Map in JS

#map #javascript #object

A Map in JavaScript is a collection of key-value pairs where keys can be of any type. It retains the order of insertion, meaning that the items are iterated in the order they were added.

Key Characteristics of a Map;

Unique Keys: Each key in a Map is unique.
Any Data Type for Keys: Unlike objects, keys can be of any data type, including functions, objects, or any primitive type.
Iterable: You can iterate over the keys, values, or entries of a Map.

Basic Operations

Creating a Map

let funnyMap = new Map();

Adding Elements to a Map

funnyMap.set('a', 1); // string key
funnyMap.set(2, 'two'); // number key
funnyMap.set(true, 'yes'); // boolean key
funnyMap.set({name: 'obj'}, 'object'); // object key

// Funny example
funnyMap.set('knock-knock', 'who’s there?');

Getting Values from a Map

console.log(funnyMap.get('a')); // 1
console.log(funnyMap.get(2)); // two
console.log(funnyMap.get(true)); // yes

// Funny example
console.log(funnyMap.get('knock-knock')); // who’s there?

Checking for a Key

console.log(funnyMap.has('a')); // true
console.log(funnyMap.has(42)); // false

// Funny example
console.log(funnyMap.has('chicken')); // false (It crossed the road)

Removing an Element

funnyMap.delete('a');
console.log(funnyMap.has('a')); // false

// Funny example
funnyMap.delete('knock-knock');
console.log(funnyMap.get('knock-knock')); // undefined (No one answered)

Getting the Size of the Map

console.log(funnyMap.size); // 3 after deletion

// Funny example
console.log(`The map has ${funnyMap.size} jokes left.`);

Clearing the Map

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

// Funny example
console.log(`All jokes are cleared from the map.`);

Iterating Over a Map

Using for...of

funnyMap.set('banana', 'yellow');
funnyMap.set('apple', 'red');
funnyMap.set('grape', 'purple');

// Iterating over keys
for (let key of funnyMap.keys()) {
    console.log(`Key: ${key}`);
}

// Iterating over values
for (let value of funnyMap.values()) {
    console.log(`Value: ${value}`);
}

// Iterating over entries
for (let [key, value] of funnyMap.entries()) {
    console.log(`Key: ${key}, Value: ${value}`);
}

// Funny example
funnyMap.set('dad joke', 'What do you call fake spaghetti? An impasta!');
for (let [key, value] of funnyMap.entries()) {
    console.log(`Here’s a ${key}: ${value}`);
}

Using forEach

funnyMap.forEach((value, key) => {
    console.log(`Key: ${key}, Value: ${value}`);
});

// Funny example
funnyMap.set('bad pun', 'I’m reading a book on anti-gravity. It’s impossible to put down!');
funnyMap.forEach((value, key) => {
    console.log(`Here’s a ${key}: ${value}`);
});

Practical Uses

Word Counting (Funny Version)

Imagine you want to count how many times certain words appear in a joke collection:

let jokeText = "Why did the scarecrow win an award? Because he was outstanding in his field!";

// Splitting text into words
let words = jokeText.split(/\W+/);

let wordCount = new Map();

words.forEach(word => {
    word = word.toLowerCase();
    if (wordCount.has(word)) {
        wordCount.set(word, wordCount.get(word) + 1);
    } else {
        wordCount.set(word, 1);
    }
});

wordCount.forEach((count, word) => {
    console.log(`Word: ${word}, Count: ${count}`);
});

// Funny example
console.log('Word counts in our joke:');
wordCount.forEach((count, word) => {
    console.log(`"${word}": ${count} times`);
});

Summary

A Map in JavaScript is a versatile data structure that can hold key-value pairs of any data type, provides easy iteration methods, and maintains the order of elements. It's useful for scenarios where you need a more flexible key management than a regular object, such as counting words in jokes or storing responses to various riddles.

Top comments (4)

Stevo82930 • Jul 19 • Edited

I really needed this enlightenment for my project right now. Thank you Khojiak! I only ever thought of Map() for passing numbers through functions because I was getting it confused with array.map() and my own ignorance. Now I will be writing my code differently.