Creating a variable name of a property in a JSON object

Question

I'm running this code.

var output = {"records": []};
for(i = 0; i < data.length; i++)
  output.records[i] = { propertyName : data[i][propertyName] }

I expected the output to be on the following form.

{ "cat" : "mjau" }
{ "dog" : "woff" }

Instead, I get to my surprise this.

{ "propertyName" : "mjau" }
{ "propertyName" : "woff" }

How can I get variable propertyName?

I'm trying to create a parser that will create a number of records that are all cat but, when called from an other place, the records should have dog property instead. I wish to avoid creating two different code pieces for that.

I've found this question, which I suspect contains the answer to my issue. However, due to ignorance, I don't get it.

where do you put those cat n dog?? i mean where do you hold those properties? — Bhushan Firake, Dec 10 '12 at 20:30
possible duplicate of [creating json object with variables](http://stackoverflow.com/questions/12979335/creating-json-object-with-variables) — jbabey, Dec 10 '12 at 20:30
@jbabey Congrats! I mentioned in my question that I found **that exact question**. I also explained that due to lack of skills, I don't understand it. What's the point of mentioning it **again**? Besides, the answers I got here are very clear and actually helpful, so **exact** duplicate it is not. :) — , Dec 10 '12 at 20:39
@BhushanFirake I'm not sure what you mean. Those are just some property names that I'll be working with. — , Dec 10 '12 at 20:44

score 3 · Answer 1 · answered Dec 10 '12 at 20:28

3

Keys in object literals won't be evaluated in JavaScript. So, you need to create an empty object ({}) and then assign the key dynamically:

output.records[i] = {};
output.records[i][propertyName] = data[i][propertyName]

answered Dec 10 '12 at 20:28

Wyatt Anderson

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score 0 · Answer 2 · answered Dec 10 '12 at 20:28

0

var a = {b:'c'}

is just like

var a = {};
a['b'] = 'c';

What you want is

a[b] = c

that is

output.records[i] = {};
output.records[i][propertyName] = data[i][propertyName];

You have in this MDN document : Working with objects.

answered Dec 10 '12 at 20:28

Denys Séguret

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score 0 · Answer 3 · answered Dec 10 '12 at 20:30

In { propertyName : data[i][propertyName] } the property name part should be constant string. It you pass a variable it wont fetch its value.

What you have to do is

for(i = 0; i < data.length; i++){
  var a = {};
  a[propertyName] = data[i][propertyName];
  output.records.push(a);
}

score -1 · Answer 4 · answered Dec 10 '12 at 20:29

-1

You can try this:

'"' + propertyName + '"' : ...

answered Dec 10 '12 at 20:29

orolo

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I notice that your approach is different from most of the others. Is it the same thing expressed with an other syntax or is it a different method? If so, which is preferable in this case? – Dec 10 '12 at 20:41
No. I misunderstood. All I'm doing in the above is wrapping the string in quotes. OP would probably get something like ""propertyName"". – orolo Dec 10 '12 at 21:55

Creating a variable name of a property in a JSON object

4 Answers4