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

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.

Please note the distinctions (compared to map for example):

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 data that implements its own  data.values()  method. And we still can call Object.values(data) 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, there exist a method  Reflect.ownKeys(obj) that returns  all  keys.

Transforming objects

Objects lack many methods that exist for arrays, e.g. map, filter and others.
If we’d like to apply them, then we can use Object.entries followed by  Object.fromEntries:

1. Use Object.entries(obj) to get an array of key/value pairs from obj.
2. Use array methods on that array, e.g. map, to transform these key/value pairs.
3. Use  Object.fromEntries(array)  on the resulting array to turn it back into an object.

For example, we have an object with prices, and would like to double them:

let prices = {
  banana: 1,
  orange: 2,
  meat: 4,
};

let doublePrices = Object.fromEntries(// convert prices to array, map each key/value pair into another pair// and then fromEntries gives back the object
  Object.entries(prices).map(entry => [entry[0], entry[1] * 2])
);

alert(doublePrices.meat); // 8

It may look difficult at first sight, but becomes easy to understand after you use it once or twice. We can make powerful chains of transforms this way.

Tasks

Sum the properties
importance: 5

There is a salaries object with arbitrary number of salaries.
Write the function  sumSalaries(salaries)  that returns the sum of all salaries using  Object.values  and the  for..of  loop.
If salaries is empty, then the result must be 0.

For instance:

let salaries = {"John": 100,"Pete": 300,"Mary": 250
};

alert( sumSalaries(salaries) ); // 650

Open a sandbox with tests.

Count properties
importance: 5

Write a function count(obj) that returns the number of properties in the object:

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

alert( count(user) ); // 2

Try to make the code as short as possible.

P.S. Ignore symbolic properties, count only “regular” ones.
Open a sandbox with tests.

Original content at https://javascript.info/keys-values-entries

© 2007—2024 Ilya Kantor, https://javascript.info