JavaScript Maps

JavaScript Maps

In JavaScript Maps is a collection of key-value pairs in JavaScript. Unlike objects, Maps allow:

✔ Any type of key (objects, functions, numbers, strings)
✔ Values to be stored in insertion order
✔ Easy size checking

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🧩 Creating a Map

let map = new Map();

1	let map = new Map();

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➕ Adding Key-Value Pairs

Use the .set() method:

map.set("name", "Alice"); map.set("age", 25); map.set(1, "Number Key");

1 2 3

map.set("name", "Alice"); map.set("age", 25); map.set(1, "Number Key");

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🔍 Getting Values

console.log(map.get("name")); // Alice
console.log(map.get(1)); // Number Key


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❓ Checking Keys

console.log(map.has("age")); // true console.log(map.has("job")); // false

1 2	console.log(map.has("age")); // true console.log(map.has("job")); // false

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🗑 Removing Items

map.delete("age"); console.log(map.has("age")); // false

1 2	map.delete("age"); console.log(map.has("age")); // false

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🧹 Clear All Items

map.clear(); console.log(map.size); // 0

1 2	map.clear(); console.log(map.size); // 0

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📏 Size of a Map

console.log(map.size);

1	console.log(map.size);

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🔁 Iterating Through a Map

Loop through keys and values:

let users = new Map([ ["id", 101], ["name", "John"], ["role", "Admin"] ]); for (let [key, value] of users) { console.log(key, ":", value); }

1 2 3 4 5 6 7 8 9

let users = new Map([ ["id", 101], ["name", "John"], ["role", "Admin"] ]);   for (let [key, value] of users) { console.log(key, ":", value); }

 

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Using forEach():

users.forEach((value, key) => { console.log(key, "=>", value); });

1 2 3

users.forEach((value, key) => { console.log(key, "=>", value); });

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📌 Useful Methods Summary

Method	Description
set(key, value)	Adds or updates entry
get(key)	Returns value for the key
has(key)	Checks if key exists
delete(key)	Removes key/value
clear()	Removes all entries
size	Number of items

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🆚 Map vs Object

Feature	Map	Object
Key type	Any type	String/symbol only
Ordered	Yes	No guarantee
Iterable	Yes	No (needs conversion)
Size tracking	size property	Must count manually

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⭐ Practical Example: Counting Occurrences

let words = ["apple", "banana", "apple", "orange", "banana"]; let counter = new Map(); for (let word of words) { counter.set(word, (counter.get(word) || 0) + 1); } console.log(counter);

1 2 3 4 5 6 7 8

let words = ["apple", "banana", "apple", "orange", "banana"]; let counter = new Map();   for (let word of words) { counter.set(word, (counter.get(word) || 0) + 1); }   console.log(counter);

 

Output:

Map(3) { 'apple' => 2, 'banana' => 2, 'orange' => 1 }

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