Error: for..in loops iterate over the entire prototype chain

ESLint报错: for…in loops iterate over the entire prototype chain, which is virtually never what you want.

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原因

for…in会遍历整个原型链，可能会出现我们不想遍历到的属性，例如

class A {
  constructor() {
    this.a = 'va'
  }
};
A.prototype.todo = () => 'todo';

let a = new A();
for(let k in a) { // for..in 遍历
  console.log(k)
}
// a  todo
console.log(Object.keys(a));
// [a]

解决

添加hasOwnProperty判断，或
改为使用Object.keys或Object.entries，这两个方法不会遍历原型链上的属性

class A {
  constructor() {
    this.a = 'va'
  }
};
A.prototype.todo = () => 'todo';

let a = new A();

for(let k in a)
  if(a.hasOwnProperty(k))
    console.log(k) // a

Object.keys(a).forEach((k) => console.log(k)); // a

Object.entries(a).forEach((e) => console.log(e)); // ['a', 'va']

备注: hasOwnProperty可以兼容到ES3，Object.keys和Object.entries分别是ES6及ES8中引入的，但是经过Babel打包后，这个区别可以忽略不计