Filter: 3 things go in, only the things you want come out

The filter function is another array comprehension like map used when a user is filtering search results they want to see, and in parsing operations. You give it an Array and a predicate function (a function that only returns true or false). The filter function will give every item to the predicate, and whenever that function returns true, it'll put that item in the new Array, else if it's false, it won't include it.

Old Sk00l for

Let's filter using the traditional for loop. We have our list of people and doggo's, and we're only wanting to keep the humans.

const group = [
    {
        name: 'Jesse',
        type: 'human'
    },
    {
        name: 'Brandy',
        type: 'human'
    },
    {
        name: 'Albus',
        type: 'dog'
    }
]

const len = group.length
const filtered = []
for(i = 0; i < len; i++) {
    const current = group[i]
    if(current.type === 'human') {
        filtered.push(current)
    }
}

Logging it out, we can see our new filtered Array is:

[ { name: 'Jesse', type: 'human' },
  { name: 'Brandy', type: 'human' } ]

Filter

We can create a somewhat pure function using filter:

import { filter } from 'lodash'

const typeIsHuman = item =>
    item.type === 'human'

const filterHumans = group =>
    filter(
        group, 
        typeIsHuman
    )

If we pass filterHumans the group, it'll give us the same filtered, new Array back like the above for loop does:

console.log(filterHumans(group))
[ { name: 'Jesse', type: 'human' },
  { name: 'Brandy', type: 'human' } ]

Conclusions

Filter: Take an Array of things in, give each thing to a function, if the function returns true when you give it an item, store it in an new Array to give back, else discard it. The filter function expects a pure predicate function, and is itself a pure function.

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