printing properties with Object.keys()

Question

I'm trying to print Object's own properties (not including prototype properties) by using Object.keys() method.

var obj = {
    name : "Jane"
}

keys = Object.keys(obj); //["name"]

when I do

console.log("length" in keys) //It is printing true?

I believe "length" is coming from the prototype . Why does it exist in keys array?

Answer 1

Object.keys() returns an Array and Arrays always have length property .

From Mozilla Documentation.

The Object.keys() method returns an array of a given object's own enumerable properties, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

Answer 2

Why does it exist in keys array?

It does exists in keys array since keys is an instance of Array and Array has the property length .

This is equivalent of saying

if ( "length" in keys)
{ 
  console.log(true)
}

Check in operator documentation here too.

The in operator returns true if the specified property is in the specified object.