JavaScript 配列のマスター: テクニック、ベスト プラクティス、および高度な使用法

配列は、複数の値を保持できる特別な変数です。

const name = ["Sakib", "Arif", "Fatema"];

配列を使用する理由

配列は、JavaScript (および他の多くのプログラミング言語) の基本的なデータ構造です。その理由は次のとおりです。

1. データの整理: 配列を使用すると、複数の値を 1 つの変数に格納できるため、データのコレクションの管理が容易になります。たとえば、ユーザー名のリストを追跡する必要がある場合は、それらをすべて配列に保存できます。

2. インデックス付きアクセス: 配列は、インデックスを使用して要素にアクセスする便利な方法を提供します。これにより、位置に基づいて特定のアイテムを簡単に取得または変更できるようになります。

3. 効率的な反復処理: JavaScript には、for、forEach、map、filter、reduce など、配列を反復処理するためのさまざまなメソッドが用意されています。これらのメソッドを使用すると、最小限のコードで配列の各要素を処理できます。

4. 動的サイズ変更: JavaScript の配列は動的です。つまり、必要に応じてサイズを拡大または縮小できます。事前にサイズを指定する必要がないため、データの処理方法が柔軟になります。

5. 組み込みメソッド: JavaScript 配列には、データの操作とクエリを実行するための豊富な組み込みメソッドのセットが付属しています。プッシュ、ポップ、シフト、シフト解除、スプライス、スライスなどのメソッドにより、一般的なタスクが簡素化されます。

6. 多彩なデータ処理: 配列は、数値、文字列、オブジェクト、さらにはその他の配列を含む、さまざまなタイプのデータを保持できます。この多用途性により、アレイは幅広いアプリケーションに適しています。

7. パフォーマンスの強化: 配列は JavaScript エンジンのパフォーマンスに合わせて最適化されており、逐次的なデータ処理と操作を伴うタスクで効率的になります。

8. 高階関数のサポート: JavaScript 配列は高階関数とシームレスに動作するように設計されており、より表現力豊かで簡潔なコードを実現できます。マップ、フィルター、リデュースなどの関数により、強力なデータ変換と集計が可能になります。

要約すると、配列は、JavaScript でデータのコレクションを効率的かつ柔軟に整理、アクセス、操作するために不可欠です。

配列の作成

構文

const array_name = [item1, item2, ...]; 

スペースと改行は重要ではありません。宣言は複数行にまたがることができます:

const developer = [
  "Fatema",
  "Sakib",
  "Riaz"
];

配列を作成して要素を指定することもできます:

const man = [];
cars[0]= "Abdur Rahman";
cars[1]= "Riyaz Khan";
cars[2]= "Jumman KL";

JavaScript キーワード new の使用

const man = new Array("Saabid", "Fatema", "Rukhsana");

配列要素へのアクセス
インデックス番号:
を参照して配列要素にアクセスします。

const man = ["Fatema", "Sakib", "Ayesha"];
let car = cars[0];

配列から文字列への変換

const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();

JavaScript で配列を文字列に変換するのは一般的な操作であり、これを実現するにはいくつかの方法があり、それぞれ異なるニーズに対応します。最も一般的に使用される方法は次のとおりです:

1. join() メソッド

join() メソッドは、要素間に指定された区切り文字を使用して、配列のすべての要素を単一の文字列に結合します。

const fruits = ['apple', 'banana', 'cherry'];
const result = fruits.join(', '); // 'apple, banana, cherry'
console.log(result);

• 構文: array.join([セパレータ])
• デフォルトの区切り文字: , (カンマ)
• カスタム区切り文字: '-'、' | などの任意の文字列を区切り文字として指定できます。 'など

2.toString() メソッド

toString() メソッドは、要素を区切るためにカンマを使用して、配列を文字列に変換します。

const numbers = [1, 2, 3, 4];
const result = numbers.toString(); // '1,2,3,4'
console.log(result);

