Simple floating point maths in JavaScript

Question

I need to do some basic floating point math stuff (adding and multiplying money) for a website UI. I know that Javascript floats aren't accurate because of how they're stored, but I also know that somehow, it's possible to get the level of accuracy I require. I know this because Google's calculator can do it (type "calculator" into the Goog)..

Anyway, I don't want to have to send my little numbers back to the server and have to wait for a response, so I'm trying to use a library called BigNumbers.js, but I can't figure out how to make it spit out numbers (or strings) no matter what I call, it returns a BigNumber object.

Here's my test code: JSFiddle

floats = [145, 1.44, 1.3];

sum = new BigNumber(0);

for(i=0; i<floats.length; i++){
    sum = sum.times(floats[i]);
}

// sum = sum.toDigits(); //returns object
// sum = sum.toString(); //returns 0

console.log(sum); // expecting 271.44, getting object

How can I achieve the expected result? If there's a better library to use, that would be an acceptable answer as well.

Thank you.

When you're working with money and something like BigNumbers, you're working with **fixed point** numbers, not floating point. (Which is the right thing to do for money.) — Pointy, Sep 22 '15 at 19:11
what about ye'olde dirty typecast? sum = sum + " "; it should convert it to string. — GrafiCode, Sep 22 '15 at 19:12
Have you tried [`toFixed()`](https://mikemcl.github.io/bignumber.js/#toFix)? Eg `console.log( sum.toFixed(2) );` — dsh, Sep 22 '15 at 19:12

Pointy · Accepted Answer · 2015-09-22T19:23:04.280

3

You'll want to initialize sum to 1 instead of 0 (and maybe change its name to product), and then call .toString() when you pass it to console.log():

console.log(sum.toString());

edit — also, as pointed out in a comment, you should set the number of decimal places (to 2, probably) and also set the rounding mode. You can do that via the BigNumber.config() call.

edited Sep 22 '15 at 19:23

answered Sep 22 '15 at 19:13

Pointy

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i cant freaking believe i've been multiplying by zero this whole time.. thanks – I wrestled a bear once. Sep 22 '15 at 19:14
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You need also to set number of decimal places `BigNumber.config({ DECIMAL_PLACES: 2 })` – MaxZoom Sep 22 '15 at 19:19
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@Adelphia I would also recommend changing your multipliers from floats to fixed point if you want the accuracy (`1.44`, `1.3` should be `new BigNumber('1.44')` and `new BigNumber('1.3')`) – dsh Sep 22 '15 at 20:25
@dsh Thanks, I've done that with the config method as suggested in the answer, and I now realize that all numbers have to be converted to BigNumber. – I wrestled a bear once. Sep 22 '15 at 20:36
@Adelphia yes, you kind-of have to go all-in :) – Pointy Sep 22 '15 at 20:37

score 1 · Answer 2 · edited Dec 17 '17 at 13:05

1

You can go just fine with the JavaScript floating values and Math.round(..) method used to round cents:

var floats = [145, 1.44, 1.3];

sum = 1;

for (i=0; i<floats.length; i++){
  sum = Math.round(sum * floats[i] * 100)/100;
}

console.log(sum.toFixed(2)); // expecting 271.44

edited Dec 17 '17 at 13:05

Jed Fox

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answered Sep 22 '15 at 21:37

MaxZoom

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yeah, idk why i didn't just do that. obviously i didn't have my head on right this morning. – I wrestled a bear once. Sep 23 '15 at 01:49
Yeah, it happens.to everyone. Good brand of coffee usually works for me :) – MaxZoom Sep 23 '15 at 14:29

Simple floating point maths in JavaScript

2 Answers2