Array of array of objects reference in Javascript

Question

I have an array as follows

var sample = [{a:1, b: 1, c:1}, {a:1, b: 1, c:1}, {a:1, b: 1, c:1}];

I then run the following code and try groupsOfItems[0].sample[0].a = 10 , groupsOfItems[0].sample[0].a , groupsOfItems[1].sample[0].a and groupsOfItems[2].sample[0].a get changed to 10.

How do I prevent this?

 var sample = [{a:1, b: 1, c:1}, {a:1, b: 1, c:1}, {a:1, b: 1, c:1}]; var groupsOfItems = []; for(let i = 0; i < 10; i++) { var item = {}; item.sample = _.clone(sample); groupsOfItems.push(item); } groupsOfItems[0].sample[0].a = 10 console.log(groupsOfItems[0].sample[0].a,groupsOfItems[1].sample[0].a,groupsOfItems[2].sample[0].a); 

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Answer 1

You need to clone the object before assiging its referece to the array's property

Replace

item.sample = sample;

with

item.sample = JSON.parse(JSON.stringify(sample));

Note that this method of cloning will be less efficient when the sample object grows. Try some other methods shown here .

Answer 2

as mentioned in this post

for(let i = 0; i < 10; i++) {
        var item = {};
        item.sample = $.extend(true, [], sample);
        groupsOfItems.push(item);
    }

Answer 3

I would avoid cloning the object in general. Cloning objects only tends to end in pain further down the track. Here's how I have achieved similar in the past without cloning.

var sample = [{   a: 1,   b: 1,  c: 1}, {  a: 1,  b: 1,  c: 1}, {  a: 1,  b: 1,  c: 1}];
var groupsOfItems = [];

var Item = function(a, b, c) {
  this.a = a;
  this.b = b;
  this.c = c;
}

for (let i = 0; i < 10; i++) {
  var item = {};
  item.sample = _.map(sample, function(item) {
    return new Item(item.a, item.b, item.c)
  });
  groupsOfItems.push(item);
}

groupsOfItems[0].sample[0].a = 10

console.log(groupsOfItems[0].sample[0].a, groupsOfItems[1].sample[0].a, groupsOfItems[2].sample[0].a);
//10 1 1

This way you're assigning them all a container for your modifications and the problem with cloning goes away.