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:
MapSetArray
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):
| 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
}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:
- Use
Object.entries(obj)to get an array of key/value pairs fromobj. - Use array methods on that array, e.g.
map, to transform these key/value pairs. - 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); // 8It 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.