What is Set in JavaScript? When to use? how to use?

Javascript needs to use Set in some cases. The following article will take you to understand Set, introduce what Set is, when to use Set, and the data operations of Set (intersection, difference set, intersection, symmetric difference set).

In many cases, you need to compare multiple lists to obtain whether they have intersection or difference, etc. There is a data type in Javascript that can achieve this very well. Demand, that is Set.

SetThe object is like an array, but contains only unique items. SetThe object is a collection of values, and its elements can be iterated in the order of insertion. The elements in Set will only appear once, that is, the elements in Set are unique.

The code address involved in the article: https://codepen.io/quintiontang/pen/rNmNbbY

What is Set

Set The object is a collection of values. Its elements can be iterated in the order of insertion. The elements will only appear once, that is, Set is not in a specific order. A stored collection of unique values. Unlike other collection types such as stacks, queues, and arrays, Sets can be used for list comparisons and for detecting the presence of an item in a set.

Set is an abstract data type that is defined by its behavior, similar to stack and queue data structures. Due to the characteristics of key-key, this is similar to Map.

Javascript Set

Set in Javascript is very basic and simple, it doesn’t provide as much as other languages General set operation functions. It uses a unique algorithm (not based on strict equality ===) to detect whether elements are identical.

This means that storing undefined, null and NaN in the collection will only be stored once, even if it is NaN != = NaN, which is usually applied to the storage of object types.

const setTest = new Set([0, -0, Infinity,null, undefined, null, NaN, NaN, Infinity,null]);
console.log(setTest);  // Set { 0, Infinity, null, undefined, NaN }

The following conclusions can be drawn from the above execution results:

• Although NaN and NaN are not equal, but in Set There will only be one
• undefined in the set and Infinity There will only be one # in the Set
set

##The use of basic Set will not be introduced in this article. You can refer to the

mozilla website.

When to use Set

When you need to compare a specific list and determine whether it is equal, you can use

Set , let’s describe the applicable occasions, mainly the set operations in the data:

Get the union of two sets
• union
Get the two sets The difference set
• difference
Get the intersection of two sets
• intersection
Get the symmetric difference set of two sets
• intersectionDifference
Judge whether two sets are subsets
• isSubset
Judge whether two sets are supersets
• isSuperset

The following will introduce the related operations of

Set based on these three occasions.

Set Operations

In mathematics, whenever we talk about sets, there are some operations that can be performed, in fact,

Set is the computer implementation of mathematical finite sets.

In order to better demonstrate the

Set operation in the code, the sample code will extend Javascript Set to inherit its properties and methods, and add other methods to it.

For the sample code, only a simple method is used to check whether it is a valid collection that is not empty.

class SetHelper extends Set {
    /**
     * 验证集合是否为有效集合
     * @param {*} set
     * @returns
     */
    _isValid = (set) => {
        return set && set instanceof Set && set.size > 0;
    };
}

Unionunion

##union

The operation will merge multiple Set Object and return the combined result. The implementation merges the current set and the given set into an array and creates it, thus returning a new set. <pre class="brush:php;toolbar:false">union(set) {     if (!this._isValid(set)) return new SetHelper();     return new SetHelper([...this, ...set]); }</pre>

Difference set

difference

difference

The operation will return a new set that is only contained in one set Elements that are in and not in another set, that is, the mathematical concept of difference set. <pre class="brush:php;toolbar:false">difference(set) {     if (!this._isValid(set)) return new SetHelper();     const differenceSet = new SetHelper();     this.forEach((item) =&gt; {         !set.has(item) &amp;&amp; differenceSet.add(item);     });     return differenceSet; }</pre>

Intersection

intersection

intersection

The operation returns a new collection containing only elements common to both collections. The implementation will iterate over the smaller collection (avoiding unnecessary checks) and check if each item exists in the larger collection and add it to the intersection, which will be returned after the traversal is complete. <pre class="brush:js;toolbar:false;">intersection(set) { const intersectionSet = new SetHelper(); if (!this._isValid(set)) return intersectionSet; const [smallerSet, biggerSet] = set.size &lt;= this.size ? [set, this] : [this, set]; smallerSet.forEach((item) =&gt; { biggerSet.has(item) &amp;&amp; intersectionSet.add(item); }); return intersectionSet; }</pre>

Symmetric difference set

intersectionDifference##intersectionDifference

The operation will return a set that contains all elements that have no intersection between the two sets. New collection.

intersectionDifference(set) {
    if (!this._isValid(set)) return new SetHelper();
    return new SetHelper([
        ...this.difference(set),
        ...set.difference(this),
    ]);
}

subset

subset

<p><code>isSubset 操作将判断两个集合是否为子集关系（当一个集合的所有项都包含在另一个集合中时）。实现上首先检查两个集合的大小，如果一个集合更大，则它不能是另一个集合的子集，然后对于每个项目，它检查它是否存在于另一个中。

isSubset(set) {
    if (!this._isValidSet(set)) return false;
    return (
        this.size <= set.size && [...this].every((item) => set.has(item))
    );
}

超集 superset

isSuperset 操作将判断两个集合是否为超集关系。超集是子集的反操作。当一个集合包含另一个较小或相等大小的集合的所有项目时，它就是一个超集。

isSuperset(set) {
    if (!this._isValidSet(set)) return false;
    return (
        this.size >= set.size && [...set].every((item) => this.has(item))
    );
}

静态 Set

静态Set 是一个始终包含它初始化元素的集合，不能添加、删除、清除元素。Javascript Set 不是静态的，它总能在创建后可以公开修改该集合的方法，如 add、delete ，为避免集合被修改，可以创建一个新的 Set ，将其修改方法重置 。

class StaticSet extends SetHelper {
    constructor(items) {
        super(items);

        this.add = undefined;
        this.delete = undefined;
        this.clear = undefined;
    }
}

使用

现在就可以使用上面定义的方法操作两个 Set，如下：

const setA = new StaticSet(new Set([1, 2, 3, 4]));
const setB = new StaticSet(new Set([3, 4, 5, 6]));
console.log([...setA.union(setB)]); // [ 1, 2, 3, 4, 5, 6 ]
console.log([...setA.difference(setB)]); // [ 1, 2 ]
console.log([...setA.intersection(setB)]); // [ 3, 4 ]
console.log([...setB.intersectionDifference(setA)]); // [ 5, 6, 1, 2 ]

总结

Set 不限于上面这些操作，之前有介绍过可以用来合并数组去重，由于 Set 和 Array 相互转换很简单，因此可以用到 Array 的场合可以优先考虑一下 Set ，因为在内存使用上， Set 比 Array 占用更少。

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