TL;DR Don't trust numbers on immature languages like JavaScript.
Problems
Principle of Least Surprise Violation
Wrong decimal representations.
Solutions
Choose Mature Languages.
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
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)
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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