Question

But the rabbit hole doesn’t end there. What if someone registered the name hasOwnProperty in our map object and set it to the value 42? Now the call to map.hasOwnProperty will try to call the local property, which holds a number, not a function.

In such a case, prototypes just get in the way, and we would actually prefer to have objects without prototypes. We saw the Object.create function, which allows us to create an object with a specific prototype. You are allowed to pass null as the prototype to create a fresh object with no prototype. For objects like map , where the properties could be anything, this is exactly what we want.

var map = Object.create(null);
map["pizza"] = 0.069;
console.log("toString" in map);
// → false
console.log("pizza" in map);
// → true

Much better! We no longer need the hasOwnProperty kludge because all the properties the object has are its own properties. Now we can safely use for / in loops, no matter what people have been doing to Object.prototype .

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.