Maps – Basics

Creates a new Map, a built-in object that stores key-value pairs.

Part of: JavaScript Coding without HTML and CSS — Module 51

Understanding Maps – Basics in JavaScript

What it does: get() looks up the value stored for the "Sam" key. set() stores 30 under the key "Tom" in the ages Map. new Map() creates an empty Map ready to store key-value pairs. set() stores a value under the given key in the Map. JavaScript requires a capital M: new Map(). JavaScript is case-sensitive, so 'set' must be lowercase. set() takes the key first, then the value to store. set() adds this key-value pair to the Map.

Common mistakes: 'Map' must start with a capital M, not lowercase. 'set' must be lowercase - a capital S causes an error.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 41–52) and word search (Modules 21–52).

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Learning

Duo

Does new Map() create a new Map object?

YesNo

Matches

let m = new map();

↓

Wrong - 'Map' must start with a capital M

Applying

Meanings

Term

new Map()

Meaning

Creates a new Map, a built-in object that stores key-value pairs.

Practice

let scores = new Map();

Testing

Gaps

let scores = Map();

Check Answer

Spots

1let m = new map();

2m.set("a", 1);

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Quick Questions

1. Does new Map() create a new Map object?

A. Yes ✓

B. No

new Map() creates an empty Map ready to store key-value pairs.

2. Does set() add a key-value pair to a Map?

A. No

B. Yes ✓

set() stores a value under the given key in the Map.

3. Does get() read a value from a Map using its key?

A. Yes ✓

B. No

get() looks up and returns the value stored for that key.

4. Is 'map' with a lowercase m valid for creating a Map?

A. Yes

B. No ✓

JavaScript requires a capital M: new Map().

5. Must 'set' be written in lowercase?

A. Yes ✓

B. No

JavaScript is case-sensitive, so 'set' must be lowercase.

Match the Pairs

new Map() → Creates a new, empty Map

scores.set("Sam", 90) → Stores 90 under the key "Sam" in the scores Map

Reads the value stored under a given key → scores.get("Sam")

Adds a new key-value pair to a Map → ages.set("Tom", 30)

let m = new map(); → Wrong - 'Map' must start with a capital M

m.Set("a", 1); → Wrong - 'set' must start with a lowercase s

Key Term

new Map()

Creates a new Map, a built-in object that stores key-value pairs.

Fill in the Blanks

1. let scores = new Map();

Hint: Type the keyword used to create a new Map.

2. scores.set("Sam", 90);

Hint: Type the method that adds a key-value pair to a Map.

3. console.log(scores.get("Sam"));

Hint: Type the method that reads a value from a Map by its key.

4. let ages = new Map();

Hint: Type the keyword used to create a new Map.

5. ages.set("Tom", 30);

Hint: Type the method that adds a key-value pair to a Map.

Spot the Error

Question 1

A let m = new map();

B m.set("a", 1);

C console.log(m);

Line A — 'Map' must start with a capital M, not lowercase.

Question 2

A let m2 = new Map();

B m2.Set("b", 2);

C console.log(m2);

Line B — 'set' must be lowercase - a capital S causes an error.

Question 3

A let m3 = new Map();

B m3.set("c", 3);

C console.log(m3.Get("c"));

Line C — 'get' must be lowercase - a capital G causes an error.

Question 4

A let a = 1;

B let b = 2;

C let m5 = new map();

Line C — 'Map' must start with a capital M, not lowercase.

Question 5

A let m6 = new Map();

B m6.set("d", 4);

C console.log(m6);

D m6.Set("e", 5);

Line D — 'set' must be lowercase - a capital S causes an error.

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