Const declares something to be constant. Can it mutate?

TL;DR: Don't rely on languages cheating about directives.

Problems

• Unexpected side effects

• Accidental complexity

Solutions

1. Use better languages.

2. Use spread operator.

Sample Code

Wrong

const array = [1, 2];

array.push(3)

//array => [1, 2, 3]
//Wasn't it constant ?
//constant != immutable ?

Right

const array = [1, 2];

const newArray = [...array,3 ]

//array => [1, 2] Didn't mutate
//newArray = [1, 2, 3]

Detection

Since this is a "language feature", we can explicitly forbid it.

Tags

• Mutability

• JavaScript

Conclusion

We should always favour immutability on our designs and take extra care with side effects.

More Info

• The evil power of mutants

Credits

Photo by Zorik D on Unsplash

Thank you, Oliver Jumpertz for this tip.

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Correctness is clearly the prime quality. If a system does not do what it is supposed to do, then everything else about it matters little.

Bertrand Meyer

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This article is part of the CodeSmell Series.