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: true console.log(set1.has(5)); // Expected output: true console.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.length-
The value of the
lengthproperty is 0. Set[Symbol.species]-
The constructor function that is used to create derived objects.
Set.prototype-
Represents the prototype for the
Setconstructor. Allows the addition of properties to allSetobjects.
Set 物件實體
All Set instances inherit from Set.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.size-
Returns 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 the values for each element in the
Setobject in insertion order. Set.prototype.values()-
Returns a new iterator object that yields the values 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 contains an array of
[value, value]for each element in theSetobject, in insertion order.This is similar to the
Mapobject, so that each entry's key is the same as its value 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 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 |
|---|
| ECMAScript® 2026 Language Specification # sec-set-objects |