• 構文: array.toString()
• 区切り文字: 区切り文字として常に , (カンマ) を使用します。

3. String() コンストラクター

String() コンストラクターを使用して、配列を文字列に変換できます。このアプローチは toString() に似ていますが、特定のコンテキストではより明示的になることがあります。

const boolArray = [true, false, true];
const result = String(boolArray); // 'true,false,true'
console.log(result);

• 構文: 文字列(配列)

4. テンプレートリテラル

カスタム書式設定の場合、テンプレート リテラルを使用して配列を文字列に変換できます。

const colors = ['red', 'green', 'blue'];
const result = `${colors[0]}, ${colors[1]}, ${colors[2]}`; // 'red, green, blue'
console.log(result);

• 構文: ${} を使用して、配列要素をテンプレート文字列に埋め込みます。

5. Array.prototype.map() と join()

特に各要素をフォーマットする必要がある場合、変換をより詳細に制御するには、map() を join() と組み合わせて使用​​できます。

const numbers = [1, 2, 3];
const result = numbers.map(num => `Number ${num}`).join(' | '); // 'Number 1 | Number 2 | Number 3'
console.log(result);

• Syntax: array.map(callback).join(separator)

Access the Full Array

let a = [1, 12, 13, 14, 6, 8,9, 5, 11, 7, 10, 15, 2, 3, 4, 22, 44, 33];
console.log(a);

let a = [1, 12, 13, 14, 6, 8,9, 5, 11, 7, 10, 15, 2, 3, 4, 22, 44, 33];
document.getElementById("demo").innerHTML = a;

Accessing the full contents of an array in JavaScript can be done in various ways depending on the context and the goal. Here are several methods to access and work with all elements of an array:

1. Direct Access by Index

You can access individual elements of an array directly using their indices. For example, to access the first element, you use index 0.

const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits[0]); // 'apple'
console.log(fruits[1]); // 'banana'
console.log(fruits[2]); // 'cherry'

2. Looping through the Array

You can use loops to iterate through each element of the array. Here are a few common looping methods:

a. for Loop

The traditional for loop gives you control over the index and can be useful for tasks like modifying elements based on their position.

const numbers = [10, 20, 30];
for (let i = 0; i < numbers.length; i++) {
  console.log(numbers[i]); // 10, 20, 30
}

b. forEach() Method

The forEach() method executes a provided function once for each array element.

const colors = ['red', 'green', 'blue'];
colors.forEach(color => {
  console.log(color); // 'red', 'green', 'blue'
});

c. for...of Loop

The for...of loop provides a more modern and readable way to iterate through array elements.

const fruits = ['apple', 'banana', 'cherry'];
for (const fruit of fruits) {
  console.log(fruit); // 'apple', 'banana', 'cherry'
}

d. map() Method

The map() method creates a new array with the results of calling a provided function on every element.

const numbers = [1, 2, 3];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6]

3. Using Array Destructuring

Array destructuring allows you to unpack elements from arrays into distinct variables.

const [first, second, third] = ['apple', 'banana', 'cherry'];
console.log(first);  // 'apple'
console.log(second); // 'banana'
console.log(third); // 'cherry'

4. console.log()

To quickly view the entire array, you can use console.log().

const array = [1, 2, 3, 4, 5];
console.log(array); // [1, 2, 3, 4, 5]

5. toString() Method

Convert the entire array to a comma-separated string.

const array = [1, 2, 3, 4];
console.log(array.toString()); // '1,2,3,4'

6. join() Method

Similar to toString(), but you can specify a separator.

const array = [1, 2, 3, 4];
console.log(array.join(' - ')); // '1 - 2 - 3 - 4'

7. Spread Operator

Use the spread operator to create a new array or pass the array elements to a function.

const numbers = [1, 2, 3];
const newNumbers = [...numbers];
console.log(newNumbers); // [1, 2, 3]

JavaScript as Object

const man = ["Fatema", "CSE", "UU", 24];
console.log(man);

In JavaScript, arrays are indeed a type of object. This concept is fundamental to understanding how arrays work in JavaScript. Here’s a deeper look into why arrays are considered objects and how that impacts their behavior:

1. Arrays as Objects

• Inheritance from Object: Arrays in JavaScript inherit from the Object prototype, which means they have all the properties and methods of objects. This includes methods like hasOwnProperty(), toString(), and others.

