How can I add elements to an Array of objects in Javascript?

Question

I am writting a program to get a list of cities and their GPS coordinates and store it in an Array named "cities", to worjk later with it. This is the fragment of code:

var city = {name:String,lat:Number,lon:Number};
var cities = Array.of(city);

city.name="London";
city.lat=51.50;
city.lon=-0,11;

cities.push(city);

city.name="Barcelona";
city.lat=41.38;
city.lon=2,15;

cities.push(city);

console.log("Number of cities "+cities.length);
console.log(cities);

The result when executing this code is the following:

Number of cities 3
[
  { name: 'Barcelona', lat: 41.38, lon: 2 },
  { name: 'Barcelona', lat: 41.38, lon: 2 },
  { name: 'Barcelona', lat: 41.38, lon: 2 }
]

That is, it seems that the "push" method overwrites all the array records, and also add an additional record. I also have declared the array cities as follows: var cities = [city]; // An array of elements of the class "city" var cities = []; with both declarations the result is the same.

That's very strange and it seems obvious, but I do not know what's happening..

¿What am I doing wrong in the code shown above?

Thanks in advance

Gogowitsch · Accepted Answer · 2020-12-05 23:14:09Z

3

The behavior you observe has nothing to do with the push() function or the array. Instead, it is the way JavaScript handles all objects: by reference.

To clone an object, in other words break the reference, you can use a call such as

city = {...city}; // spread
city = Object.assign({}, city);
city = jQuery.extend({}, city);

The problem you have is that when changing the properties within the city object, you modify the references already in the array at the same time.

answered Dec 5, 2020 at 23:14

Gogowitsch

1,2331 gold badge14 silver badges35 bronze badges

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

Comments

Terry · Accepted Answer · 2020-12-05 23:14:56Z

That is because your city object is being passed by reference. The object in your array will be pointing to the same instance, so modifying it will modify all instances of that object in the array: regardless of how many times it's being inserted.

What you can do is to create a function that returns a fresh object:

function getCity(name, lat, long) {
  return { name, lat, long };
}

const cities = [];

cities.push(getCity('London', 51.50, 0.11));
cities.push(getCity('Barcelona', 41.38, 2.15));

console.log("Number of cities " + cities.length);
console.log(cities);

Alternatively, just create the objects in place:

function getCity(name, lat, long) {
  return { name, lat, long };
}

const cities = [];

cities.push({
  name: 'London',
  lat: 51.50,
  long: -0.11
});
cities.push({
  name: 'Barbelona',
  lat: 41.38,
  long: 2.15
});

console.log("Number of cities " + cities.length);
console.log(cities);

Joseph · Accepted Answer · 2020-12-06 10:43:30Z

Thank you very much!! it was a beginner's mistake....the way how javascript manage objects, and nothing to do with the push() method.

I have used a class and a constructor and instantiate a new "fresh" object like this and it works !!. Here is the code:

class City {
    constructor(name, lat,lon) {
        this.name = name;
        this.lat = lat;
        this.lon = lon;
    }
    };

var cities = [];

var newcity = new City("London",51.50,-0.11);

cities.push(newcity);

var newcity = new City("Barcelona",41.38,2.15);

cities.push(newcity);

var newcity = new City("Rome",41.90,12.49);

cities.push(newcity);

console.log("Number of cities "+cities.length);
console.log(cities);

and the result correct:

Number of cities 3
[
  City { name: 'London', lat: 51.5, lon: -0.11 },
  City { name: 'Barcelona', lat: 41.38, lon: 2.15 },
  City { name: 'Rome', lat: 41.9, lon: 12.49 }
]

Collectives™ on Stack Overflow

How can I add elements to an Array of objects in Javascript?

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related