根据Asaf Katz的回答，以下是一个打字版本:

export function union<T> (...iterables: Array<Set<T>>): Set<T> {
  const set = new Set<T>()
  iterables.forEach(iterable => {
    iterable.forEach(item => set.add(item))
  })
  return set
}

被认可的答案很好，但每次都会创建一个新的集合。

如果要更改现有对象，请使用helper函数

在不久的将来，你可以只使用setA.union(setB)

Set

function concatSets(set, ...iterables) {
    for (const iterable of iterables) {
        for (const item of iterable) {
            set.add(item);
        }
    }
}

用法:

const setA = new Set([1, 2, 3]);
const setB = new Set([4, 5, 6]);
const setC = new Set([7, 8, 9]);
concatSets(setA, setB, setC);
// setA will have items 1, 2, 3, 4, 5, 6, 7, 8, 9

Map

function concatMaps(map, ...iterables) {
    for (const iterable of iterables) {
        for (const item of iterable) {
            map.set(...item);
        }
    }
}

用法:

const mapA = new Map().set('S', 1).set('P', 2);
const mapB = new Map().set('Q', 3).set('R', 4);
concatMaps(mapA, mapB);
// mapA will have items ['S', 1], ['P', 2], ['Q', 3], ['R', 4]

出于我不理解的原因，您不能直接使用内置方法将一个Set的内容添加到另一个Set。像联合、交叉、合并等操作…是非常基本的集合操作，但不是内置的。幸运的是，您可以相当容易地自己构建这些。

[2021年新增]-现在有一个提议为这些类型的操作添加新的Set/Map方法，但实施的时间还不清楚。他们似乎处于规范过程的第二阶段。

要实现合并操作(将一个Set的内容合并到另一个Set中，或将一个Map的内容合并到另一个Map中)，你可以使用单个.forEach()行来完成:

var s = new Set([1,2,3]);
var t = new Set([4,5,6]);

t.forEach(s.add, s);
console.log(s);   // 1,2,3,4,5,6

对于Map，你可以这样做:

var s = new Map([["key1", 1], ["key2", 2]]);
var t = new Map([["key3", 3], ["key4", 4]]);

t.forEach(function(value, key) {
    s.set(key, value);
});

或者，在ES6语法中:

t.forEach((value, key) => s.set(key, value));

[2021年新增]

由于现在有一个新的Set方法的官方提议，你可以使用这个polyfill提议的.union()方法，它将在ES6+版本的ECMAScript中工作。注意，根据规范，这将返回一个新Set，它是另外两个Set的并集。它不会将一个集合的内容合并到另一个集合中，这实现了建议中指定的类型检查。

if (!Set.prototype.union) {
    Set.prototype.union = function(iterable) {
        if (typeof this !== "object") {
            throw new TypeError("Must be of object type");
        }
        const Species = this.constructor[Symbol.species];
        const newSet = new Species(this);
        if (typeof newSet.add !== "function") {
            throw new TypeError("add method on new set species is not callable");
        }
        for (item of iterable) {
            newSet.add(item);
        }
        return newSet;
    }
}

或者，这里有一个更完整的版本，它对ECMAScript过程进行了建模，以更完整地获得物种构造函数，并且已经适应于在甚至可能没有Symbol或有Symbol的旧版本Javascript上运行。种类:

if (!Set.prototype.union) {
    Set.prototype.union = function(iterable) {
        if (typeof this !== "object") {
            throw new TypeError("Must be of object type");
        }
        const Species = getSpeciesConstructor(this, Set);
        const newSet = new Species(this);
        if (typeof newSet.add !== "function") {
            throw new TypeError("add method on new set species is not callable");
        }
        for (item of iterable) {
            newSet.add(item);
        }
        return newSet;
    }
}

function isConstructor(C) {
    return typeof C === "function" && typeof C.prototype === "object";
}

function getSpeciesConstructor(obj, defaultConstructor) {
    const C = obj.constructor;
    if (!C) return defaultConstructor;
    if (typeof C !== "function") {
        throw new TypeError("constructor is not a function");
    }

