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

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。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 length property is 0.
Set[Symbol.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 all Set objects.

`Set` 物件實體

All Set instances inherit from Set.prototype.

屬性

Set.prototype[Symbol.toStringTag]: The initial value of the Symbol.toStringTag property is the string "Set". This property is used in Object.prototype.toString().
Set.prototype.size: Returns the number of values in the Set object.

方法

Set.prototype.add()

Inserts a new element with a specified value in to a Set object, if there isn't an element with the same value already in the Set.

Set.prototype.clear()

Removes all elements from the Set object.

Set.prototype.delete()

Removes the element associated to the value and returns a boolean asserting whether an element was successfully removed or not. Set.prototype.has(value) will return false afterwards.

Set.prototype.has()

Returns a boolean asserting whether an element is present with the given value in the Set object or not.

Set.prototype[Symbol.iterator]()

Returns a new iterator object that yields the values for each element in the Set object in insertion order.

Set.prototype.values()

Returns a new iterator object that yields the values for each element in the Set object 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 the Set object, in insertion order.

This is similar to the Map object, so that each entry's key is the same as its value for a Set.

Set.prototype.forEach()

Calls callbackFn once for each value present in the Set object, in insertion order. If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackFn.

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

瀏覽器相容性

參見

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