• Prototype Chain: Arrays have their own prototype chain that extends from Array.prototype, which is itself an object. This prototype chain provides arrays with their specific methods, like push(), pop(), map(), and more.
  

const arr = [1, 2, 3];
console.log(arr.constructor === Array); // true
console.log(arr instanceof Object); // true
console.log(arr instanceof Array); // true

2. Array-Specific Properties

• Length Property: Arrays have a length property that automatically updates as elements are added or removed. This is specific to arrays and is not present in general objects.

const fruits = ['apple', 'banana'];
console.log(fruits.length); // 2
fruits.push('cherry');
console.log(fruits.length); // 3

• Index-Based Access: Arrays use numeric indices to access elements, whereas objects use string keys. This is a key distinction between arrays and regular objects.

const arr = ['a', 'b', 'c'];
console.log(arr[0]); // 'a'

3. Arrays vs. Objects

• Enumerability: Arrays have numeric indices and are often used in scenarios where the order of elements is important. Objects use string keys and are typically used for key-value pairs where the order is less significant.

• Prototype Methods: Arrays come with a set of methods specific to array manipulation, such as concat(), slice(), and reduce(). Objects have methods and properties from Object.prototype, such as hasOwnProperty().
  

const obj = { a: 1, b: 2 };
console.log(Object.keys(obj)); // ['a', 'b']

const arr = [1, 2, 3];
console.log(arr.map(x => x * 2)); // [2, 4, 6]

4. Arrays as Objects in Practice

• Additional Properties: You can add custom properties to arrays just like you can with objects, though it's not common practice. This does not affect array behavior but can lead to unexpected results when using array methods or properties.

const arr = [1, 2, 3];
arr.customProperty = 'value';
console.log(arr.customProperty); // 'value'

• Array Methods: Methods such as forEach(), filter(), and map() operate on arrays but are not part of the base Object prototype. They are defined on Array.prototype.

const numbers = [1, 2, 3];
numbers.forEach(num => console.log(num)); // 1 2 3

5. Practical Implications

• Use Cases: Arrays are best used when you need to store ordered collections of data and perform operations that involve sequence and index-based access. Objects are more suited for storing data with named properties where the order is not a priority.

• Performance: Arrays can be optimized for sequential access and manipulation due to their special handling in JavaScript engines. Objects are optimized for key-based access.

The length Property

The length property in JavaScript is a special property found on arrays and strings, and it plays a crucial role in managing collections of data. Here's a comprehensive overview of how the length property works:

1. length Property in Arrays

The length property of an array returns the number of elements in the array. It is a dynamic property that automatically updates when elements are added or removed from the array.

Basic Usage

const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.length); // 3

Dynamic Updates

• Adding Elements: When you add elements to an array, the length property increases.

  const numbers = [1, 2, 3];
  numbers.push(4);
  console.log(numbers.length); // 4

• Removing Elements: When you remove elements, the length property decreases.

  const numbers = [1, 2, 3];
  numbers.pop();
  console.log(numbers.length); // 2

• Directly Setting Length: You can also set the length property manually. This will truncate the array or pad it with empty slots.

  const numbers = [1, 2, 3, 4, 5];
  numbers.length = 3;
  console.log(numbers); // [1, 2, 3]

  numbers.length = 5;
  console.log(numbers); // [1, 2, 3, <2 empty items>]

• Truncation: Setting length to a smaller number truncates the array to that length.
• Padding: Setting length to a larger number adds empty slots (undefined values) to the array.

