JavaScript adding decimal numbers issue

Thursday, 27 July 2017

So I am making a script that adds two numbers (decimal numbers) together, which I have encountered a problem.

I made the script, it turns out pretty good:

0.1 + 0.5  //0.6

0.2 + 0.3  //0.5

But soon I see:

0.1 + 0.2  //0.30000000000000004
0.01 + 0.06  //0.06999999999999999

And it does not look right to me. I know it is a shortcoming of using float point with finite bits, but I can't find a way to fix that.

Math.ceil   //No
Math.floor  //No
.slice      //No

UPDATE

Is it possible to multiply the numbers by 1000 first, then add them then divide it by 1000?

Answer

Use toFixed to convert it to a string with some decimal places shaved off, and then convert it back to a number.

+(0.1 + 0.2).toFixed(12) // 0.3

It looks like IE's toFixed has some weird behavior, so if you need to support IE something like this might be better:

Math.round((0.1 + 0.2) * 1e12) / 1e12

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