Creating JS object with Object.create(null)?

Question

I know a lot of ways to create JS objects but I didn\'t know the Object.create(null)\'s one.

Question:

is it exactly the same as:

Answer 1

They are definitely not equivalent. I'm writing this answer to more fully explain why it makes a difference.

1. var p = {};

   Creates an object that inherits the properties and methods from Object.

2. var p2 = Object.create(null);

   Creates an object that doesn't inherit anything.

If you are using an object as a map, and you create an object using method 1 above, then you have to be extra careful when doing lookups in the map. Because the properties and methods from Object are inherited, your code may run into a case where there are keys in the map that you never inserted. For example, if you did a lookup on toString, you would find a function, even though you never put that value there. You can work around that like this:

if (Object.prototype.hasOwnProperty.call(p, 'toString')) {
    // we actually inserted a 'toString' key into p
}

_{Note that it is fine to assign something to p.toString, it will simply override the inherited toString function on p.}

Note that you can't just do p.hasOwnProperty('toString') because you may have inserted a key "hasOwnProperty" into p, so we force it to use the implementation in Object.

On the other hand, if you use method 2 above, then you won't have to worry about things from Object appearing in the map.

You can't check for the existence of a property with a simple if like this:

// Unreliable:
if (p[someKey]) {
    // ...
}

The value might be an empty string, might be false, or null, or undefined, or 0, or NaN, etc. To check whether a property exists at all, you would still need to use Object.prototype.hasOwnProperty.call(p, someKey).