Return the index of object that exist in both array when comparing Javascript

Question

------Edited---------------

I have 2 arrays of objects A, B

var a = [{id:1},{id:2},{id:3},{id:4},{id:5}];
var b = [{id:1},{id:2},{id:3}];

I want to delete object which are both in A and in B

//after the operation
 a = [{id:1},{id:2},{id:3}];

I thought in order to do it, I need to get the index of them in A to use A.splice() but I just cannot wrap my head around how I can do it.

This is the code I use to get the objects itself from Difference between two array of objects in JavaScript

 var onlyInA = A.filter(function(current){
    return res.nodes.filter(function(current_b){
        return current_b._id == current._id
    }).length == 0

If you want to remove them, than just remove them in the filter line... Do not loop over once and do it again to remove.... — epascarello, Nov 08 '16 at 15:37
btw, you need the objects, which exists in both arrays, not only in one. — Nina Scholz, Nov 08 '16 at 16:03
@NinaScholz: Thank you, but one aspect I would like to get out is the index of those object in array A. Due to some limitations, I can only remove from A, not assign them to those new value. — Loredra L, Nov 08 '16 at 16:26

epascarello · Accepted Answer · 2016-11-08T17:42:46.623

2

Filter in filter would not be the way to go. You want to use filter and some.

var a = [{id:1},{id:2},{id:3},{id:4},{id:5}];
var b = [{id:1},{id:2},{id:5},{id:6},{id:7}];

var result = a.filter( function(oa) {
  return b.some( function(ob){
    return oa.id===ob.id;
  });
});

a.length = 0;
a.push.apply(a, result);

console.log(JSON.stringify(a));

If you want to use splice(), than loop over it backwards.

var a = [{id:1},{id:2},{id:3},{id:4},{id:5}];
var b = [{id:1},{id:2},{id:5},{id:6},{id:7}];

for (var i=a.length-1;i>=0;i--) {
    var inB = b.some( function(ob){
      return a[i].id===ob.id;
    });
    if(!inB) {
      a.splice(i,1)
    }
}
console.log(a);

edited Nov 08 '16 at 17:42

answered Nov 08 '16 at 15:42

epascarello

204,599
20
195
236

Sorry,I am still a bit confused about how to use filter in conjution with splice to delete the element which is not in A – Loredra L Nov 08 '16 at 16:30
In my version, due to some limitations, I cannot assign the array but rather have to remove element from it :( – Loredra L Nov 08 '16 at 16:38
Well you can just do... `a.length=0; a.push.apply(a, result)`, I will edit the answer – epascarello Nov 08 '16 at 16:42

score 2 · Answer 2 · answered Nov 08 '16 at 16:00

You could use a hash table for the id of the objects and filter a with it.

var a = [{ id: 1 }, { id: 2 }, { id: 3 }, { id: 4 }, { id: 5 }],
    b = [{ id: 1 }, { id: 2 }, { id: 3 }],
    hash = Object.create(null);

b.forEach(function (item) {
    hash[item.id] = true;
});

a = a.filter(function (item) {
    return hash[item.id];
});

console.log(a);

Return the index of object that exist in both array when comparing Javascript

2 Answers2