How to update or add value in an array using JavaScript?

Question

I have an array of arrays in a variable using in JavaScript. I would like to update the value with the sub-array without having to copy the array temporary.

Here is an example.

I have this array

var data= [{
             '123':  [{'a': 10, 'b': 20, 'c': 30, 'd': 40}],
             '456':  [{'a': 1, 'b': 2, 'c': 3, 'd': 4}]
          }];

I want to be able to change the value of a inside of the 123 key if a existing. Otherwise, I want to create that key. Also, if 'a' does not exists in the sub-array, I want to add it instead of throwing an error.

I am trying to update

data['123']['assigned_at'] = 'value';

but since assigned_at does not exist I am getting this error

Unable to set property 'assigned_at' of undefined or null reference.

How can I correctly update the array correctly?

var data = [{
  '123': [{
    'a': 10,
    'b': 20,
    'c': 30,
    'd': 40
  }],
  '456': [{
    'a': 1,
    'b': 2,
    'c': 3,
    'd': 4
  }]
}];

data['123']['assigned_at'] = 'value';

It appears data is an array containing a single object which in return holds the keys 123, 456. Is that a typo? Oh and 123 and 456 are again arrays with only a single object - so the full path would be: data[0][123][0]["a"] — nuala
– nuala, Commented Mar 7, 2016 at 21:45

Amit · Accepted Answer · 2016-03-07 21:42:17Z

6

Your problem is not that assigned_at does not exist, it's that data['123'] does not exist, hence undefined.

Try data[0]['123'].

BTW: you seem to not understand arrays and/or objects since you put a single object in each of your arrays.

answered Mar 7, 2016 at 21:42

Amit

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2 Comments

Junior Over a year ago

Sorry, I think my example just throw everything off. Here is how I define my array var data = []; Then I add a key and array to it data['123']['assigned_at'] = 'value'. I hope this better explain what I am trying to do.PS: data[0]['123'] is still now working.

Amit Over a year ago

@MikeA how about you ask about your problem instead of telling that you have something which you really don't? And while you're at it, create a MCVE

jdabrowski · Accepted Answer · 2016-03-07 22:05:47Z

5

You probably want the simpliest form of the data which would be a JSON in this case, so the brackets are unnecessary:

var data= {
             '123': {'a': 10, 'b': 20, 'c': 30, 'd': 40},
             '456': {'a': 1, 'b': 2, 'c': 3, 'd': 4}
          };

Now you can get the items like this:

data['123'];
// and the nested objects:
data['123']['a'];

What you had was an Array with one JSON element in it, so you would get it with the following code:

data[0]['123']
// and for the nested objects:
data[0]['123'][0]['a']

edited Mar 7, 2016 at 22:05

answered Mar 7, 2016 at 21:43

jdabrowski

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Comments

sulimmesh · Accepted Answer · 2016-03-07 21:49:12Z

So you're problem is that you think you are using a hashmap when in reality you are using an array. So when you write something like:

var data= [{
         '123':  [{'a': 10, 'b': 20, 'c': 30, 'd': 40}],
         '456':  [{'a': 1, 'b': 2, 'c': 3, 'd': 4}]
      }];

console.log(data["123"])

You'll get an error because you are trying to index an array with a string. As you can see by looking at your data object, it's an array wrapping a hashmap (which is honestly unnecessary). If you really were to use this object correctly you'd need to write

var data= [{
         '123':  [{'a': 10, 'b': 20, 'c': 30, 'd': 40}],
         '456':  [{'a': 1, 'b': 2, 'c': 3, 'd': 4}]
      }];

console.log(data[0]["123"])

which will print [{'a': 10, 'b': 20, 'c': 30, 'd': 40}].

If you wanted to assign a new key into data[0]["123"], since you wrap the value in an array as well, you'd need to write

var data= [{
         '123':  [{'a': 10, 'b': 20, 'c': 30, 'd': 40}],
         '456':  [{'a': 1, 'b': 2, 'c': 3, 'd': 4}]
      }];

data[0]["123"][0]["assigned_at"] = 52 //or w/e you want here

I'd recommend dropping the [] wrapping around your values and just writing

var data= {
         '123':  {'a': 10, 'b': 20, 'c': 30, 'd': 40},
         '456':  {'a': 1, 'b': 2, 'c': 3, 'd': 4}
      };

data["123"]["assigned_at"] = 52 //or w/e

Which will work. I think you've just caused yourself a little confusion by adding the [] to your values is all.

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