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array-filter.md

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layout lesson
title Filter
permalink /filter/

Imagine trying to filter out numbers from a list, less than 18

let myListOfAges = [22, 12, 43, 44, 22, 54, 16, 87, 12];

let myListOfOldEnoughAges = [];

for(let i = 0; i < myListOfAges.length; i++) {
  if(myListOfAges[i] >= 18) {
    myListOfOldEnoughAges.push(myListOfAges[i]);
  }
}

Challenges

  • Takes multiple lines to handle even the simplest filters
  • Can be hard to quickly understand what is happening
  • Complexity increases significantly with more complex conditions

What is Array.filter?

  • A functional programming tool found natively in javascript
  • Creates a new array based off the data in a base array
  • Does not modify the original array
  • Only the items from the original array that pass a test are copied into a new array

How does it work?

  • With the same example as above
let myListOfAges = [22, 12, 43, 44, 22, 54, 16, 87, 12];

// dont forget that 'eachAge >= 18' resolves to true or false,
// and that with arrow functions, the value on the right hand of the arrow is automatically returned
let myListOfOldEnoughAges = myListOfAges.filter(eachAge => eachAge >= 18);
  • filter exists as a method on all arrays, it takes one uncalled function as an argument
  • This uncalled function is called for each item in the array, as filter iterates over each item
  • This function is provided the value currently being iterated over as the first parameter, in the above case, that means that eachAge represents 22 the first time it is called, then 12, and then 43 and so on.
  • This function expects either a true or a false to be returned inside of it.
  • In the above example, the first time eachAge is equal to 22, and 22 >= 18 resolves to true. That true is returned, and that means that 22 is kept in the next array

Common Mistakes

  • Filter does not modify the original array, it creates and returns a brand new one (which generally, is a very good thing)
const allFriends = [{name: 'Sumit', isSuperFriend: true}, {name: 'Joanne', isSuperFriend: false}, {name: 'Purvi', isSuperFriend: false}];
const superFriends = allFriends.filter(friend => friend.isSuperFriend);

allFriends does not get modified, so later references to all of your friends are still available

  • You should not create a new array and push into it in your filter 'loop'
// DO NOT DO THIS
const allFriends = [{name: 'Sumit', isSuperFriend: true}, {name: 'Joanne', isSuperFriend: false}, {name: 'Purvi', isSuperFriend: false}];
const superFriends = [];

allFriends.filter(friend => {
  if(friend.isSuperFriend){
    superFriends.push(friend);
  }
})

In the above, the filter is not being correctly used, it is being used as an iterator, no different than forEach or a for loop. Instead we should be ensuring that the functions being used inside of our filter are pure.

Useful Tips

  • Chaining together multiple filters, combined with other array methods, can make elaborate requirements seem trivial
// Out of a list of all my friends, get the first superFriend's with dog(s) as pets
const allFriends = [
  {name: 'Sumit', isSuperFriend: true, pets: ['Cat', 'Bird']}, 
  {name: 'Joanne', isSuperFriend: false, pets: []}, 
  {name: 'Purvi', isSuperFriend: true, pets: ['Dog']},
  {name: 'Pete', isSuperFriend: true, pets: ['Monkey', 'Dog']},
  {name: 'Manal', isSuperFriend: false, pets: ['Dog']},
];

const dogOwningSuperFriends = 
  allFriends.filter(friend => friend.isSuperFriend) // resolves to just super friends
          .filter(friend => friend.pets.includes('Dog')); // takes the list of super friends and filters it further

The above does not include the friend Manal, because she is not a super friend, even though she has a dog

  • Filtering by the position in an array can be a very useful thing
const peopleWaitingInLine = ['Della', 'Lisamarie', 'Melanie', 'Ahmed', 'Lucy', 'Purvi'];

// the first three people in line get a prize!!!!
const firstThreeInLine = peopleWaitingInLine.filter((person, index) => index < 3);
  • The second parameter passed into the filter callback function is the index of the item currently being iterated over, you can use this in your filters!