You declare something a constant. But you can mutate it.
TL;DR: Use inmutable constants
Problems
The Least Surprise Principle violation
Coupling
Solutions
Enforce mutability
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
More Info
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
This article is part of the CodeSmell Series.
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