2. length Property in Strings

The length property of a string returns the number of characters in the string.

Basic Usage

const message = 'Hello, World!';
console.log(message.length); // 13

3. Special Considerations

• Sparse Arrays: Arrays can be sparse, meaning they may have "holes" where indices are not explicitly assigned values. The length property reflects the highest index plus one, not the number of actual elements.

  const arr = [1, , 3];
  console.log(arr.length); // 3

Here, arr has a length of 3, but only two indices (0 and 2) have values. The index 1 is "empty" but still counts towards the length.

• Negative Indexes: The length property does not support negative indexes. Negative indices are not part of the standard JavaScript array indexing.

4. Practical Uses

• Iteration: Knowing the length of an array or string is essential for iterating through elements.

  const arr = ['a', 'b', 'c'];
  for (let i = 0; i < arr.length; i++) {
    console.log(arr[i]); // 'a', 'b', 'c'
  }

• Validation: Use the length property to validate input, such as ensuring a user input string meets minimum length requirements.

  function validatePassword(password) {
    return password.length >= 8;
  }

• Padding and Truncation: Adjust the size of arrays and strings to fit specific requirements by setting the length property.

  const arr = [1, 2, 3];
  arr.length = 5; // Adds two empty slots

Adding Array Elements

Adding elements to an array in JavaScript can be done using several methods, depending on where you want to add the elements and how you want to manipulate the array. Here’s a detailed look at the various techniques for adding elements to arrays:

1. Using push()

The push() method adds one or more elements to the end of an array and returns the new length of the array.

const fruits = ['apple', 'banana'];
fruits.push('cherry'); // Adds 'cherry' to the end
console.log(fruits); // ['apple', 'banana', 'cherry']

• Syntax: array.push(element1, element2, ..., elementN)

2. Using unshift()

The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.

const fruits = ['banana', 'cherry'];
fruits.unshift('apple'); // Adds 'apple' to the beginning
console.log(fruits); // ['apple', 'banana', 'cherry']

• Syntax: array.unshift(element1, element2, ..., elementN)

3. Using splice()

The splice() method can add elements at any position in the array. It can also be used to remove elements.

const fruits = ['apple', 'cherry'];
fruits.splice(1, 0, 'banana'); // At index 1, remove 0 elements, add 'banana'
console.log(fruits); // ['apple', 'banana', 'cherry']

• Syntax: array.splice(start, deleteCount, element1, element2, ..., elementN)
  • start: The index at which to start adding elements.
  • deleteCount: The number of elements to remove (0 if you are only adding).
  • element1, ..., elementN: The elements to add.

4. Using the Spread Operator

The spread operator (...) allows you to add elements from one array into another array. This is particularly useful for combining arrays.

const fruits = ['apple', 'banana'];
const moreFruits = ['cherry', 'date'];
const allFruits = [...fruits, ...moreFruits];
console.log(allFruits); // ['apple', 'banana', 'cherry', 'date']

• Syntax: const newArray = [...array1, ...array2, ...array3]

5. Using concat()

The concat() method creates a new array by combining multiple arrays or values.

const fruits = ['apple', 'banana'];
const moreFruits = ['cherry', 'date'];
const allFruits = fruits.concat(moreFruits);
console.log(allFruits); // ['apple', 'banana', 'cherry', 'date']

• Syntax: array1.concat(array2, array3, ..., value1, value2, ...)

6. Using Array Destructuring with Spread Operator

You can use array destructuring with the spread operator to add elements to specific positions in an array.

const fruits = ['apple', 'date'];
const newFruits = ['banana', ...fruits, 'cherry'];
console.log(newFruits); // ['banana', 'apple', 'date', 'cherry']

• Syntax: const newArray = [element1, ...oldArray, elementN]

7. Inserting Multiple Elements

You can use splice() to insert multiple elements at a specific index.

const numbers = [1, 2, 5];
numbers.splice(2, 0, 3, 4); // Insert 3 and 4 at index 2
console.log(numbers); // [1, 2, 3, 4, 5]

8. Handling Empty Slots

When using length to add elements, be aware that it will add empty slots.

const arr = [1, 2, 3];
arr.length = 5; // Adds two empty slots
console.log(arr); // [1, 2, 3, <2 empty items>]

Nested Arrays and Objects

Nested arrays and objects in JavaScript are powerful features that allow you to create complex data structures. These can be used to represent multi-dimensional data, hierarchies, or any scenario where data is organized in layers.

