## DEV Community

Christian Falucho

Posted on • Updated on

# Day 7 of #100DaysOfCode!

### Today's progress

Today I learned about functional programming.

### What I learned

Functional programming is a style of programming in which solutions are simple. A function takes an input, processes it and returns an output.

Three important factors of functional programming:

• Isolated function - does not depend on outside variables (global variables)
• Pure functions - same input always give the same output
• Function has limited side effects - this means any changes or mutations is controlled.

This allows for greater control, less mutation or changing of variables and objects.

For instance, let's say we have a global variable called `animals` and it stores an array of different kinds of animals.

``````let animals = ['lion', 'eagle', 'cheetah', 'bear', 'giraffe']
``````

Now, let's create two functions. One function adds a new animal and the other function removes an animal.

``````// adds an animal to arr
// make a copy of the array of animals
let newArr = [...arr]

// push new animal to new arr
newArr.push(animalName);

// return the new array
return newArr;
}
// removes an animal from arr
function remove(arr, animalName){
//make a copy of the array of animals
let newArr = [...arr]

// grab the index of the animal name
// store into variable
animal_index = newArr.indexOf(animalName)

// if the animal's index exist
// remove it, use splice
if(animal_index >= 0){
newArr.splice(0, 1)
}

// return the new array
return newArr;
}
``````

You'll notice in the above code that we created two functions and in both functions take two parameters. The first parameter is the array and the second parameter takes in a string animal.

Inside the functions we added a line of code `let newArr = [...arr]` that makes a copy of the global array by passing it through our function argument `function add(arr, animalName)` and `function remove(arr, animalName)`

``````let newArr = [...arr]
``````

Now, when I test this using the functions above. I will get one function that adds a new animal and the other function that removes an animal from the array all without mutating or changing the original array.

``````let addAnimal = add(animal, 'monkey')

//output: ['lion', 'eagle', 'cheetah', 'bear', 'giraffe', 'monkey']
``````

function remove(arr, animalName)

``````let removedAnimal = remove(animal, 'lion')

console.log(removedAnimal)
//output: ['eagle', 'cheetah', 'bear', 'giraffe']
``````

When I run a `console.log` on the global variable animals from earlier. The global variable did not change at all.

``````let animals = ['lion', 'eagle', 'cheetah', 'bear', 'giraffe']

console.log(animals)
//output: ['lion', 'eagle', 'cheetah', 'bear', 'giraffe']
``````

This is the benefit of using functional programming as a way to copy the global variable within the scope of the function. From there you can mutate or change it without affecting the global variable and in addition return a new copy of the variable or object.