TypeScript Map

参考文档: https://howtodoinjava.com/typescript/maps/

The Map is a new data structure introduced in ES6 so it is available to JavaScript as well as TypeScript. A Map allows storing key-value pairs (i.e. entries), similar to the maps in other programming languages e.g. Java HashMap.

As Map is a collection, meaning it has a size, and an order and we can iterate over its keys and values.

Creating a Map

Use Map type and new keyword to create a map in TypeScript:

let myMap = new Map<string, number>();

To create a Map with initial key-value pairs, pass the key-value pairs as an array to the Map constructor.

let myMap = new Map<string, string>([
        ["key1", "value1"],
        ["key2", "value2"]
    ]);

Add, Retrieve, Delete Entries from Map

The common operations available in a Map are:

  1. <strong>map.set(key, value)</strong> – adds a new entry in the Map.
  2. <strong>map.get(key)</strong> – retrieves the value for a given key from the Map.
  3. <strong>map.has(key)</strong> – checks if a key is present in the Map. Returns true or false.
  4. <strong>map.size</strong> – returns the count of entries in the Map.
  5. <strong>map.delete(key)</strong> – deletes a key-value pair using its key. If key is found and deleted, it returns true, else returns false.
  6. map.clear() – deletes all entries from the Map.
let nameAgeMapping = new Map<string, number>();

//1. Add entries
nameAgeMapping.set("Lokesh", 37);
nameAgeMapping.set("Raj", 35);
nameAgeMapping.set("John", 40);

//2. Get entries
let age = nameAgeMapping.get("John");		// age = 40

//3. Check entry by Key
nameAgeMapping.has("Lokesh");		        // true
nameAgeMapping.has("Brian");		        // false

//4. Size of the Map
let count = nameAgeMapping.size; 	        // count = 3

//5. Delete an entry
let isDeleted = nameAgeMapping.delete("Lokesh");	        // isDeleted = true

//6. Clear whole Map
nameAgeMapping.clear();				//Clear all entries

Iterating over Map

The Map entries iterate in the insertion order. A for-each loop returns an array of [key, value] pairs for each iteration.

Use 'for...of' loop to iterate over map keys, values, or entries.

  1. map.keys() – to iterate over map keys
  2. map.values() – to iterate over map values
  3. map.entries() – to iterate over map entries
  4. map – use object destructuring to iterate over map entries
let nameAgeMapping = new Map<string, number>();
 
nameAgeMapping.set("Lokesh", 37);
nameAgeMapping.set("Raj", 35);
nameAgeMapping.set("John", 40);

//1. Iterate over map keys

for (let key of nameAgeMapping.keys()) {
    console.log(key);                   //Lokesh Raj John
}

//2. Iterate over map values
for (let value of nameAgeMapping.values()) {
    console.log(value);                 //37 35 40
}

//3. Iterate over map entries
for (let entry of nameAgeMapping.entries()) {
    console.log(entry[0], entry[1]);    //"Lokesh" 37 "Raj" 35 "John" 40
}

//4. Using object destructuring
for (let [key, value] of nameAgeMapping) {
    console.log(key, value);            //"Lokesh" 37 "Raj" 35 "John" 40
}