How to update javascript object

Question

I have a js object like

[{"name":"John","group":"t1..." .....}]

so to create new user from same t1, I loop through the obj using

var new_user;
for(var i=0; i<users.length; i++) {
  if(users[i].group === 'group name goes here') {
    new_user = users[i];
    break;
  }
}

then is simply push it using users.push(new_user); then I try to change the just the name using

var l = users.length - 1; users[l].name = 'Jack';

it changes the name for the all the users.

[{"name":"Jack","group":"t1..." .....},{"name":"Jack","group":"t1..." .....}]

I know I am doing something wrong. I appreciate any help.

Are you sure that all of the other elements had `name` other than `Jack` before you do `users[l].name = 'Jack'`? — ekad, Dec 02 '14 at 13:36
It's not that you're doing anything wrong, but how JavaScript handles the values. See http://stackoverflow.com/questions/518000/is-javascript-a-pass-by-reference-or-pass-by-value-language for more details... — War10ck, Dec 02 '14 at 13:38
at the end you are changing all elements, when you do new_user = users[i]; you copy the ref — rahpuser, Dec 02 '14 at 13:38
How about something like this - > http://jsfiddle.net/6whsLvuo/ — Mark Walters, Dec 02 '14 at 13:55

score 7 · Answer 1 · answered Dec 02 '14 at 13:38

Problem here is that you assign the a new variable name to an old object. That is a reference and not a copy of the object. new_user = users[i] creates an reference that new_user is linked to users[i]. You have to copy the users[i] object, in order to only change the new_user. Underscore has a copy/clone function that you can use.

score 1 · Accepted Answer · answered Dec 02 '14 at 13:59

@Haneev's answer gives the concept of why the name of all users elements was changed by your code. Below is a code example of how you copy users[i] using slice method

var users = [{"name":"John","group":"t1"}];

var new_user;
for(var i=0; i<users.length; i++) {
  if(users[i].group === 't1') {
    // make a copy of users[i] and assign to new_user
    new_user = users.slice(i);
    break;
  }
}

// add new_user to users
users.push(new_user);

var l = users.length - 1; 
users[l].name = 'Jack';

alert(users[0].name); // this will display John
alert(users[1].name); // this will display Jack

Working demo: http://jsfiddle.net/hkLkkepo/

How to update javascript object

2 Answers2