CODEWITHSUNDEEP
javascriptfloating-pointieee-754
Mark Jeronimus
Mark Jeronimus

Reputation: 9543

Convert between double and float precision in javascript

How do I convert a number from double-precision to single-precision and back?

For example, I have this in Java/C/C#:

double x = 0.00001;
float f = (float)x; // should be 0.000009999999747378752 not 0.0000100000000000000008180305391403
int n = Math.floor(1001 * f / x); // should be 1000 not 1001.

(don't ask. this is a simplified version of what some hairy C code looks like and I need to port it 1:1)

Another example:

double y = Math.floor((double)(float)5.999999876); // should be 6.0

What I already tried:

var f:float = x; // syntax error
var f = x as float; // syntax error
var f = parseFloat(x); // returns full double-precision (0.0000100000000000000008180305391403)
var f = toFixed(x, ...); // Can't use this as float is not defined by number of decimal places but number of binary places.

I'm using Rhino (as part of Java 7), and it should also be compatible with Nashorn (as part of Java 8). I have access to the entire public Java API, if this helps.

[Edit] I did some more experiments and it seems like the problem is not in the float conversion but in the float operations. I really need the FPU in my processor to perform a single-precision fmul here. 1001 * f doesn't work if f contains the float precision version of 0.00001 in a double-precision number. The only way I get an exact result is if I perform a 32-bit multiplication on 1001f * 0.00001f and then obtain the result.

Upvotes: 3

Views: 6046

Answers (3)

Eyal
Eyal

Reputation: 5848

This will convert a number to float in javascript:

new Float32Array([15.603179021718429])[0]

The answer is:

15.603178977966309

Oops, even easier, use Math.fround(). New in ES6.

Upvotes: 5

Sphygmomanometer
Sphygmomanometer

Reputation: 595

JavaScript itself doesn't provide any such feature. I recommend you use big.js if you absolutely need precision conversions.

Upvotes: 3

AdityaParab
AdityaParab

Reputation: 7100

All numerical values in JavaScript are Numbers They are all 64-bit floating point numbers.

Read more here

Upvotes: 0

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