Code Smell 127 — Mutable Constants

You declare something a constant. But you can mutate it.

TL;DR: Use immutable constants

Problems

• Mutability
• The Least Surprise Principle violation
• Coupling

Solutions

1. Enforce mutability
2. Avoid constants. They are hard to mock in tests.

Context

We learned to declare constants in our first course on computer programming.

As always, it is not important if something is constant.

It is important if it does not mutate.

Sample Code

Wrong

const DISCOUNT_PLATINUM = 0.1;
const DISCOUNT_GOLD = 0.05;
const DISCOUNT_SILVER = 0.02;
//Since variables are constants we cannot reassign them
const DISCOUNT_PLATINUM = 0.05; //Error
//We can group them
const ALL_CONSTANTS = {
  DISCOUNT: {
    PLATINUM = 0.1;
    GOLD = 0.04;
    SILVER = 0.02;  
  },
};
const ALL_CONSTANTS = 3.14; //Error
ALL_CONSTANTS.DISCOUNT.PLATINUM = 0.08; //NOT AN ERROR. WTF!

const ALL_CONSTANTS = Object.freeze({
  DISCOUNT: 
    PLATINUM = 0.1;
    GOLD = 0.05;
    SILVER = 0.02; 
});
const ALL_CONSTANTS = 3.14; //Error
ALL_CONSTANTS.DISCOUNT.PLATINUM = 0.12; //NOT AN ERROR. WTF!

Right

export const ALL_CONSTANTS = Object.freeze({
  DISCOUNT: Object.freeze({
    PLATINUM = 0.1;
    GOLD = 0.05;
    SILVER = 0.02;  
  }),
});
const ALL_CONSTANTS = 3.14; //Error
ALL_CONSTANTS.DISCOUNT.PLATINUM = 0.12; //ERROR
//Code works, but it is coupled and we cannot test it
Class TaxesProvider {
  applyPlatinum(product);
}
//Now we can couple to a interface (the protocol of taxes provider)
//Since class has no setters it is constant an immuatable
//And we can replace it on tests

Detection

[X] Semi-Automatic

We can perform mutation testing to find changed values.

Tags

• Constants

Conclusion

Mutability is very important.

We need to enforce it with the right tools.

Relations

Code Smell 86 — Mutable Const Arrays

Code Smell 107 — Variables Reuse

Code Smell 02 — Constants and Magic Numbers

More Info

• The Evil Power of Mutants

Credits

This smell was inspired by This

Photo by Sangharsh Lohakare on Unsplash

You start digging in the code. The more you dig, the more stuff you turn up. Eventually you dig yourself into a hole you can’t get out of. To avoid digging your own grave, refactoring must be done systematically.

Eric Gamma

Software Engineering Great Quotes

This article is part of the CodeSmell Series.

How to Find the Stinky parts of your Code