JavaScriptで配列を操作する方法

プログラミング言語の重要な部分。ほとんどの場合、配列に対していくつかの操作を行う必要があるため、この記事を参照してください。

この記事では、JavaScriptで配列を操作するさまざまな方法を紹介します[^^]

JavaScriptの配列とは何ですか?

先に進む前に、配列の実際の意味を理解する必要があります。

JavaScriptでは、配列はさまざまなデータ型を格納するために使用される変数です。基本的に、さまざまな要素を1つのボックスに格納し、後で変数を使用して評価できます。

配列の宣言:

let myBox = []; // Initial Array declaration in JS

配列には複数のデータ型を含めることができます

let myBox = ['hello', 1, 2, 3, true, 'hi'];

配列は、メソッドと呼ばれるいくつかのアクションを使用して操作できますこれらのメソッドのいくつかは、配列に追加、削除、変更、さらに多くのことを行うことを可能にします。

この記事でいくつかお見せします、ロールしましょう:)

注意:この投稿では矢印関数を使用しました。これが何を意味するのかわからない場合は、ここをお読みください。矢印機能はES6の機能です

toString()

JavaScriptメソッドはtoString()、配列をコンマで区切られた文字列に変換します。

let colors = ['green', 'yellow', 'blue']; console.log(colors.toString()); // green,yellow,blue

join()

JavaScriptjoin()メソッドは、すべての配列要素を1つの文字列に結合します。

これはtoString()メソッドに似ていますが、ここではデフォルトのコンマの代わりに区切り文字を指定できます。

let colors = ['green', 'yellow', 'blue']; console.log(colors.join('-')); // green-yellow-blue

concat

このメソッドは、2つの配列を組み合わせるか、配列に項目を追加してから、新しい配列を返します。

let firstNumbers = [1, 2, 3]; let secondNumbers = [4, 5, 6]; let merged = firstNumbers.concat(secondNumbers); console.log(merged); // [1, 2, 3, 4, 5, 6]

押す()

このメソッドは、配列の最後に項目を追加し、元の配列を変更します。

let browsers = ['chrome', 'firefox', 'edge']; browsers.push('safari', 'opera mini'); console.log(browsers); // ["chrome", "firefox", "edge", "safari", "opera mini"]

ポップ()

このメソッドは、配列の最後の項目を削除して返します

let browsers = ['chrome', 'firefox', 'edge']; browsers.pop(); // "edge" console.log(browsers); // ["chrome", "firefox"]

シフト()

このメソッドは、配列の最初の項目を削除して返します

let browsers = ['chrome', 'firefox', 'edge']; browsers.shift(); // "chrome" console.log(browsers); // ["firefox", "edge"]

unshift()

このメソッドは、配列の先頭にアイテムを追加し、元の配列を変更します。

let browsers = ['chrome', 'firefox', 'edge']; browsers.unshift('safari'); console.log(browsers); // ["safari", "chrome", "firefox", "edge"]
一度に複数のアイテムを追加することもできます

スプライス()

このメソッド、要素を追加、削除、挿入することにより、配列を変更します。

構文は次のとおりです。

array.splice(index[, deleteCount, element1, ..., elementN])
  • Indexこれが配列内の要素を削除するための開始点です
  • deleteCountそのインデックスから削除される要素の数です
  • element1, …, elementN追加する要素です

アイテムの削除

splice()を実行した後、アイテムが削除された配列を返し、元の配列から削除します。
let colors = ['green', 'yellow', 'blue', 'purple']; colors.splice(0, 3); console.log(colors); // ["purple"] // deletes ["green", "yellow", "blue"]
注意:deleteCountには、範囲内の最後のインデックスは含まれません。

If the second parameter is not declared, every element starting from the given index will be removed from the array:

let colors = ['green', 'yellow', 'blue', 'purple']; colors.splice(3); console.log(colors); // ["green", "yellow", "blue"] // deletes ['purple']

In the next example we will remove 3 elements from the array and replace them with more items:

let schedule = ['I', 'have', 'a', 'meeting', 'tommorrow']; // removes 4 first elements and replace them with another schedule.splice(0, 4, 'we', 'are', 'going', 'to', 'swim'); console.log(schedule); // ["we", "are", "going", "to", "swim", "tommorrow"]

Adding items

To add items, we need to set the deleteCount to zero

let schedule = ['I', 'have', 'a', 'meeting', 'with']; // adds 3 new elements to the array schedule.splice(5, 0, 'some', 'clients', 'tommorrow'); console.log(schedule); // ["I", "have", "a", "meeting", "with", "some", "clients", "tommorrow"]

slice()

This method is similar to splice() but very different. It returns subarrays instead of substrings.

