The Map object is a relatively new standard built-in object that holds [key, value] pairs in the order that they're inserted.

Map對(duì)象是一個(gè)相對(duì)較新的標(biāo)準(zhǔn)內(nèi)置對(duì)象，按插入順序保存[key, value]對(duì)。

The keys and values in the Map object can be any value (both objects and primitive values are valid).

Map對(duì)象中的鍵和值可以是任何值(對(duì)象和基本值均有效)。

句法 (Syntax)

new Map([iterable])

參量 (Parameters)

iterable An Array or other iterable object whose elements are key-value pairs.

iterable一個(gè)數(shù)組或其他可迭代對(duì)象，其元素為鍵值對(duì)。

例 (Example)

const myMap = new Map(); myMap.set('foo', 1); myMap.set('bar', 2); myMap.set('baz', 3);myMap.get('foo'); // returns 1 myMap.get('baz'); // returns 3 myMap.get('hihi'); // return undefinedmyMap.size; // 3console.log(myMap); // Map { 'foo' => 1, 'bar' => 2, 'baz' => 3 }

It's easy to create a new Map from existing 2D arrays of key-value pairs:

根據(jù)現(xiàn)有的鍵值對(duì)的2D數(shù)組創(chuàng)建新的Map很容易：

const myArr = [['foo', 1], ['bar', 2], ['baz', 3]]; const arrMap = new Map(myArr);console.log(arrMap); // Map { 'foo' => 1, 'bar' => 2, 'baz' => 3 }

You can also convert an object into a Map with the help of the Object.entries:

您還可以借助Object.entries將對(duì)象轉(zhuǎn)換為Map ：

const myObj = {foo: 1,bar: 2,baz: 3 } const objMap = new Map(Object.entries(myObj));console.log(objMap); // Map { 'foo' => 1, 'bar' => 2, 'baz' => 3 }

Map.prototype.get (Map.prototype.get)

Returns the value of the specified key from a Map object.

從Map對(duì)象返回指定鍵的值。

句法 (Syntax)

myMap.get(key);

參量 (Parameters)

key Required.

密鑰必填。

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);myMap.get('foo'); // returns 1 myMap.get('baz'); // returns 3 myMap.get('hihi'); // return undefined

Map.prototype.set (Map.prototype.set)

Sets or updates an element with the specified key and value in a Map object. The set() method also returns the Map object.

使用Map對(duì)象中的指定鍵和值設(shè)置或更新元素。 set()方法還返回Map對(duì)象。

句法 (Syntax)

myMap.set(key, value);

參量 (Parameters)

• key Required

  key必

• value Required

  需要的值

例 (Example)

const myMap = new Map();// sets new elements myMap.set('foo', 1); myMap.set('bar', 2); myMap.set('baz', 3);// Updates an existing element myMap.set('foo', 100);myMap.get('foo'); // returns 100

Because set() returns the Map object it operated on, the method can be easily chained. For example, the code above can be simplified to:

由于set()返回對(duì)其進(jìn)行操作的Map對(duì)象，因此可以輕松地鏈接該方法。 例如，上面的代碼可以簡(jiǎn)化為：

const myMap = new Map();// sets new elements myMap.set('foo', 1).set('bar', 2).set('baz', 3).set('foo', 100); // Updates an existing elementmyMap.get('foo'); // returns 100

Map.prototype.size (Map.prototype.size)

Returns the number of elements in a Map object.

返回Map對(duì)象中的元素?cái)?shù)。

句法 (Syntax)

myMap.size();

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);myMap.size(); // 3

Map.prototype.keys (Map.prototype.keys)

Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.

返回一個(gè)新的Iterator對(duì)象，該對(duì)象按插入順序包含Map對(duì)象中每個(gè)元素的鍵。

句法 (Syntax)

myMap.keys()

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);const iterator = myMap.keys();console.log(iterator.next().value); // 'foo' console.log(iterator.next().value); // 'bar' console.log(iterator.next().value); // 'baz'

Map.prototype.values (Map.prototype.values)

Returns an iterator object that contains the values for each element in the Map object in the order they were inserted.

返回一個(gè)迭代器對(duì)象，該迭代器對(duì)象按插入順序包含Map對(duì)象中每個(gè)元素的值。

句法 (Syntax)

myMap.values()

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);const iterator = myMap.values(); console.log(iterator.next().value); // 1 console.log(iterator.next().value); // 2 console.log(iterator.next().value); // 3

Map.prototype.delete (Map.prototype.delete)

Removes the specified element from a Map object. Returns whether the key was found and successfully deleted.

