JavaScript 中 Map 和 WeakMap 的区别是什么?


Map 和 WeakMap 的区别

Map 和 WeakMap 的功能机制相同,但它们之间存在一些细微差别。

1) WeakMap 仅接受对象作为键,而Map 除了对象之外,还接受基本数据类型,例如字符串、数字等。

2) 如果没有对充当键的对象的引用,WeakMap 对象不会阻止垃圾回收。因此,在WeakMap 中没有方法可以检索键,而在Map 中则存在诸如 Map.prototype.keys() 之类的方法来获取键。

3) WeakMap 中不存在 size 属性。

Map

它用于将键与值关联起来,无论数据类型如何,例如字符串、数字、对象等。

示例

在线演示

<html>
<body>
<script>

// Creates a new Map object
   var map = new Map();    

// Defines an object that will be used a key in the ma                
   var objKey = {name: 'tutorialspoint'};    
   document.write("</br>");

// Adds a new element having a String as its key and a String as its value
   map.set('first', 'a');                    
   document.write("</br>");

// Adds a new element having a Number as its key and an Array as its value
   map.set(3, ['c']);      
   document.write("</br>");

// Adds a new element having an Object as its key and a Number as its value
   map.set(objKey, 3);

// Adds a new element having an Array as its key and a String as its value
   map.set(['add', 'mapping'], 'd');          

// Checks whether an element having a key of "2" exists in the map.
   document.write(map.has(2));
   document.write("</br>");

// Checks whether an element having a key of "first" exists in the map.
   document.write(map.has('first'));
   document.write("</br>");

// Retrieves the element having key of "first". Prints "a"
   document.write(map.get('first'));
   document.write("</br>");

// Retrieves the element having as a key the value of objKey.
   document.write(map.get(objKey));
   document.write("</br>");

// Retrieves the element having key of "empty". Prints "undefined"
   document.write(map.get('empty'));
   document.write("</br>");

// Retrieves the map size. Prints "4"
   document.write(map.size);
   document.write("</br>");

// deletes all the value  
   map.clear();
   document.write(map.size);
</script>
</body>
</html>

输出

false
true
a
3
undefined
4
0

WeakMap

在下面的示例中,我们可以发现WeakMap 仅接受对象,而不接受任何基本值(字符串、数字)。

示例

在线演示

<html>
<body>
<script>

// Creates a new WeakMap object
   var weakMap = new WeakMap();

// Defines an object that will be used a key in the map
   var obj4 = {d: 4};

// Defines another object that will be used a key in the map
   var obj5 = {e: 5};

// Adds a new element having an Object as its key and a String as its value
   weakMap.set(obj4, 'fourth');

// Adds a new element having an Object as its key and a String as its value
   weakMap.set(obj5, 'fifth');

// Adds a new element having a Function as its key and a Number as its value
   weakMap.set(function(){}, 7);

// Checks whether an element having as its key the value of objKey4 exists in the weak map.
   document.write(weakMap.has(obj4));
   document.write("</br>");

// Retrieve the value of element associated with the key having the value of objKey4. Prints "first"
   document.write(weakMap.get(obj4));
   document.write("</br>");

// Deletes the element having key of objKey4. Prints "true"
   document.write(weakMap.delete(obj4));
   document.write("</br>");

// Deletes all the elements of the weak map
   weakMap.clear();

</script>
</body>
</html>

输出

true
fourth
true

更新于: 2019年7月30日

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