Why can't I get properties count of navigator object in JavaScript?

Question

Run in your browser (ES5+)

var propCount = Object.keys(navigator).length;
console.log(propCount); // 0

If you do it for a plain object like that

let obj = {
    foo: 'bar',
    breaking: 'bad'
}

let propCount = Object.keys(obj).length;

console.log(propCount); // 2

Why does it happen?

Sorry if it might relate to another problem like when Object.keys(obj) is only counting it for simple objects which do not contain functions/arrays, but this the 1st time I encountered with it.

And would like to know the reason of it.

Answer 1

Object.keys() function returns properties of the object that are directly assigned to it. If you do the following:

console.log(navigator.hasOwnProperty('permissions')); // false

If you want to see properties of navigator do the following:

for(let i in navigator){
    console.log(i);
}

It will list all the properties of navigator object because for ... in ... loop includes object's prototype.

Answer 2

That's because most properties of navigator are set on the Navigator prototype, which the navigator instance inherits, and Object.keys only returns the properties set on the navigator object itself.

You can get those properties from the prototype with this:

Object.keys(Object.getPrototypeOf(navigator));

On a side note, Firefox has the following properties in the navigator object itself:

[ "doNotTrack", "mozPay", "mozContacts", "mozApps" ]