從Map對(duì)象移除指定的元素。 返回是否找到并成功刪除密鑰。

句法 (Syntax)

myMap.delete(key);

參量 (Parameters)

key Required.

密鑰必填。

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);myMap.size(); // 3 myMap.has('foo'); // true;myMap.delete('foo'); // Returns true. Successfully removed.myMap.size(); // 2 myMap.has('foo'); // Returns false. The "foo" element is no longer present.

Map.prototype.entries (Map.prototype.entries)

Returns a new Iterator object that contains the [key, value] pairs for each element in the Map object in insertion order.

返回一個(gè)新的Iterator對(duì)象，該對(duì)象包含按插入順序的Map對(duì)象中每個(gè)元素的[key, value]對(duì)。

句法 (Syntax)

myMap.entries()

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);const iterator = myMap.entries();console.log(iterator.next().value); // ['foo', 1] console.log(iterator.next().value); // ['bar', 2] console.log(iterator.next().value); // ['baz', 3]

Map.prototype.clear (Map.prototype.clear)

Removes all elements from a Map object and returns undefined.

從Map對(duì)象移除所有元素，并返回undefined 。

句法 (Syntax)

myMap.clear();

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);myMap.size(); // 3 myMap.has('foo'); // true;myMap.clear(); myMap.size(); // 0 myMap.has('foo'); // false

Map.prototype.has (Map.prototype.has)

Given a Map with elements inside, the has() function allows you to determine whether or not an element exists inside the Map, based on a key that you pass.

給定一個(gè)Map內(nèi)部包含元素， has()函數(shù)允許您根據(jù)傳遞的鍵來確定Map中是否存在元素。

The has() function returns a Boolean primitive (either true or false), which indicates that the Map contains the element or not.

has()函數(shù)返回一個(gè)Boolean原語(yǔ) ( true或false )，它指示Map是否包含該元素。

You pass a key parameter to the has() function, which will be used to look for an element with that key inside the Map.

您將key參數(shù)傳遞給has()函數(shù)，該函數(shù)將用于在Map中查找具有該鍵的元素。

Example:

例：

// A simple Map const campers = new Map();// add some elements to the map // each element's key is 'camp' and a number campers.set('camp1', 'Bernardo'); campers.set('camp2', 'Andrea'); campers.set('camp3', 'Miguel');// Now I want to know if there's an element // with 'camp4' key: campers.has('camp4'); // output is `false`

The campers Map does not currently have an element with a 'camp4' key. Therefore, the has('camp4') function call will return false.

campers地圖當(dāng)前沒有帶有'camp4'鍵的元素。 因此， has('camp4')函數(shù)調(diào)用將返回false 。

// If we add an element with the 'camp4' key to the map campers.set('camp4', 'Ana');// and try looking for that key again campers.has('camp4'); // output is `true`

Since the map now does have an element with a 'camp4' key, the has('camp4') function call will return true this time!

由于地圖現(xiàn)在確實(shí)具有帶有'camp4'鍵的元素，因此has('camp4')函數(shù)調(diào)用這次將返回true ！

In a more real-world scenario, you might not manually add the elements to the Map yourself, so the has() function would become really useful in those cases.

在更真實(shí)的場(chǎng)景中，您可能不會(huì)自己手動(dòng)將元素添加到Map中，因此has()函數(shù)在這些情況下將非常有用。

Map.prototype.forEach (Map.prototype.forEach)

Executes the passed function on each key-value pair in the Map object. Returns undefined.

在Map對(duì)象中的每個(gè)鍵值對(duì)上執(zhí)行傳遞的函數(shù)。 返回undefined 。

句法 (Syntax)

myMap.forEach(callback, thisArg)

參量 (Parameters)

• callback Function to execute for each element.

  callback為每個(gè)元素執(zhí)行的功能。

• thisArg Value to use as this when executing callback.

  thisArg在執(zhí)行回調(diào)時(shí)用作此值。

例 (Example)

const myMap = new Map(); myMap.set('foo',1); myMap.set('bar',2); myMap.set('baz',3);function valueLogger(value) {console.log(`${value}`); }myMap.forEach(valueLogger); // 1 // 2 // 3

翻譯自: https://www.freecodecamp.org/news/javascript-standard-objects-maps/

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