I create a new array from an existing one (big array with 100.000 objects). In the new array I want only elements where the value of "city" is for example New York City.

    var newData = [];

    for (var i = 0; i < data.length; i++) {

        if(data[i].city === "New York City") {

            newData[i] = {"city": data[i].city, "longitude": 
            data[i].longitude, "latitude": data[i].latitude, "state": 
            data[i].state};

        }
     }

I must be doing something wrong since a lot of elements in the new array are null …

The new array then looks something like this:

[null,null,null,null,null, {"city":"New York", "logitude": 
-73.935242, "latitude": 40.730610, "state": "NY"},
null,null,null,null,null,null,"city":"New York", "logitude": 
-73.935242, "latitude": 40.730610, "state": "NY"}]

What am I doing wrong? How could I achieve my goal?

Thanks in advance guys!

I create a new array from an existing one (big array with 100.000 objects). In the new array I want only elements where the value of "city" is for example New York City.

    var newData = [];

    for (var i = 0; i < data.length; i++) {

        if(data[i].city === "New York City") {

            newData[i] = {"city": data[i].city, "longitude": 
            data[i].longitude, "latitude": data[i].latitude, "state": 
            data[i].state};

        }
     }

I must be doing something wrong since a lot of elements in the new array are null …

The new array then looks something like this:

[null,null,null,null,null, {"city":"New York", "logitude": 
-73.935242, "latitude": 40.730610, "state": "NY"},
null,null,null,null,null,null,"city":"New York", "logitude": 
-73.935242, "latitude": 40.730610, "state": "NY"}]

What am I doing wrong? How could I achieve my goal?

Thanks in advance guys!

• javascript
• arrays
• key-value

• 1 Why not use filter? – evolutionxbox Commented Jun 5, 2017 at 10:43
• @evolutionxbox: Looks like they want to create new objects. – T.J. Crowder Commented Jun 5, 2017 at 10:43
• You could try using console.log(data[i]) inside the if statement, to see what part of the object is being outputted. Excluding the null result, does the new array contain all the correct results? – WizardCoder Commented Jun 5, 2017 at 10:48

2 Answers 2

The elements won't be null, they'll be missing (which shows up as undefined when you try to access them). The reason is that you're increasing i every time, even when you skip an entry.

To fix it, use push instead:

var newData = [];

for (var i = 0; i < data.length; i++) {

    if (data[i].city === "New York City") {
        newData.push({
            "city": data[i].city,
            "longitude": data[i].longitude,
            "latitude": data[i].latitude,
            "state": data[i].state
        });
    }
}

If you want the two arrays to share objects, you could use filter instead:

var newData = data.filter(function(entry) {
    return entry.city === "New York City";
});

but if you want the new array to have new objects that are different from the originals, your for loop is fine.

You can use Array.prototype.filter method:

newData.filter(function (el) {
  return el.city === "New York City";
});

or if you need other filter parameter:

newData.filter(function (el) {
  return el.city === "New York City" && el.state === "NY" ;
});

This method is part of the new ECMAScript 5th Edition standard.

From documentation:

filter() calls a provided callback function once for each element in an array, and constructs a new array of all the values for which callback returns a value that coerces to true. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values. Array elements which do not pass the callback test are simply skipped, and are not included in the new array.