Javascript push Object to global array overwrites previous values?

Question

I'm having this problem in Javascript: I want to get the longitude and latitude values from an array of Objects. This all works fine, but when I try to save it into a global array, it overwrites every previous value. The result is an array with 8 times the last pushed object.

Global array: var _coordinates = [];

function getCoordinates()
{
   mark = {};

   for(var key in _data)
   {
       if(_data.hasOwnProperty(key)){
           mark["lng"] = _data[key].long;
           mark["lat"] = _data[key].lat;
       }

       console.log(mark); // Returns different coordinates (yay)
       _coordinates.push(mark);

   }   

   console.log(_coordinates); // All coordinates are the same (meh)
}

This is my first time asking a question here. So if I forgot something, please say so.

What do you want the global and local variables to be, here? currently, mark is global, _coordinates is assumed + global, _data is assumed + global. — Paul S.
– Paul S., Commented Aug 21, 2013 at 15:35
so problem is you are working on the same instance of mark — Arun P Johny
– Arun P Johny, Commented Aug 21, 2013 at 15:35

Darin Dimitrov · Accepted Answer · 2013-08-21 15:35:39Z

6

You could try to declare and instantiate the mark object inside the for loop because right now you are modifying the same instance all the time:

function getCoordinates() {
    for(var key in _data) {
        var mark = {};
        if(_data.hasOwnProperty(key)) {
            mark["lng"] = _data[key].long;
            mark["lat"] = _data[key].lat;
        }
        _coordinates.push(mark);
    }   

    console.log(_coordinates);
}

answered Aug 21, 2013 at 15:35

Darin Dimitrov

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3 Comments

Darin Dimitrov Over a year ago

@OttoAllmendinger, are you sure? Did you test it? Because this seems to work perfectly fine: jsfiddle.net/bGGPX Why would this demo work if you were right?

George Over a year ago

@OttoAllmendinger The var declaration is indeed hoisted, but because mark is still being reassigned to a new object every loop, this still works.

Otto Allmendinger Over a year ago

Whoops, you're right. Been bitten to often by it, now I'm too paranoid ;-)

NPE · Accepted Answer · 2013-08-21 15:35:52Z

2

The problem is that you are repeatedly modifying the same object. Your array ends up containing eight references to that one object.

To fix, move

mark = {};

into the for loop.

answered Aug 21, 2013 at 15:35

NPE

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Comments

Otto Allmendinger · Accepted Answer · 2013-08-21 15:42:32Z

1

You are mutating the (global) variable mark in every loop. Do this instead

_coordinates.push({lng: _data[key].long, lat: data[key].lat});

edited Aug 21, 2013 at 15:42

answered Aug 21, 2013 at 15:36

Otto Allmendinger

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