# Code Smell 116 - Variables Declared With 'var'

> TL;DR: Choose wisely your variable names, scope, and mutability.

# Problems

- [Mutability](https://maximilianocontieri.com/the-evil-powers-of-mutants)

- Readability

# Solutions

1. Declare const all variables unless you need to change them

# Context

Most languages don't need variable declarations.

Some other languages allow us to state mutability. 

We should be strict and explicit with our declarations.

# Sample Code

## Wrong

[Gist Url]: # (https://gist.github.com/mcsee/f310bf19719788bfe2dbca3dab16a2c3)
```javascript
var pi = 3.14
var universeAgeInYears = 13.800.000.000

pi = 3.1415 // no error
universeAgeInYears = 13.800.000.001 // no error
```

## Right

[Gist Url]: # (https://gist.github.com/mcsee/c0c6bf0726100b918e5aa04955519465)
```javascript
const pi = 3.14 //Value cannot mutate or change 
let universeAgeInYears = 13.800.000.000 //Value can change

pi = 3.1415 // error. cannot define
universeAgeInYears = 13.800.000.001 // no error
```

# Detection
 
[X] Manual

With [mutation testing](https://en.wikipedia.org/wiki/Mutation_testing) by forcing a 'const' declaration, we can check if a value remains constant and be more declarative by explicitly enforcing it.

# Tags

- Mutability

- Javascript

# Conclusion

Readability is always very important. 

We need to explicitly state our intentions and usages.

# Relations

%[https://maximilianocontieri.com/code-smell-86-mutable-const-arrays]

# More Info

- [Differences between let, const, and var](https://twitter.com/SajalShlan/status/1495010893683314689)

- [The Evil Power of Mutants](https://maximilianocontieri.com/the-evil-powers-of-mutants) 
  
* * *

> Just as it is a good practice to make all fields private unless they need greater visibility, it is a good practice to make all fields final unless they need to be mutable.

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

* * *

This article is part of the CodeSmell Series.

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