Set
物件可讓你儲存任何類型的唯一值(unique),不論是基本型別(primitive)值或物件參考(references)。
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語法
new Set([iterable]);
參數
iterable
- 若一個可迭代物件被傳入,其所有的元素將會被加入至新的
Set
。若你沒有指定此參數,或參數值為null
,則新的Set
會是空的。
回傳值
一個新的 Set
物件。
描述
Set
對象是數值的收集器。您可以按插入順序迭代收集器中的元素。在 Set
裡的元素只會出現一次; 意即在Set
裡的元素都是獨一無二
值的相等性
因為在 Set 裡每個值都是獨立的,所以都會檢查值的相等性。在早期的ECMAScript規範版本中,此處算法跟基於===操作符中使用的算法並不相同。具體來說,在 Set
裡+0(在嚴格模式是和-0相等)和-0是不同的值。然而在 ECMAScript 2015規範中這點已被更改。請參閱 瀏覽器兼容性 中的"Value equality for -0 and 0"。
另外,NaN和undefined都可以被放置在Set 中, NaN之間被視為相同的值(儘管 NaN !== NaN)。
Set.length
- The value of the
length
property is 0. get Set[@@species]
- The constructor function that is used to create derived objects.
Set.prototype
- Represents the prototype for the
Set
constructor. Allows the addition of properties to allSet
objects.
Set 物件實體
All Set
instances inherit from Set.prototype
.
屬性
方法
範例
使用 Set 物件
var mySet = new Set();
mySet.add(1); // Set [ 1 ]
mySet.add(5); // Set [ 1, 5 ]
mySet.add(5); // Set [ 1, 5 ]
mySet.add('some text'); // Set [ 1, 5, 'some text' ]
var o = {a: 1, b: 2};
mySet.add(o);
mySet.add({a: 1, b: 2}); // o is referencing a different object so this is okay
mySet.has(1); // true
mySet.has(3); // false, 3 has not been added to the set
mySet.has(5); // true
mySet.has(Math.sqrt(25)); // true
mySet.has('Some Text'.toLowerCase()); // true
mySet.has(o); // true
mySet.size; // 5
mySet.delete(5); // removes 5 from the set
mySet.has(5); // false, 5 has been removed
mySet.size; // 4, we just removed one value
console.log(mySet);// Set [ 1, "some text", Object {a: 1, b: 2}, Object {a: 1, b: 2} ]
迭代 Sets
// iterate over items in set
// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}
for (let item of mySet) console.log(item);
// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}
for (let item of mySet.keys()) console.log(item);
// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}
for (let item of mySet.values()) console.log(item);
// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}
//(key and value are the same here)
for (let [key, value] of mySet.entries()) console.log(key);
// convert Set object to an Array object, with Array.from
var myArr = Array.from(mySet); // [1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}]
// the following will also work if run in an HTML document
mySet.add(document.body);
mySet.has(document.querySelector('body')); // true
// converting between Set and Array
mySet2 = new Set([1, 2, 3, 4]);
mySet2.size; // 4
[...mySet2]; // [1, 2, 3, 4]
// intersect can be simulated via
var intersection = new Set([...set1].filter(x => set2.has(x)));
// difference can be simulated via
var difference = new Set([...set1].filter(x => !set2.has(x)));
// Iterate set entries with forEach
mySet.forEach(function(value) {
console.log(value);
});
// 1
// 2
// 3
// 4
實作基本的 set 操作
Set.prototype.isSuperset = function(subset) {
for (var elem of subset) {
if (!this.has(elem)) {
return false;
}
}
return true;
}
Set.prototype.union = function(setB) {
var union = new Set(this);
for (var elem of setB) {
union.add(elem);
}
return union;
}
Set.prototype.intersection = function(setB) {
var intersection = new Set();
for (var elem of setB) {
if (this.has(elem)) {
intersection.add(elem);
}
}
return intersection;
}
Set.prototype.difference = function(setB) {
var difference = new Set(this);
for (var elem of setB) {
difference.delete(elem);
}
return difference;
}
//Examples
var setA = new Set([1, 2, 3, 4]),
setB = new Set([2, 3]),
setC = new Set([3, 4, 5, 6]);
setA.isSuperset(setB); // => true
setA.union(setC); // => Set [1, 2, 3, 4, 5, 6]
setA.intersection(setC); // => Set [3, 4]
setA.difference(setC); // => Set [1, 2]
與 Array 物件關聯
var myArray = ['value1', 'value2', 'value3'];
// Use the regular Set constructor to transform an Array into a Set
var mySet = new Set(myArray);
mySet.has('value1'); // returns true
// Use the spread operator to transform a set into an Array.
console.log([...mySet]); // Will show you exactly the same Array as myArray
與 Strings 關聯
var text = 'India';
var mySet = new Set(text); // Set ['I', 'n', 'd', 'i', 'a']
mySet.size; // 5
規範
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Set' in that specification. |
Standard | Initial definition. |
ECMAScript (ECMA-262) The definition of 'Set' in that specification. |
Living Standard |
瀏覽器相容性
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