What is the Set data structure of ES6

ES6: Set

Understand Set

ES6 provides the data structure Set. Similar to an array, but without duplicate values.

• Set itself is a constructor used to generate the Set data structure

const s = new Set();
[2, 3, 5, 4, 5, 2, 2].forEach(x => s.add(x));for(let i of s ) {    console.log(i); //2 3 4 5 }

• Set can accept an array ( Or array-like object) as a parameter, used to initialize

var set = new Set([1, 2, 3, 4, 4]);
[...set]; // [1, 2, 3, 4]

can be used for array deduplication[...new Set(array)]

Array.from()The method can convert the Set structure into an arrayArray.from(new Set(array))

• When adding a value to a Set, no type conversion will occur (similar to exact equality ===), but please note that in the Set NaN is equal to itself. The other two objects are always not equal.

let set = new Set();let a = NaN；let b = NaN;
set.add(a);
set.add(b);
set; //{NaN} 只能加入一个，说明Set内部两个NaN是相等的

Set instance properties and methods

• Properties:

• Set.prototype.constructor: Constructor, the default is Set function

• Set.prototype.size: Returns the total number of members of the instance

• Operation method (for the specific implementation of the method, see: My simple study of JS collections):

• add(value ): Add a value and return the Set structure itself

• delete(value): Delete a value and return a Boolean value

• has(value): Returns a Boolean value indicating whether it is a member

• clear(): Clear all members , no return value

s.add(1).add(2).add(2); //链式写法s.size(); //2s.has(3); //falses.delete(2);
s.has(2); //false

• 遍历方法

• keys()：返回键名的遍历器（什么是遍历器？Iterator）

• values()：返回键值的遍历器

• entries()：返回键值对的遍历器

• forEach()：使用回调函数遍历每个成员

这里要注意Set的键名和键值是同一个值，所以key()和values()行为是一致的。

let set = new Set(['red', 'green', 'no']);for(let item of set.keys()) {    console.log(item); //red green no}for(let item of set.values()) {    console.log(item); //red green no}for(let item of set.entries()) {    console.log(item); //['red': 'red'] ['green': 'green'] ['no': 'no']}//对每个成员执行某种操作，参数依次为键值、键名、集合本身new Set([1, 2, 3]).forEach((value, key) => console.log(value * 2)); //2 4 6

操作集合

let a = new Set([1, 2, 3]);let b = new Set([4, 3, 2]);//并集let union = new Set([...a, ...b]); //{1, 2, 3, 4}//交集let intersect = new Set([...a].filter(x => b.has(x))); //{2, 3}//差集let difference = new Set([...a].filter(x => !b.has(x))); //{1}

号外：扩展运算符（...）内部使用for...of循环，所以应该知道for of是干嘛的吧

数组的map()和filter()可用于Set

let set = new Set([1, 2, 3]);set = new Set([...set].map(x => x * 2)); //set: {2, 4, 6}let set = new Set([1, 2, 3, 4, 5]);set = new Set([...set].filter(x => (x % 2) == 0)); //set {2, 4}

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