Array modification while iteration

Question

When trying to modify the current item while iterating over an array the modification fails. Below is the sample code.

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

for(var i in s_arr) {

  if(s_arr[i].a == 5) {

    s_arr[i].b = 0;
    console.log('First modification: ' +JSON.stringify(s_arr[i]));
    arr.push(s_arr[i]);

    s_arr[i].b = 9;
    console.log('Second modification: ' +JSON.stringify(s_arr[i]));
    arr.push(s_arr[i]);
  }

}

console.log('Final: ' +JSON.stringify(arr));

After running the above script node test.js, below is the result.

First modification: {"a":5,"b":0}
Second modification: {"a":5,"b":9}
Final: [{"a":5,"b":9},{"a":5,"b":9}]

Expected result is as below.

First modification: {"a":5,"b":0}
Second modification: {"a":5,"b":9}
Final: [{"a":5,"b":0},{"a":5,"b":9}]

However when adding new object while iteration & assigning each values of current item (object) extensively works.

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

for(var i in s_arr) {

  if(s_arr[i].a == 5) {

    s_arr[i].b = 9;
    console.log('Second modification: ' +JSON.stringify(s_arr[i]));
    arr.push(s_arr[i]);

    var a = {};
    a.a = s_arr[i].a;
    a.b = 0;
    arr.push(a);

    var b = {};
    b.a = s_arr[i].a;
    b.b = 9;
    arr.push(b);
  }

}

console.log('Final: ' +JSON.stringify(arr));

Below is the result of modified script.

Final: [{"a":5,"b":0},{"a":5,"b":9}]

Why is the first script when ran shows modification of object right but the final array shows which consists of modified objects is not as expected?

maybe because it's the same object ? .. it's not pushing a copy of this object in the array, so you add the same reference to the object twice. — Hacketo
– Hacketo, Commented Nov 26, 2015 at 10:55
Objects always passed by reference, you should clone it manually — vp_arth
– vp_arth, Commented Nov 26, 2015 at 10:58

vp_arth · Accepted Answer · 2015-11-26 11:58:56Z

2

Objects always passed by reference in JS.

arr.push(s_arr[i]); does not create copy of object, it just save reference to it in the arr array;
So any changes inside object you will see in the array too.

You should explicit clone your object to prevent changes.
For example you can use serialize-deserialize pair:

arr.push(JSON.parse(JSON.stringify(s_arr[i])));

Your

var a = {};
a.a = s_arr[i].a;
a.b = 0;
arr.push(a);

will work too, because you create new object instance here and use only scalar properties to fill it.

edited Nov 26, 2015 at 11:58

answered Nov 26, 2015 at 11:00

vp_arth

15k4 gold badges43 silver badges72 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Array modification while iteration

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related