Map in JS
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.
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