如何使对象在数组中找到自己的索引？

Question

I have a simple array of objects with a id property and I need the ids to be equal to their index. I tried this

var arr = [

  {
    id: arr.indexOf(this),
    length: this.length
  },
   {
    id: arr.indexOf(this),
    length: this.length
  },
   {
    id: arr.indexOf(this),
    length: this.length
  }

]

But that doesn't work. Do I have to use a function outside the array to do this? None of the built in methods would help?

Answer 1

 // If this.length =3 var arr = [ {length: 3}, {length: 3}, {length: 3} ] arr.forEach((item, index)=> item.id =index) console.log(arr) 

Answer 2

Every time you push to the array you can get the current array length and set it as id:

var array = []; 
array.push({id: array.length});

Also you can use a loop:

var array = [];
for(var i = 0; i < howeveryouwant; i++){
     array[i] = {id: i};
}

And if the initialization is static you could just set the ids:

 var arr = [

 {
    id:0,
    length: this.length 
 },
 {
    id: 1,
    length: this.length //this will be 0
 },
 {
    id: 2,
    length: this.length //this also will be 0
 }

];

Answer 3

You could also use map which takes the index and array as 2nd and 3rd parameter respectively:

const arr = [{title: 'A'}, {title: 'B'}, {title: 'C'}].map( (el, i, a) => ({id: i, ...el, length: a.length}));

console.log(arr);

Answer 4

Following answer tries to address how you can use a class to create above structure and not have to loop to mutate length.

Idea:

• Create a function( MyList ) which has an array that holds the objects.
• Create another function( MyTempObject ) which only holds values that are specific to object.
• Move any common property, like length to prototype.
• In MyList , make the array as private and create a public property that returns a copy of this array. This will ensure that value is not mutable and can only be mutated using exposed APIs.

 function MyList () { var values = []; function MyListItem(id) { this.id = id; } Object.defineProperty(MyListItem.prototype, 'length', { get: function() { return values.length; } }); Object.defineProperty(this, 'value', { get: function() { return values.slice(); } }); this.add = function(id) { values.push(new MyListItem(id)); } } var myList = new MyList(); myList.add(1); console.log(myList.value) console.log(myList.value[0].length) myList.add(2); console.log(myList.value) console.log(myList.value[0].length) myList.add(3); console.log(myList.value) console.log(myList.value[0].length) 

---

One reason, you might wish to use this approach is in a scenario where you add/delete entry in this array. You will have to loop again and again to update the length in every object.