对象

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A vector type

封装一个类，实现坐标的加减法，以及get属性的length返回坐标离原点的距离。 

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// Your code here.
class Vec{
    constructor(x,y){
      this.x = x;
      this.y = y;
    }
    plus(other){
      return new Vec(this.x+other.x,this.y+other.y);  
    }
    minus(other){
      return new Vec(this.x-other.x,this.y-other.y); 
    }
    get length(){
       return Math.sqrt(this.x**2+this.y**2);
    }
}
console.log(new Vec(1, 2).plus(new Vec(2, 3)));
// → Vec{x: 3, y: 5}
console.log(new Vec(1, 2).minus(new Vec(2, 3)));
// → Vec{x: -1, y: -1}
console.log(new Vec(3, 4).length);
// → 5

Groups

封装一个Group类，实现相当于Set的功能。 

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class Group {
  // Your code here.
  constructor(){
    this.members = [];  
  }
  add(value){
   if(!this.has(value)){
     this.members.push(value);  
   }
  }
  delete(value){
    this.members = this.members.filter(member => member!==value);
  }
  has(value){
    return this.members.includes(value);  
  }
  static from(iterable){
    let group = new Group();
    for(let value of iterable){
      group.add(value);
    }
    return group;
  }
  
}

let group = Group.from([10, 20]);
console.log(group.has(10));
// → true
console.log(group.has(30));
// → false
group.add(10);
group.delete(10);
console.log(group.has(10));
// → false

Iterable groups

让上面的数组类可迭代。

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// Your code here (and the code from the previous exercise)
class Group {
  // Your code here.
  constructor(){
    this.members = [];  
  }
  add(value){
   if(!this.has(value)){
     this.members.push(value);  
   }
  }
  delete(value){
    this.members = this.members.filter(member => member!==value);
  }
  has(value){
    return this.members.includes(value);  
  }
  static from(iterable){
    let group = new Group();
    for(let value of iterable){
      group.add(value);
    }
    return group;
  }
   [Symbol.iterator](){
     let index = 0;
     let members = this.members;
     return {
       next(){
         if(index < members.length){
           return {value:members[index++],done:false};
         }else {
           return {done:true};
         }
       }
     };
   } 
}
for (let value of Group.from(["a", "b", "c"])) {
  console.log(value);
}
// → a
// → b
// → c