    // use try/catch here to handle backward compatibility when Symbol does not exist
    let S;
    try {
        S = C[Symbol.species];
        if (!S) {
            // no S, so use C
            S = C;
        }
    } catch (e) {
        // No Symbol so use C
        S = C;
    }
    if (!isConstructor(S)) {
        throw new TypeError("constructor function is not a constructor");
    }
    return S;
}

供参考，如果你想要一个内置Set对象的简单子类，它包含一个.merge()方法，你可以使用这个:

// subclass of Set that adds new methods
// Except where otherwise noted, arguments to methods
//   can be a Set, anything derived from it or an Array
// Any method that returns a new Set returns whatever class the this object is
//   allowing SetEx to be subclassed and these methods will return that subclass
//   For this to work properly, subclasses must not change behavior of SetEx methods
//
// Note that if the contructor for SetEx is passed one or more iterables, 
// it will iterate them and add the individual elements of those iterables to the Set
// If you want a Set itself added to the Set, then use the .add() method
// which remains unchanged from the original Set object.  This way you have
// a choice about how you want to add things and can do it either way.

class SetEx extends Set {
    // create a new SetEx populated with the contents of one or more iterables
    constructor(...iterables) {
        super();
        this.merge(...iterables);
    }
    
    // merge the items from one or more iterables into this set
    merge(...iterables) {
        for (let iterable of iterables) {
            for (let item of iterable) {
                this.add(item);
            }
        }
        return this;        
    }
    
    // return new SetEx object that is union of all sets passed in with the current set
    union(...sets) {
        let newSet = new this.constructor(...sets);
        newSet.merge(this);
        return newSet;
    }
    
    // return a new SetEx that contains the items that are in both sets
    intersect(target) {
        let newSet = new this.constructor();
        for (let item of this) {
            if (target.has(item)) {
                newSet.add(item);
            }
        }
        return newSet;        
    }
    
    // return a new SetEx that contains the items that are in this set, but not in target
    // target must be a Set (or something that supports .has(item) such as a Map)
    diff(target) {
        let newSet = new this.constructor();
        for (let item of this) {
            if (!target.has(item)) {
                newSet.add(item);
            }
        }
        return newSet;        
    }
    
    // target can be either a Set or an Array
    // return boolean which indicates if target set contains exactly same elements as this
    // target elements are iterated and checked for this.has(item)
    sameItems(target) {
        let tsize;
        if ("size" in target) {
            tsize = target.size;
        } else if ("length" in target) {
            tsize = target.length;
        } else {
            throw new TypeError("target must be an iterable like a Set with .size or .length");
        }
        if (tsize !== this.size) {
            return false;
        }
        for (let item of target) {
            if (!this.has(item)) {
                return false;
            }
        }
        return true;
    }
}

module.exports = SetEx;

这意味着在它自己的文件setex.js中，然后你可以在node.js中使用require()并取代内置的Set。

我已经创建了一个小片段，使用ES6中的一个函数合并任意数量的set。你可以改变“设置”为“地图”，让它与地图一起工作。

const mergeSets = (...args) => {
    return new Set(args.reduce((acc, current) => {
        return [...acc, ...current];
    }, []));
};

const foo = new Set([1, 2, 3]);
const bar = new Set([1, 3, 4, 5]);

mergeSets(foo, bar); // Set(5) {1, 2, 3, 4, 5}
mergeSets(foo, bar, new Set([6])); // Set(6) {1, 2, 3, 4, 5, 6}

不，它们没有内置操作，但你可以很容易地创建自己的操作:

Map.prototype.assign = function(...maps) {
    for (const m of maps)
        for (const kv of m)
            this.add(...kv);
    return this;
};

Set.prototype.concat = function(...sets) {
    const c = this.constructor;
    let res = new (c[Symbol.species] || c)();
    for (const set of [this, ...sets])
        for (const v of set)
            res.add(v);
    return res;
};

将集合转换为数组，将它们平直，最后构造函数将惟一化。

const union = (...sets) => new Set(sets.map(s => [...s]).flat());