1. Nested Arrays

A nested array is an array that contains other arrays as its elements. This can be useful for representing matrices, grids, or hierarchical data.

Example of a Nested Array

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];

// Accessing elements
console.log(matrix[0][0]); // 1
console.log(matrix[1][2]); // 6

// Iterating through a nested array
for (const row of matrix) {
  for (const value of row) {
    console.log(value);
  }
}

2. Nested Objects

A nested object is an object that contains other objects as its properties. This is useful for representing hierarchical data or entities with multiple attributes.

Example of a Nested Object

const person = {
  name: 'John Doe',
  age: 30,
  address: {
    street: '123 Main St',
    city: 'Anytown',
    zipCode: '12345'
  },
  hobbies: ['reading', 'gaming']
};

// Accessing nested properties
console.log(person.address.street); // '123 Main St'
console.log(person.hobbies[1]); // 'gaming'

// Iterating through a nested object
for (const key in person) {
  if (typeof person[key] === 'object' && !Array.isArray(person[key])) {
    console.log(`Nested object ${key}:`);
    for (const subKey in person[key]) {
      console.log(`  ${subKey}: ${person[key][subKey]}`);
    }
  } else {
    console.log(`${key}: ${person[key]}`);
  }
}

3. Combining Nested Arrays and Objects

You can combine arrays and objects to create more complex structures. For example, you might have an array of objects, where each object contains nested arrays or other objects.

Example of Combining Nested Arrays and Objects

const classRoom = [
  {
    name: 'Math',
    students: [
      { name: 'Alice', score: 95 },
      { name: 'Bob', score: 88 }
    ]
  },
  {
    name: 'Science',
    students: [
      { name: 'Charlie', score: 92 },
      { name: 'David', score: 85 }
    ]
  }
];

// Accessing data
console.log(classRoom[0].students[1].name); // 'Bob'
console.log(classRoom[1].students[0].score); // 92

// Iterating through the combined structure
for (const subject of classRoom) {
  console.log(`Subject: ${subject.name}`);
  for (const student of subject.students) {
    console.log(`  Student: ${student.name}, Score: ${student.score}`);
  }
}

4. Manipulating Nested Structures

• Accessing: Use dot notation or bracket notation for objects, and indices for arrays.

• Updating: Assign new values to nested properties or elements.
  

  person.address.city = 'New City'; // Update a nested property
  classRoom[0].students[0].score = 97; // Update a nested element

• Adding: Add new properties or elements as needed.

  person.phone = '555-5555'; // Add a new property
  classRoom.push({ name: 'History', students: [] }); // Add a new subject

• Deleting: Use delete for properties and .splice() for array elements.

  delete person.phone; // Remove a nested property
  classRoom[1].students.splice(1, 1); // Remove a student

5. Practical Use Cases

• Data Representation: Represent complex data structures such as configuration settings, hierarchical data (e.g., organizational charts), and multi-dimensional datasets.

• APIs and Databases: Often used in API responses and database queries to represent complex records.

• Form Data: Useful for handling nested form data, such as forms with sections or groups of fields.

