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JavaScript Tutorial · Objects and Arrays · Lesson 26 of 44

JavaScript Map and Set

Key-value maps with any key type, and sets of unique values.

Set — unique values

A Set stores each value once. It's the quickest way to remove duplicates or check membership:

Example 1Try it yourself
const tags = new Set(["js", "css", "js", "html", "css"]);
console.log(tags.size);
console.log(tags.has("js"), tags.has("go"));
tags.add("go");
tags.delete("css");
console.log([...tags]);
console.log([...new Set([3, 1, 3, 2, 1])]);

Output

3
true false
[ 'js', 'html', 'go' ]
[ 3, 1, 2 ]

Map — key-value pairs

A Map is like an object but keys can be any type (objects, numbers…), it remembers insertion order, and it has a size. Prefer Map for dictionaries that change at runtime:

Example 2Try it yourself
const visits = new Map();
visits.set("/home", 10);
visits.set("/about", 3);
visits.set("/home", visits.get("/home") + 1);
console.log(visits.get("/home"), visits.size);
for (const [path, count] of visits) {
  console.log(path, count);
}
const objKey = { id: 1 };
const meta = new Map([[objKey, "an object as a key"]]);
console.log(meta.get(objKey));

Output

11 2
/home 11
/about 3
an object as a key

Counting with a Map

Example 3Try it yourself
const words = "the cat and the hat and the bat".split(" ");
const counts = new Map();
for (const w of words) {
  counts.set(w, (counts.get(w) ?? 0) + 1);
}
console.log(counts);

Output

Map(5) { 'the' => 3, 'cat' => 1, 'and' => 2, 'hat' => 1, 'bat' => 1 }

Hash maps and sets are the most common tools in coding interviews — practise with Two Sum and Contains Duplicate.

Test yourself

What is the quickest way to remove duplicates from an array arr?

  1. arr.unique()
  2. [...new Set(arr)]
  3. new Map(arr)
  4. arr.distinct()
Show answer

Answer: [...new Set(arr)]

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