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:

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 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 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.
  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 to array, map, and then fromEntries gives back the object
  Object.entries(prices).map(([key, value]) => [key, value * 2])
);

alert(doublePrices.meat); // 8

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

Tasks

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.

function sumSalaries(salaries) {

  let sum = 0;
  for (let salary of Object.values(salaries)) {
    sum += salary;
  }

  return sum; // 650
}

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

alert( sumSalaries(salaries) ); // 650

Or, optionally, we could also get the sum using Object.values and reduce:

// reduce loops over array of salaries,
// adding them up
// and returns the result
function sumSalaries(salaries) {
  return Object.values(salaries).reduce((a, b) => a + b, 0) // 650
}

Open the solution with tests in a sandbox.

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.

function count(obj) {
  return Object.keys(obj).length;
}

Open the solution with tests in a sandbox.

Tutorial map

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