bind: one of the tests replaces all three with functions that throw before calling myCall. It must also leave thisArg exactly as it found it — after myCall returns, thisArg has no extra own property, neither a string-keyed one nor a symbol-keyed one.

function whoAmI() { return this.name; }

myCall(whoAmI, { name: "Ada" });        // "Ada"
myCall(whoAmI.bind({ name: "x" }), { name: "y" }); // "x" (bind wins)

function greet(greeting, punct) {
  return greeting + ", " + this.name + punct;
}

myCall(greet, { name: "Ada" }, "Hi", "!"); // "Hi, Ada!"