为什么此吸气剂返回未定义？

Question

Here's an example of a getter that iterates over an array and is expected to return an element for which a is true . But test.active returns undefined .

var test = {
  arr: [
    {id: 1, a: false},
    {id: 2, a: false},
    {id: 3, a: true},
    {id: 4, a: false},
    {id: 5, a: false},
  ],
  get active() {
    this.arr.forEach(item => {
      if(item.a) return item
    })
  }
}

Why?

Answer 1

Your return statement is in an inner function, causing that function to return. Your outer function does in fact return nothing.

If you want the outer function to return, use a normal for loop instead.

 var test = { arr: [ {id: 1, a: false}, {id: 2, a: false}, {id: 3, a: true}, {id: 4, a: false}, {id: 5, a: false}, ], get active() { for (var i = 0, e = this.arr.length; i < e; i++) { var item = this.arr[i]; if (item.a) return item; } } } console.log(test.active); 

It might help if you understood how forEach works.

Internally, it looks much like the following, though this is very simplified.

 function forEach (array, block) { var i, length = array.length; for (i = 0; i < length; i++) { // This is where your return would end up, unused, in a different context. block(array[i], i); } } forEach(['a', 'b', 'c', 'd'], item => { return 'is meaningless here'; }); 

Answer 2

Alternatively, you can use Array.prototype.find() function to test for a given condition and return the found element.

var test = {
  arr: [
    {id: 1, a: false},
    {id: 2, a: false},
    {id: 3, a: true},
    {id: 4, a: false},
    {id: 5, a: false},
  ],
  get active() {
    return this.arr.find(item => {
      return (item.a === true);
    });
  }
}

alert(test.active.id)

https://jsfiddle.net/arqxcbkv/1/