# Code Smell 02 - Constants and Magic Numbers

> TL;DR: Avoid Magic numbers without explanation. We don't know their source and are afraid to change them.

# Problems

- [Coupling](https://maximilianocontieri.com/coupling-the-one-and-only-software-design-problem)
- Low testability
- Low readability

# Solutions
1) Rename the constant with a semantic and name (meaningful and intention revealing).

2) Replace constants with parameters, so you can mock them from the outside.

3) The constant definition is often a different object than the constant (ab)user.

# Examples
- Algorithms Hyper Parameters

# Sample Code

## Wrong

[Gist Url]: # (https://gist.github.com/mcsee/dec9856bf69a06c367d2e683b179577a)
```ruby
def energy(mass)
    mass * 299792458 ** 2
end
```

## Right

[Gist Url]: # (https://gist.github.com/mcsee/2e4c88a516078500ce833dbfbd3d9b0e)
```ruby
# Storing magnitudes without units is another smell
class PhysicsConstants
   LIGHT_SPEED = 299792458.freeze
end

def energy(mass)
    mass * PhysicsConstants::LIGHT_SPEED ** 2
end
```

# Detection

Many linters can detect number literals in attributes and methods.

# Tags
- Hard coded
- Constants

# More info
- [Refactoring Guru](https://refactoring.guru/es/replace-magic-number-with-symbolic-constant)
- [How to Decouple a Legacy System](https://maximilianocontieri.com/how-to-decouple-a-legacy-system)

# Credits

Photo by [Kristopher Roller](https://unsplash.com/@krisroller) on [Unsplash](https://unsplash.com/s/photos/magic)

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> In a purely functional program, the value of a [constant] never changes, and yet, it changes all the time! A paradox!

_Joel Spolsky_

%[https://maximilianocontieri.com/software-engineering-great-quotes]

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This article is part of the CodeSmell Series.

%[https://maximilianocontieri.com/how-to-find-the-stinky-parts-of-your-code]