javascript maps

 

JavaScript Maps (Map Object)

1) What is a Map?

A Map is a built-in JavaScript object used to store key–value pairs where:

• Keys can be any data type (string, number, object, function).

• Insertion order is preserved.

• It is different from a plain object {}.

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2) Create a Map

const map = new Map();

With initial values:

const map = new Map([
  ["name", "Sushant"],
  ["age", 23],
  [true, "active"]
]);

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3) Common Methods

set(key, value)

map.set("city", "Pune");

get(key)

map.get("name"); // "Sushant"

has(key)

map.has("age"); // true

delete(key)

map.delete("age");

size

map.size; // number of entries

clear()

map.clear();

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4) Looping over a Map

for...of

for (let [key, value] of map) {
  console.log(key, value);
}

keys(), values(), entries()

map.keys();
map.values();
map.entries();

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5) Map vs Object (Important)

Feature	Map	Object
Key types	Any	String / Symbol
Order	Preserved	Not guaranteed
Size	map.size	Manual
Iteration	Easy	Needs extra methods

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6) Real-life Example

const userVisits = new Map();

userVisits.set("Sushant", 5);

userVisits.set("Rahul", 2);

console.log(userVisits.get("Sushant")); // 5

7) When to Use Map

• Dynamic keys

• Frequent add/remove operations

• Need guaranteed order

• Better performance than objects in such cases

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8) Common Mistake

map["name"] = "ABC"; // ❌ wrong
map.set("name", "ABC"); // ✅ correct

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If you want:

• practice questions

• Map vs Array.map() difference

• interview questions

• real projects using Map

state the next topic.