Adding object in javascript array at runtime

Question

The mRegion object adds last object multiple times, however the objBeacon prints different objects. What is wrong with the mRegion?

      var mRegion = new Array();

      var objBeacon = {
          id: '10',
name:'name',   
          description: 'description'
      };
      $.ajax(settings).done(function(response) {
                  // populate beacon registry in an array
                  for (var i in response.items[0].devices) {
                      objBeacon.id = response.items[0].devices[i].id;
                      objBeacon.name = response.items[0].devices[i].name;
                      objBeacon.description = response.items[0].devices[i].description;


                      console.log("value of i is" + i);
                      console.log(objBeacon);

                      mRegion.push(objBeacon);
                  }
                  console.log(mRegion);

There is no error, however the same object is being added in array — Rohit
– Rohit, Commented Aug 29, 2016 at 11:27

Michał Młoźniak · Accepted Answer · 2016-08-29 11:03:24Z

2

Objects in javascript are passed by reference. You only have one variable objBeacon and each array element is pointing to this variable. Whenever you change objBeacon, all references will change.

var mRegion = [];

$.ajax(settings).done(function(response) {
  // populate beacon registry in an array
  for (var i in response.items[0].devices) {
    mRegion.push({
      id: response.items[0].devices[i].id,
      uid: '00',
      major: 1,
      minor: 1,
      name: response.items[0].devices[i].name,
      description: response.items[0].devices[i].description
    });
  }
});

answered Aug 29, 2016 at 11:03

Michał Młoźniak

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2 Comments

Rohit Over a year ago

will it make a difference if we use var mRegion = new Array(); instead of var mRegion = [];

Michał Młoźniak Over a year ago

No, but the array constructor has some quirks with how it handles input parameters. That's why it is highly recommended to always use array literal [] when creating new arrays.

Quentin · Accepted Answer · 2016-08-29 10:59:26Z

0

You only ever create one object and assign a reference to it to objBeacon.

Each time you go around the loop you modify the single object you have and push an additional reference to it into the array.

If you want an array of different objects, you need to create a new object each time you go around the loop.

answered Aug 29, 2016 at 10:59

Quentin

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Comments

lippoliv · Accepted Answer · 2016-08-29 10:59:54Z

As you are using objects your using "references" instead of "clones".

That code should work (even it is not very beautifull)

var mRegion = new Array();


$.ajax(settings).done(function(response) {
    // populate beacon registry in an array
    for (var i in response.items[0].devices) {
        var objBeacon = {
            id: '10',
            uid: '00',
            major: 1,
            minor: 1,
            name: 'name',
            description: 'description'
        };

        objBeacon.id = response.items[0].devices[i].id;
        objBeacon.name = response.items[0].devices[i].name;
        objBeacon.description = response.items[0].devices[i].description;


        console.log("value of i is" + i);
        console.log(objBeacon);

        mRegion.push(objBeacon);
    }
    console.log(mRegion);
});

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