strange Javascript Float division result

Question

var coeff='0.1';
var amount='12.2';

var res = Math.floor(parseFloat(amount) / parseFloat(coeff));

console.log(res);

Why the result of this is 121 (I was expecting 122)?

EDIT: my question was ambiguous: no trouble with the floor function. I was just wondering why 12.2 / 0.1 is not equal to 122.

what does `console.log(parseFloat(amount) / parseFloat(coeff))` show? — Jaromanda X, Sep 08 '15 at 14:31
Because in my project I get these values from string (I init them as strings to make this question simpler). — David Dahan, Sep 08 '15 at 14:32
@DavidD. hint: check the docs on exactly what `Math.floor()` does, then look at the console result jaromanda X clued you to — Jared Smith, Sep 08 '15 at 14:33
floor(121.9) is 121, floor is not a round-up/round-down function, it is a "take everything after the decimal away" function. — Roy Dictus, Sep 08 '15 at 14:33
@JaredSmith I saw this.. the question is more why `console.log(parseFloat(amount) / parseFloat(coeff))` is 121.99999 instead of 122.0 — David Dahan, Sep 08 '15 at 14:34
see http://stackoverflow.com/questions/1458633/how-to-deal-with-floating-point-number-precision-in-javascript - it has to do with floating point precision and has been asked many times - your original code masked the fact — Jaromanda X, Sep 08 '15 at 14:35
@RoyDictus : take a look my answer to JaredSmith please. I have no trouve with floor, but with the result of division. — David Dahan, Sep 08 '15 at 14:35
Yeah, that IEEE 754 lack of floating point precision will bite you in a lot of languages besides javascript — Jared Smith, Sep 08 '15 at 14:40

score 0 · Answer 1 · answered Sep 08 '15 at 14:33

0

the result is 121.99999999999999

The Math.floor() function returns the largest integer less than or equal to a given number.

so it cuts off the .99999. you might want to use

Math.round(parseFloat(amount) / parseFloat(coeff));

instead

answered Sep 08 '15 at 14:33

Jesse

3

explain the 121.99999999 instead of 122.0 (12.2 / 0.1 is 122 after all) – Jaromanda X Sep 08 '15 at 14:35
@JaromandaX From what I understand, javascript has a weird usage of bytes in your system. Don't ask me why though – Timberman Oct 23 '22 at 16:32
@Timberman, there's no weird usage of bytes in my system by javascript or anything else. 7 years ago, I was pointing out that the answer did not fully answer the question - sure, it's obvious that `Math.floor(121.99999)` is `121` - everyone knows that, but why would the result of `12.2/0.1` be 121.99999 in the first place – Jaromanda X Oct 23 '22 at 22:12

1 Answers1