Map in JavaScript - Part 2

Published July 27, 2020

We have looked at the basics of Map in JavaScript including creating a new map, adding a new element, deleting an element, clearing all elements, and checking if an element is present in a map In the last blog post.

Now let's look at more operations we can do with Map in JavaScript.

Loop over Map in JavaScript

To loop over the map in javascript we can use the forEach() method or the for..of statement.

Since the map has a key and a corresponding value the programmer can loop over the key or the value using these methods:

  • To loop over only the keys we can use the keys() method available in a map object with the for...of statement. This works because keys() method returns an iterable for keys.
// create a map
const map = new Map();

// add elements to map
map.set("John", "John Doe");
map.set(true, "true");

// loop over only the keys in map
for (let key of map.keys()) {
  console.log(key);
}

/*

OUTPUT
======

John
true

*/
  • To loop over only the values we can use the values() method in a map object with the for...of statement. The method returns an iterable for values.
// create a map
const map = new Map();

// add elements to map
map.set("John", "John Doe");
map.set(true, "true");

// loop over only the values in map
for (let value of map.values()) {
  console.log(value);
}

/*

OUTPUT
======

John Doe
true

*/
  • To loop over both key keys and values at the same time we can use the entries() method available in the map object. The method returns an iterable of the form [key, value].
// create a map
const map = new Map();

// add elements to map
map.set("John", "John Doe");
map.set(true, "true");

// loop over all entries in map
for (let entry of map.entries()) {
  console.log(entry);
}

/*

OUTPUT
======

["John", "John Doe"]
[true, "true"]

*/

You don't have to use the entries() method available in the map object. You can simply use the map object in the for..of statement without any method like this,

// create a map
const map = new Map();

// add elements to map
map.set("John", "John Doe");
map.set(true, "true");

// loop over all entries in map
// same as using the map.entries()
for (let entry of map) {
  console.log(entry);
}

/*

OUTPUT
======

["John", "John Doe"]
[true, "true"]

*/
  • Another way to loop over a map is to use the forEach() method available in the map object.
// create a map
const map = new Map();

// add elements to map
map.set("John", "John Doe");
map.set(true, "true");

// loop over the elements
map.forEach((key, value) => {
  console.log(`${key}: ${value}`);
});

/*

OUTPUT
======

John Doe: John
true: true

*/

As you can see from all the looping strategies the looping goes in the same order as they were inserted to the map collection. Map collection preserves the order which we cannot guarantee with an object.

Convert an object to a map

To convert an object to a map we can use the Object.entries() method while creating a new map collection.

Since the map constructor function accepts key/value pair in [key, value] format.

// create new map using the constructor function
// passing key/value pair in [key, value] format
const map = new Map([
  ["John", "John Doe"],
  [true, 1],
]);

If we have an object like this,

// an object
const john = {
  name: "John Doe",
  age: 23,
  isEmployed: true,
};

We can convert this object into [key, value] format using the Object.entries() method.

// an object
const john = {
  name: "John Doe",
  age: 23,
  isEmployed: true,
};

// convert to [key, value] format
const johnEntries = Object.entries(john);

Then we can pass the johnEntries to the Map constructor function.

// an object
const john = {
  name: "John Doe",
  age: 23,
  isEmployed: true,
};

// convert to [key, value] format
const johnEntries = Object.entries(john);

// make a map from object using johnEntries
const map = new Map(johnEntries);

console.log(map); // {"name" => "John Doe", "age" => 23, "isEmployed" => true}

Convert a map to an object

To convert a map to an object we can use the Object.fromEntries() method and pass the map object to it.

// create a map
const map = new Map([
  ["John", "John Doe"],
  [true, 1],
]);

// convert map to an object
const mapObj = Object.fromEntries(map);

console.log(mapObj);

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