This method copies a given part of an array and returns that copied part as a new array. It does not change the original array.

The syntax is:

array.slice(start, end)

Here’s a basic example:

let numbers = [1, 2, 3, 4] numbers.slice(0, 3) // returns [1, 2, 3] console.log(numbers) // returns the original array

The best way to use slice() is to assign it to a new variable.

let message = 'congratulations' const abbrv = message.slice(0, 7) + 's!'; console.log(abbrv) // returns "congrats!"

split()

This method is used for strings. It divides a string into substrings and returns them as an array.

Here’s the syntax:string.split(separator, limit);

  • The separator here defines how to split a string either by a comma.
  • The limit determines the number of splits to be carried out
let firstName = 'Bolaji'; // return the string as an array firstName.split() // ["Bolaji"]

another example:

let firstName = 'hello, my name is bolaji, I am a dev.'; firstName.split(',', 2); // ["hello", " my name is bolaji"]
NB: If we declare an empty array, like this  firstName.split(''); then each item in the string will be divided as substrings:
let firstName = 'Bolaji'; firstName.split('') // ["B", "o", "l", "a", "j", "i"]

indexOf()

This method looks for an item in an array and returns the index where it was found else it returns -1

let fruits = ['apple', 'orange', false, 3] fruits.indexOf('orange'); // returns 1 fruits.indexOf(3); // returns 3 friuts.indexOf(null); // returns -1 (not found)

lastIndexOf()

This method works the same way indexOf() does except that it works from right to left. It returns the last index where the item was found

let fruits = ['apple', 'orange', false, 3, 'apple'] fruits.lastIndexOf('apple'); // returns 4

filter()

This method creates a new array if the items of an array pass a certain condition.

The syntax is:

let results = array.filter(function(item, index, array) { // returns true if the item passes the filter });

Example:

Checks users from Nigeria

const countryCode = ['+234', '+144', '+233', '+234']; const nigerian = countryCode.filter( code => code === '+234'); console.log(nigerian); // ["+234", "+234"]

map()

This method creates a new array by manipulating the values in an array.

Example:

Displays usernames on a page. (Basic friend list display)

const userNames = ['tina', 'danny', 'mark', 'bolaji']; const display = userNames.map(item => { return '
  • ' + item + '
  • '; }) const render = '
      ' + display.join('') + '
    '; document.write(render);

    another example:

    // adds dollar sign to numbers const numbers = [10, 3, 4, 6]; const dollars = numbers.map( number => '$' + number); console.log(dollars); // ['$10', '$3', '$4', '$6'];

    reduce()

    This method is good for calculating totals.

    reduce() is used to calculate a single value based on an array.

    let value = array.reduce(function(previousValue, item, index, array) { // ... }, initial);

    example:

    To loop through an array and sum all numbers in the array up, we can use the for of loop.
    const numbers = [100, 300, 500, 70]; let sum = 0; for (let n of numbers) { sum += n; } console.log(sum);

    Here’s how to do same with reduce()

    const numbers = [100, 300, 500, 70]; const sum = numbers.reduce((accummulator, value) => accummulator + value , 0); console.log(sum); // 970
    If you omit the initial value, the total will by default start from the first item in the array.
    const numbers = [100, 300, 500, 70]; const sum = numbers.reduce((accummulator, value) => accummulator + value); console.log(sum); // still returns 970

    The snippet below shows how the reduce() method works with all four arguments.

    source: MDN Docs

    More insights into the reduce() method and various ways of using it can be found here and here.

    forEach()

    This method is good for iterating through an array.

