ES6 high order array methods explained

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Introduction

ECMAScript 6 introduces some new array methods that is useful to loop through an array items. In this post, I’ll try to explain how I understand some ES6 high order array methods like map(), filter(), reduce(), forEach(), etc. Hopefully you will understand more about these methods after reading this post. Let’s get started.

High Order Array Methods

Find

The first method is Array.prototype.find(). Just like the name suggest, it will find the item that you are looking for. It will return the item if the function returns true. Here’s an example.

const users = [
  {
    id: 1,
    name: "John Doe",
    admin: false
  },
  {
    id: 2,
    name: "Lukas Smith",
    admin: true
  },
  {
    id: 3,
    name: "Erina Matsumoto",
    admin: false
  }
]

// will return {id: 1, name: "John Doe", admin: false}
users.find(user => user.id === 1)

As you can see from the example, Array.prototype.find() returns the first item because we give it a condition to check if the item that is currently looping has the id of 1.

What if there are multiple matches? Well, .find() will only return the first match it found. So if you would type

user.find(user => !user.admin)

It will return the first match which is {id: 1, name: "John Doe", admin: false}. If you want it to spit an array with matching condition, then you would use .filter() which we will talk about later.

Array.prototype.find() will return undefined if the given condition is not fulfilled (the item that you’re looking for is not available). You can play around with this here.

Filter

Like I said earlier, if you want multiple results based off on your condition then you would use Array.prototype.filter(). Let’s try it with the code below.

const cities = [
  { name: "Hiroshima", country: "Japan" },
  { name: "Sendai", country: "Japan" },
  { name: "London", country: "England" },
  { name: "Brighton", country: "England" },
  { name: "Jakarta", country: "Indonesia" },
  { name: "Bandung", country: "Indonesia" }
]

// will return all cities in Japan
cities.filter(city => city.country === "Japan")

As you would expect, it returns an array with matching items. The code above will give us this result.

[
  { name: "Hiroshima", country: "Japan" },
  { name: "Sendai", country: "Japan" },
]

You can play around with the code snippet above here

Some

What if you want to check if an array have some condition that you want? Well, Array.prototype.some() got you covered. This method checks if an array have some condition that you want. Take a look at this example.

const items = [
  { name: "Phone", discount: true },
  { name: "Laptop", discount: false  },
  { name: "Keyboard", discount: false  },
  { name: "Mouse", discount: true  },
  { name: "Monitor", discount: false },
]

// will return true
items.some(item => item.discount)

As you can see, it returns true because there are some items that is currently in a discount. Go ahead and try to change them all to false on repl.it.

Every

OK great, now you can check whether or not an array has some items that match you requirements. There’s also a method that will return true if every one of them matches your condition. Check the code below.

const students = [
  { name: "Erika", passed: true },
  { name: "Himawari", passed: false  },
  { name: "Irene", passed: false  },
  { name: "Suzuka", passed: true  },
  { name: "Riku", passed: true },
]

// will return false
students.every(student => student.passed)

It returns false because we need every single one of the student passed. As always, you can play around with the code above here.

ForEach

Array.prototype.forEach() is useful if you want to just loop through an array and don’t want to return anything. For example, if you just want to console.log() every item of an array.

const songs = [
  "Road of Resistance",
  "Pretender",
  "Megitsune",
  "Feel It Still",
  "Arkadia"
]

// will not return anything
songs.forEach(song => console.log(song))

You can play around with this method here.

Map

Similar to .forEach(), Array.prototype.map() will also loop through each item in an array but it will return a new array. Here’s an example.

const numbers = [1, 2, 3, 4, 5]

// will return multiplied value
numbers.map(num => num * 2)

I use this method more often than .forEach() because most of the time I need to process that item and returns a new array with the result. You can play around with this here.

Reduce

Finally, Array.prototype.reduce. This method is quite hard for me to understand initially. It’s sort of like merges the array. The first argument is the function handler that takes 2 arguments and the second argument is the initial value. The function’s first argument called accumulator has the result of current iteration. The second argument called currentValue has the current item in an array. Let’s look at the code and hope it will make more sense.

const numbers = [1, 1, 1, 1, 1]

// will return 5
numbers.reduce((acc, curr) => acc + curr)

Let’s make an analogy of that to make things simpler. Suppose you have a jar. That jar is the accumulator. The first time you have it, it’s empty. Then you started to pick an item from a box. The box is an array and the item is the currentValue. You pick the item one by one, adding the total of the items in the jar (accumulator) with the item that you are picking (currentValue). Eventually, you will end up with 5 items. Same goes to the code above.

If you give it a second argument which is initialValue, then it will start from that value instead of 0. Just like if you have a jar filled with some items already.

const numbers = [1, 1, 1, 1, 1]

// will return 10
numbers.reduce((acc, curr) => (acc + curr), 5)

Reduce method is quite hard to understand at first, don’t worry. Reduce deserves an article in itself, check out this article to understand reduce() even more.

References

There are so many references for these methods out there already. MDN is one of the best reference out there if you want something more technical and detail. Also checkout this tweet (and follow him too! He shared a lot of useful stuff in a form of simple yet concise sketch).

Conclusion

These methods that ES6 introduces is so useful. We no longer need to create a for loop that iterates through the whole array and then do something. Let me know what you think about this. See ya next time ツ

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