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JS Bits - Destructuring arrays in JavaScript

Sedat Can Yalçın — Thu, 13 Aug 2020 00:01:02 +0000

Destructuring

Destructuring is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables. (https://dev.to/sarah_chima/destructuring-assignment---arrays-16f)

Rather than getting a whole data, with destructuring we can only retrieve the values we want.

Destructuring arrays

We have a function groceries which returns us our list of items that we wish to buy next time we go to a supermarket.

The data that groceries function returns is as follows; [bread, apples, cheese]

In a traditional sense we would get a reference to each item in this way;

const groceriesList = groceries();
const bread = groceriesList[0];
const apples = groceriesList[1];
const cheese = groceriesList[2];

Destructuring allows us to achieve this in an elegant and simple way

const [
    bread,
    apples,
    cheese
] = groceries()

If you also want a reference to the groceries list all you need to do is;

const [
    bread,
    apples,
    cheese
] = groceriesList = groceries()

But, what happens if groceries returns us an array with four values?

Simply, we would only get the first three values without touching the fourth value.

What happens when we want to retrieve three values but groceries function returns two values?

Let's say the array doesn't have cheese value and we want to reference to this value with the cheese variable.

The code would not break and the cheese variable will simply be undefined

Undefined values

Another case is when a value is undefined.

Imperatively if a value can be undefined and we want a default value to it when it's undefined.

We would do;

const name = names[0] !== undefined ? names[0] : 'unregistered'

Declaratively, using destructuring we do;

const [
name = 'unregistered'
] = names

What if we want to get the first three into their own variables and the rest into one variable?

In order to achieve this we use the spread operator.

const [
    bread,
    apples,
    cheese
    // get the first three as normal then
    ...others // gather the rest of them and put them into variable others
] = groceries()

Variable swapping with destructuring

Destructuring allows a handy trick for swapping two variables without the need for a temp variable.

[x,y] = [y,x]

Destructuring function paramaters

You can destructure an array which is a parameter to a function

const arr = [1,2,3,4,5]
const firstThree = ([one = 10, two, three]) => {
    // do something
}

The parameter 10 has a default value of 10 if it receives undefined

Destructuring nested arrays

We can destructure nested array using another pair of brackets inside our brackets

const data = [1,[2,3],4]
const [
    one,
    [two, three] = [], // fallback to an empty array if undefined
    four

]

Thank you for reading!
If you have any questions please let me know!

JS Bits - Array Methods

Sedat Can Yalçın — Sun, 19 Jul 2020 18:28:36 +0000

Welcome to JS Bits

Hello everyone, welcome to the first post on my new series JS Bits where I explain and also show you use cases where you can use them.

This post was originally posted on my blog.

Find it here

Array Methods

Array.prototype.find(callback)

Takes a callback function that returns the value of the first element that satisfies the condition.

const arr = [5, 12, 8, 130, 44, 130];
const found = arr.find(element => element > 50);
// found is equal to 130. It found the value 130 in the third index and returned it.

Array.prototype.findIndex(callback)

Similar to find method, returns the index of the first value that satisfies the condition

const arr = [1,2,3,4,5];

const isLargeNumber = arr.findIndex((element) => element > 3); // return the index of value 4 which is 3

Array.prototype.includes(valueToFind[, fromIndex])

Returns true or false whether the array includes the given value.

const arr = [3,2,5,6]
console.log(arr.includes(5)) // returns true

As an optional argument includes takes a parameter fromIndex which means where to begin the searching for the valueToFind

const arr = [3,2,5,6]
console.log(arr.includes(5, 2)) // starts searching the value 5 in arr beginning from the second index which returns true.

Array.prototype.flat([depth])

Creates a new flattend array with all the sub-array (nested parts) values taken out and concentinated in to the higher depth.

You will understand it better with this example;

// we have a nested array like this
const arr = [1,2,[3,4], 5, 6, [[7]] 
// this array has [3,4] which goes one level deeper and empty array of [[]] which goes two levels deeper
const newArr = arr.flat() // default value is one, returns the array: [1,2,3,4,5,6,[7]]
const otherARr = arr.flat(2) // goes to the depth of two and returns: [1,2,3,4,5,6,7]

Array.prototype.flatMap(callback(currentValue[, index[, array]])

It is very common now to use functional programming or FP methods in JavaScript like map().

If you want to use flat() on an array you are mapping. You have to first map(), which creates a new array and then call the flat() method.

flatMap() combines a map and a flat with a depth of one by first mapping each element and then running flat on the newly created array.

const arr = [1, 2, 3, 4];
const doubleArr = arr.flatMap(x => [x * 2]);
// [2, 4, 6, 8]

A very good use case of flatMap() is adding or removing items during a map().

const returnDoubles = [1,2,3,4,5,6].flatMap((v) => {
    if(v % 2 === 0) {
        return [v];
    } else {
        return [];
    }
})
// [2,4,6]

Keep in mind that if you want to run flatMap() with a depth different than one(1). You have to call flat()
additionally with a map().

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