[用JS來寫演算法和了解資料結構] Data Structures - Hash Table - Day6 Map

Map

The Map object holds key-value pairs and remembers the original insertion order of the keys.

• MDN MAP

• elements in an array do not repeat -> use Set

• elements in an object do not repeat -> use Map
  both have size, has(), and keys().

• 與Object有點類似，但Map裡面的key是唯一的，重複set會覆蓋舊的value

• Map的key除了原始型別外，可以是任何值object或function

const map1 = new Map();
const map2 = new Map([
  ['a', 1],
  ['b', 2],
]);

map1.set('a', 1);
map1.set('b', 2);
map1.set('a', 97);

map1.size // 2
map1.get('a');  //97
map1.delete('b');   // true
map1.delete('c');   // false
map2.has('a'); // true

map1.clear(); // {size: 0}

map2.keys(); // {'a', 'b'}
map2.values(); // {1, 2}
map2.entries(); // {'a' => 1, 'b' => 2}

map2.forEach((value, key)=>{
  console.log(key, value);
});

for (let [key, value] of map2) {
  console.log(key, value);
}

Clone and merge a Map

let students = {
  Aaron: { age: '29', country: 'Taiwan' },
  Jack: { age: '26', country: 'USA' },
  Johnson: { age: '32', country: 'Korea' },
};
const studentMap = new Map(Object.entries(students));
const cloneMap = new Map(studentMap);

cloneMap.get('Aaron'); // { age: '29', country: 'Taiwan' }
studentMap === cloneMap; // false  (Useful for shallow comparison)


const first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

const second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// Merge maps with an array. The last repeated key wins.
const mergedMap = new Map([...first, ...second, [1, "1"]]);
// Map => Array
const mapToArr = [...mergedMap.entries()];

學習與參考資料

• MDN MAP
• [[JS] JavaScript Map]https://pjchender.dev/javascript/js-map/
• Master the Coding Interview: Data Structures + Algorithms