    It applies a function on all items in an array

    const colors = ['green', 'yellow', 'blue']; colors.forEach((item, index) => console.log(index, item)); // returns the index and the every item in the array // 0 "green" // 1 "yellow" // 2 "blue"

    iteration can be done without passing the index argument

    const colors = ['green', 'yellow', 'blue']; colors.forEach((item) => console.log(item)); // returns every item in the array // "green" // "yellow" // "blue"

    every()

    This method checks if all items in an array pass the specified condition and returntrue if passed, else false.

    check if all numbers are positive
    const numbers = [1, -1, 2, 3]; let allPositive = numbers.every((value) => { return value >= 0; }) console.log(allPositive); // would return false

    some()

    This method checks if an item (one or more) in an array pass the specified condition and return true if passed, else false.

    checks if at least one number is positive
    const numbers = [1, -1, 2, 3]; let atLeastOnePositive = numbers.some((value) => { return value >= 0; }) console.log(atLeastOnePositive); // would return true

    includes()

    This method checks if an array contains a certain item. It is similar to .some(), but instead of looking for a specific condition to pass, it checks if the array contains a specific item.

    let users = ['paddy', 'zaddy', 'faddy', 'baddy']; users.includes('baddy'); // returns true

    If the item is not found, it returns false

    There are more array methods, this is just a few of them. Also, there are tons of other actions that can be performed on arrays, try checking MDN docs herefor deeper insights.

    Summary

    • toString() converts an array to a string separated by a comma.
    • join() combines all array elements into a string.
    • concat combines two arrays together or add more items to an array and then return a new array.
    • push() adds item(s) to the end of an array and changes the original array.
    • pop() removes the last item of an array and returns it
    • shift() removes the first item of an array and returns it
    • unshift() adds an item(s) to the beginning of an array and changes the original array.
    • splice() changes an array, by adding, removing and inserting elements.
    • slice() copiesa given part of an array and returns that copied part as a new array. It does not change the original array.
    • split() divides a string into substrings and returns them as an array.
    • indexOf() looks for an item in an array and returns the index where it was found else it returns -1
    • lastIndexOf() looks for an item from right to left and returns the last index where the item was found.
    • filter() creates a new array if the items of an array pass a certain condition.
    • map() creates a new array by manipulating the values in an array.
    • reduce() calculates a single value based on an array.
    • forEach() iterates through an array, it applies a function on all items in an array
    • every() checks if all items in an array pass the specified condition and return true if passed, else false.
    • some() checks if an item (one or more) in an array pass the specified condition and return true if passed, else false.
    • includes() checks if an array contains a certain item.

    Let’s wrap it here; Arrays are powerful and using methods to manipulate them creates the Algorithms real-world applications use.

    Let's do a create a small function, one that converts a post title into a urlSlug.

    URL slug is the exact address of a specific page or post on your site.

    When you write an article on Freecodecamp Newsor any other writing platform, your post title is automatically converted to a slug with white spaces removed, characters turned to lowercase and each word in the title separated by a hyphen.

    Here’s a basic function that does that using some of the methods we learnt just now.

    const url = '//bolajiayodeji.com/' const urlSlug = (postTitle) => { let postUrl = postTitle.toLowerCase().split(' '); let postSlug = `${url}` + postUrl.join('-'); return postSlug; } let postTitle = 'Introduction to Chrome Lighthouse' console.log(urlSlug(postTitle)); // //bolajiayodeji.com/introduction-to-chrome-lighthouse

    in postUrl, we convert the string to lowercase then we use the split()method to convert the string into substrings and returns it in an array

    ["introduction", "to", "chrome", "lighthouse"]

    in post slug we join the returned array with a hyphen and then concatenate it to the category string and main url.

    let postSlug = `${url}` + postUrl.join('-'); postUrl.join('-') // introduction-to-chrome-lighthouse

    That’s it, pretty simple, right? :)

    If you’re just getting started with JavaScript, you should check this repository here, I’m compiling a list of basic JavaScript snippets ranging from

    • Arrays
    • Control flow
    • Functions
    • Objects
    • Operators

    Don’t forget to Star and share! :)

    PS: This article was first published on my blog here