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Set
Baseline Widely available *
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
* Some parts of this feature may have varying levels of support.
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）。
嘗試一下
const set1 = new Set([1, 2, 3, 4, 5]);console.log(set1.has(1));// Expected output: trueconsole.log(set1.has(5));// Expected output: trueconsole.log(set1.has(6));// Expected output: false語法
new Set([iterable]);參數
回傳值
一個新的Set 物件。
描述
Set 對象是數值的收集器。你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。請參閱瀏覽器相容性中的「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.lengthThe value of the
lengthproperty is 0.Set[Symbol.species]The constructor function that is used to create derived objects.
Set.prototypeRepresents the prototype for the
Setconstructor. Allows the addition of properties to allSetobjects.
Set 物件實體
AllSet instances inherit fromSet.prototype.
屬性
Set.prototype[Symbol.toStringTag]The initial value of the
Symbol.toStringTagproperty is the string"Set". This property is used inObject.prototype.toString().Set.prototype.sizeReturns the number of values in the
Setobject.
方法
Set.prototype.add()Inserts a new element with a specified value in to a
Setobject, if there isn't an element with the same value already in theSet.Set.prototype.clear()Removes all elements from the
Setobject.Set.prototype.delete()Removes the element associated to the
valueand returns a boolean asserting whether an element was successfully removed or not.Set.prototype.has(value)will returnfalseafterwards.Set.prototype.has()Returns a boolean asserting whether an element is present with the given value in the
Setobject or not.Set.prototype[Symbol.iterator]()Returns a new iterator object that yields thevalues for each element in the
Setobject in insertion order.Set.prototype.values()Returns a new iterator object that yields thevalues for each element in the
Setobject in insertion order.Set.prototype.keys()An alias for
Set.prototype.values().Set.prototype.entries()Returns a new iterator object that containsan array of
[value, value]for each element in theSetobject, in insertion order.This is similar to the
Mapobject, so that each entry'skey is the same as itsvalue for aSet.Set.prototype.forEach()Calls
callbackFnonce for each value present in theSetobject, in insertion order. If athisArgparameter is provided, it will be used as thethisvalue for each invocation ofcallbackFn.
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 okaymySet.has(1); // truemySet.has(3); // false, 3 has not been added to the setmySet.has(5); // truemySet.has(Math.sqrt(25)); // truemySet.has("Some Text".toLowerCase()); // truemySet.has(o); // truemySet.size; // 5mySet.delete(5); // removes 5 from the setmySet.has(5); // false, 5 has been removedmySet.size; // 4, we just removed one valueconsole.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.fromvar 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 documentmySet.add(document.body);mySet.has(document.querySelector("body")); // true// converting between Set and ArraymySet2 = new Set([1, 2, 3, 4]);mySet2.size; // 4[...mySet2]; // [1, 2, 3, 4]// intersect can be simulated viavar intersection = new Set([...set1].filter((x) => set2.has(x)));// difference can be simulated viavar difference = new Set([...set1].filter((x) => !set2.has(x)));// Iterate set entries with forEachmySet.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;};//Examplesvar setA = new Set([1, 2, 3, 4]), setB = new Set([2, 3]), setC = new Set([3, 4, 5, 6]);setA.isSuperset(setB); // => truesetA.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 Setvar 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 |
|---|
| ECMAScript® 2026 Language Specification # sec-set-objects |