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Object.keys, values, entries

Let's step away from the individual data structures and talk about the iterations over them.

In the previous chapter we saw methods map.keys(), map.values(), map.entries().

These methods are generic, there is a common agreement to use them for data structures. If we ever create a data structure of our own, we should implement them too.

They are supported for:

  • Map
  • Set
  • Array (except arr.values())

Plain objects also support similar methods, but the syntax is a bit different.

Object.keys, values, entries

For plain objects, the following methods are available:

  • Object.keys(obj) -- returns an array of keys.
  • Object.values(obj) -- returns an array of values.
  • Object.entries(obj) -- returns an array of [key, value] pairs.

...But please note the distinctions (compared to map for example):

Map Object
Call syntax map.keys() Object.keys(obj), but not obj.keys()
Returns iterable "real" Array

The first difference is that we have to call Object.keys(obj), and not obj.keys().

Why so? The main reason is flexibility. Remember, objects are a base of all complex structures in JavaScript. So we may have an object of our own like order that implements its own order.values() method. And we still can call Object.values(order) on it.

The second difference is that Object.* methods return "real" array objects, not just an iterable. That's mainly for historical reasons.

For instance:

let user = {
  name: "John",
  age: 30
};
  • Object.keys(user) = [name, age]
  • Object.values(user) = ["John", 30]
  • Object.entries(user) = [ ["name","John"], ["age",30] ]

Here's an example of using Object.values to loop over property values:

let user = {
  name: "John",
  age: 30
};

// loop over values
for (let value of Object.values(user)) {
  alert(value); // John, then 30
}

Object.keys/values/entries ignore symbolic properties

Just like a for..in loop, these methods ignore properties that use Symbol(...) as keys.

Usually that's convenient. But if we want symbolic keys too, then there's a separate method Object.getOwnPropertySymbols that returns an array of only symbolic keys. Also, the method Reflect.ownKeys(obj) returns all keys.