JavaScript - How To Detect Number As A Decimal (Including 1.0)

Question

It feels like I am missing something obvious here. This has been asked a number of times - and the answer usually boils down to:

var num = 4.5;
num % 1 === 0; // false - 4.5 is a decimal

But, this fails for

var num = 1.0; // or 2.0, 3.0, ...
num % 1 // 0

Unfortunately, these doesn't work either

num.toString() // 1
typeof num // "number"

I am writing a JavaScript color parsing library, and I want to process input differently if it is given as 1.0 or 1. In my case, 1.0 really means 100% and 1 means 1.

Otherwise, both rgb 1 1 1 and rgb 1 255 255 will be parsed as rgb 255 255 255 (since I am right now taking anything <= 1 to mean a ratio).

Answer 1

Those numbers aren't actually decimals or integers. They're all floats. The only real difference between 1 and 1.0 is the notation that was used to create floats of equal values.

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Edit: to help illustrate, consider:

1 === 1.0; // true
parseInt('1') == parseInt('1.0'); // true
parseFloat('1') === parseFloat('1.0'); // true
parseInt('1') === parseFloat('1'); // true
// etc...

Also, to demonstrate that they are really the same underlying data type:

typeof(1); // 'number'
typeof(1.0); // 'number'

Also, note that 'number' isn't unambiguous in JavaScript like it would be in other languages, because numbers are always floats.

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Edit 2: One more addition, since it's relevant. To the best of my knowledge, the only context in JavaScript in which you actually have "real and true" integers that aren't really represented as floats, is when you're doing bitwise operations. However, in this case, the interpreter converts all the floats to integers, performs the operation, and then converts the result back to a float before control is returned. Not totally pertinent to this question, but it helps to have a good understanding of Number handling in JS in general.