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I am developing a REST API and returning JSON. One of the fields is called submissionPercent and I need it to be a number but with exactly 2 decimal places

  • If the submissionPercent is 20, I need to return 20.00.
  • If the submissionPercent is 20.238, I need to return 20.24.

But submissionPercent should be a number not a String. If I use toFixed or toPrecision, then what I get is a String.

If possible, how do I achieve this?

Peter Bratton
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arahant
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1 Answers1

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var n = 20.238;

Math.round(n * 100) / 100
// => 20.24

Or more generally:

function roundWithPrecision(num, precision) {
  var multiplier = Math.pow(10, precision);
  return Math.round( num * multiplier ) / multiplier;
}

roundWithPrecision(20.238, 2)
// => 20.24

roundWithPrecision(20.238, 1)
// => 20.2

As others have pointed out, 20 and 20.00 are exactly the same, so if you want the user to see two digits after the decimal point even if they're zero, you'll have to use a string formatting function like toFixed.

In the case of a JSON API, it's up to the consumer to decide how to store the value upon decoding it. Even if you were to output JSON that said { val: 20.00 } a consumer will store it with whatever width it chooses. It might end up in a 32-bit float or a 64-bit float. It might end up in a string. It might end up in a MySQL database in a VARCHAR(6) column. You can't control how the consumer treats your data, and this is by design.

There is no advantage to outputting { val: 20.00 } over { val: 20 } or { val: 20.00000 } or { val: 2000e-2 }. If you want the API consumer to assume two digits of precision, you should put that information in your API documentation. I would encourage this, as it will help other developers make decisions when implementing API clients.

Jordan Running
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  • yes. but for 20 it gives me 20 not 20.00, as I was hoping was possible. – arahant Feb 11 '14 at 20:06
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    Note: multiplying, rounding and then dividing reduces the usable numeric range, if you require large precision numbers. – Xotic750 Feb 11 '14 at 20:06
  • "20.00" is only possible format as a `String`, which is what @Bergi was trying to explain. – Xotic750 Feb 11 '14 at 20:07
  • @arahant In what context? In every sense, `20` *is* `20.00` and vice versa. `var num = 20` is *exactly* the same as `var num = 20.00` – Jordan Running Feb 11 '14 at 20:14
  • @arahant In the case of a JSON API the consumer will decide how to represent the number internally. This is by design. If you want to ensure that a developer writing an API client knows to expect non-integer values you should include that information in your API documentation. – Jordan Running Feb 11 '14 at 20:22
  • @Jordan, yes I know 20 and 20.00 is the same. I just was wondering if it was possible to have it returned as 20.00 – arahant Feb 11 '14 at 20:26
  • @arahant See my updated answer. In a JSON API there is absolutely no advantage of having `20.00` instead of `20`. – Jordan Running Feb 11 '14 at 20:31
  • Throw Js into the fire, this is such an abhorrent shortcoming. Oh, hi there, Python! – Adrian M. Aug 15 '22 at 23:43