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Cover image for Code Smell 71 - Magic Floats Disguised as Decimals

Code Smell 71 - Magic Floats Disguised as Decimals

Maxi Contieri
Learn something new every day. - I am a senior software engineer working in industry, teaching and writing on software design, SOLID principles, DDD and TDD.
・2 min read

TL;DR Don't trust numbers on immature languages like JavaScript.

Problems

Solutions

  1. Choose Mature Languages.

  2. Represent Decimals with Decimals.

Sample Code

Wrong

console.log(0.2 + 0.1) 
// 0.30000000000000004

//We are adding two decimal numbers
// 2/10  +  1/10 
// Result should be 3/10 as we learnt at school
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Right

class Decimal {
  constructor(numerator) {
    this.numerator = numerator;    
  }
   plus(anotherDecimal) {
      return new Decimal(this.numerator + anotherDecimal.numerator);
  }
   toString() {
      return "0." + this.numerator;
   }}     

console.log((new Decimal(2).plus(new Decimal(1))).toString());
// 0.3

//We can represent the numbers with a Decimal class (storing only the numerator)
//or with a generic Fraction class (storing both the numerator and denominator)
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Detection

Since this is a language feature, it is difficult to detect. We can ask our linters to prevent us from manipulating numbers this way.

Tags

  • JavaScript

  • Premature Optimization

Conclusion

My first programming language was Commodore 64's basic back in 1985.

I was very surprised to discover that 1+1+1 was not always 3. Then they introduced integer types.

JavaScript is 30 years younger, and it has the same immaturity problems.

Relations

More info

Here is the technical (and accidental) explanation.

Please, don't argue telling this is fine and expected since this is the binary representation.

These numbers are decimal, we should represent them as decimals.

If you think representing them as floats is a great performance improvement, you are wrong.

Premature optimization is the root of all evil.

Credits

Photo by Stephen Radford on Unsplash


The purpose of computing is insight, not numbers.

Richard Hamming



This article is part of the CodeSmell Series.

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