April 16, 2026 at 3:38:01 PM UTC

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JavaScript addition using the new and upcoming Math.sumPrecise():

❌ 1 + 0.1 - 1 = 0.10000000000000009
✅ Math.sumPrecise([1,.1,-1]) = 0.1

❌ 1e20 + 0.1 - 1e20 = 0
✅ Math.sumPrecise([1e20,.1,-1e20]) = 0.1

❌ 0.1 + 0.2 = 0.30000000000000004
❌ Math.sumPrecise([.1,.2]) = 0.30000000000000004

Read more on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sumPrecise

23 replies

Amelia Bellamy-Royds 6 min later

@zachleat Coming soon to a JS API near you:
Math.sumPreciserFinalv2UseThisOne()
1. Zach Leatherman 7 min later
  
  @AmeliaBR and I would welcome its arrival!! 😅
  1. Philipp Tootloff 13 min later
    
    @zachleat @AmeliaBR Coming soon:
    Math.sumPreciseUnsafe
    Math.sumUnsafe
    Math.sumImpreciseSafe
    1. Zach Leatherman 19 min later
      
      @kleinfreund @AmeliaBR please do not drink the HTML Sanitizer API
      1. Philipp Tootloff 22 min later
        
        @zachleat @AmeliaBR But that's what the bartender handed me after ordering an API?
        API? IPA? Ah, dang it
        
        Zach Leatherman 23 min later
        
        @kleinfreund @AmeliaBR HTML Sanitizer IPA would be a good name though 😅
2. Chris 7 hr later
  
  @AmeliaBR @zachleat Math.sumPreciserFinalv2.00000000001
  1. Amelia Bellamy-Royds 11 hr later
    
    @lamplightdev @zachleat (this broke me. literal laugh-crying here)
Andrew Douglass 23 min later

@zachleat I love that due to backwards compatibility we can’t simply fix the plus sign. Some library out there (probably MooTools) certainly used floating point math errors in a load bearing way that would cause the web to fall apart if changed.
1. Zach Leatherman 25 min later
  
  @ardouglass one counterexample is that the `parseInt()` second argument default was changed years and years ago 😅
Andrew Douglass 29 min later

@zachleat wait…
“This still cannot avoid the 0.1 + 0.2 precision problem”
1. Zach Leatherman 36 min later
  
  @ardouglass yeah, but to be fair that problem is somewhat common across programming languages
Christopher Kirk-Nielsen 39 min later

@zachleat Dang I thought the last scenario would be solved by this. 😬 Should have been called `sumLossless`, given that it is not actually precise for that typical `.1+.2` example. Ah well, what can you do!
1. Zach Leatherman 43 min later
  
  @chriskirknielsen still, an improvement! (but same)
  1. Christopher Kirk-Nielsen 1 hr later
    
    @zachleat Yeah we'll take it! I also wish there was a `Math.evaluate` for arithmetic in general, but that's a whole 'nother can o' worms…
Lou Cyx 1 hr later

@zachleat OMG this is great! I guess is time to retire my library then 😂
https://coven.to/math
1. Lou Cyx 1 hr later
  
  @zachleat never mind! Am I missing something here?
  1. Zach Leatherman 2 hr later
    
    @loucyx hm? I included that exact example in my post 😅 the MDN docs go into it more
    1. Lou Cyx 3 hr later
      
      @zachleat my bad! I read the top part of your post and go so excited and went straight to try it out, next time I'll read the whole thing. Sorry about that!
      1. Zach Leatherman 3 hr later
        
        @loucyx look, we’ve all been there!! 🙌
2. Zach Leatherman 2 hr later
  
  @loucyx We still need Precise Math utilities!
Frank // Mottokrosh 22 hr later

@zachleat Math.sumVibe() when?
1. Zach Leatherman 22 hr later
  
  @mottokrosh even immaculate vibes can be imprecise