class Group {

constructor() {

this.members = [];

}

add(value) {

if (!this.has(value)) {

this.members.push(value);

}

}

delete(value) {

this.members = this.members.filter(v => v !== value);

}

has(value) {

return this.members.includes(value);

}

static from(collection) {

let group = new Group;

for (let value of collection) {

group.add(value);

}

return group;

}

[Symbol.iterator]() {

return new GroupIterator(this);

}

}

class GroupIterator {

constructor(group) {

this.group = group;

this.position = 0;

}

next() {

if (this.position >= this.group.members.length) {

return {done: true};

} else {

let result = {value: this.group.members[this.position],

done: false};

this.position++;

return result;

}

}

}

for (let value of Group.from(["a", "b", "c"])) {

console.log(value);

}

// → a

// → b

// → c