# Map, Filter, Reduce vs For Loops (syntax)

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The intention of this post is not to say one paradigm is better than the other. It is just to show common imperative patterns you run into and their functional equivalents.

Sometimes things are easier to learn if you can relate it to something you are already familiar with and be given a map on how to transition back and forth.

Previously, I had done something similar at Functional vs Imperative Patterns in JavaScript

# Map

`map` takes a list and runs a function on each item in the list, returning the same length list.

Imperative

``````const double = x => x * 2
const input = [ 1, 2, 3 ]
const output = []
for (let i = 0; i < input.length; i++) {
output.push(double(input[i]))
}

output //=> [ 2, 4, 6 ]
``````

Functional

``````const double = x => x * 2
const input = [ 1, 2, 3 ]
const output = input.map(double)

output //=> [ 2, 4, 6 ]
``````

# Filter

`filter` takes a list and returns a list containing all items that match the predicate. In this example `isEven` is the predicate.

Imperative

``````const isEven = x => x % 2 === 0
const input = [ 1, 2, 3, 4, 5 ]
const output = []
for (let i = 0; i < input.length; i++) {
if (isEven(input[i])) {
output.push(input[i])
}
}

output //=> [ 2, 4, 6 ]
``````

Functional

``````const isEven = x => x % 2 === 0
const input = [ 1, 2, 3, 4, 5 ]
const output = input.filter(isEven)

output //=> [ 2, 4, 6 ]
``````

# Reduce

`reduce` takes a list and returns any data structure. It could be another list or an object.

## Simple

Imperative

``````const add = (x, y) => x + y
const input = [ 1, 2, 3 ]
const initial = 0
let output = initial
for (let i = 0; i < input.length; i++) {
}

output //=> 6
``````

Functional

``````const add = (x, y) => x + y
const input = [ 1, 2, 3 ]
const initial = 0

output //=> 6
``````

## Complex

Imperative

``````const isEven = x => x % 2 === 0
const double = x => x * 2
const input = [ 1, 2, 3, 4, 5 ]
const initial = []

let output = initial
for (let i = 0; i < input.length; i++) {
if (isEven(input[i])) {
output.push(double(input[i]))
}
}

output //=> [ 4, 8 ]
``````

Functional

It could be written like (below) but know that it will iterate over the Array twice.

``````const isEven = x => x % 2 === 0
const double = x => x * 2
const input = [ 1, 2, 3, 4, 5 ]
const initial = []

let output =
input
.filter(isEven)
.map(double)

output //=> [ 4, 8 ]
``````

Alternatively, you can create a reducer that can both `filter` and `map` in a single iteration.

``````const isEven = x => x % 2 === 0
const double = x => x * 2
const input = [ 1, 2, 3, 4, 5 ]
const initial = []

const reducer = (filter, map) => (acc, x) => {
if (filter(x)) {
acc.push(map(x))
}
return acc
}

const output = input.reduce(reducer(isEven, double), initial)

output //=> [ 4, 8 ]
``````

Source: dev

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