JavaScript Array Methods

JavaScript arrays come with a rich set of built-in methods that help you manipulate and interact with array data. These methods can be broadly categorized into several types, including those for modifying arrays, accessing elements, and iterating over elements. Here’s a comprehensive overview of common array methods:

1. Adding and Removing Elements

• push(): Adds one or more elements to the end of an array.

  const fruits = ['apple', 'banana'];
  fruits.push('cherry'); // ['apple', 'banana', 'cherry']

• pop(): Removes the last element from an array and returns it.

  const fruits = ['apple', 'banana', 'cherry'];
  const lastFruit = fruits.pop(); // 'cherry'

• unshift(): Adds one or more elements to the beginning of an array.

  const fruits = ['banana', 'cherry'];
  fruits.unshift('apple'); // ['apple', 'banana', 'cherry']

• shift(): Removes the first element from an array and returns it.

  const fruits = ['apple', 'banana', 'cherry'];
  const firstFruit = fruits.shift(); // 'apple'

• splice(): Adds or removes elements from a specific index.

  const fruits = ['apple', 'banana', 'cherry'];
  fruits.splice(1, 1, 'blueberry'); // ['apple', 'blueberry', 'cherry']

• Syntax: array.splice(start, deleteCount, item1, item2, ...)

2. Accessing and Searching Elements

• indexOf(): Returns the first index at which a given element can be found, or -1 if not found.

  const fruits = ['apple', 'banana', 'cherry'];
  const index = fruits.indexOf('banana'); // 1

• includes(): Checks if an array contains a specific element.

  const fruits = ['apple', 'banana', 'cherry'];
  const hasBanana = fruits.includes('banana'); // true

• find(): Returns the first element that satisfies a provided testing function.

  const numbers = [1, 2, 3, 4];
  const firstEven = numbers.find(num => num % 2 === 0); // 2

• findIndex(): Returns the index of the first element that satisfies a provided testing function.

  const numbers = [1, 2, 3, 4];
  const index = numbers.findIndex(num => num % 2 === 0); // 1

3. Iterating Over Elements

• forEach(): Executes a provided function once for each array element.

  const fruits = ['apple', 'banana', 'cherry'];
  fruits.forEach(fruit => console.log(fruit));

• map(): Creates a new array with the results of calling a provided function on every element.

  const numbers = [1, 2, 3];
  const doubled = numbers.map(num => num * 2); // [2, 4, 6]

• filter(): Creates a new array with all elements that pass the test implemented by the provided function.

  const numbers = [1, 2, 3, 4];
  const evens = numbers.filter(num => num % 2 === 0); // [2, 4]

• reduce(): Applies a function against an accumulator and each element to reduce it to a single value.

  const numbers = [1, 2, 3, 4];
  const sum = numbers.reduce((acc, num) => acc + num, 0); // 10

• reduceRight(): Similar to reduce(), but starts from the right end of the array.

  const numbers = [1, 2, 3, 4];
  const product = numbers.reduceRight((acc, num) => acc * num, 1); // 24

• some(): Tests whether at least one element in the array passes the provided function.

  const numbers = [1, 2, 3, 4];
  const hasEven = numbers.some(num => num % 2 === 0); // true

• every(): Tests whether all elements in the array pass the provided function.

  const numbers = [2, 4, 6];
  const allEven = numbers.every(num => num % 2 === 0); // true

4. Sorting and Reversing

• sort(): Sorts the elements of an array in place and returns the array.

  const numbers = [4, 2, 3, 1];
  numbers.sort(); // [1, 2, 3, 4]

• Note: sort() sorts elements as strings by default. For numerical sorting, use a compare function.

  const numbers = [4, 2, 3, 1];
  numbers.sort((a, b) => a - b); // [1, 2, 3, 4]

• reverse(): Reverses the elements of an array in place.

  const numbers = [1, 2, 3];
  numbers.reverse(); // [3, 2, 1]

5. Array Transformation

• concat(): Merges two or more arrays into a new array.

  const arr1 = [1, 2];
  const arr2 = [3, 4];
  const combined = arr1.concat(arr2); // [1, 2, 3, 4]

• slice(): Returns a shallow copy of a portion of an array into a new array.

  const numbers = [1, 2, 3, 4];
  const sliced = numbers.slice(1, 3); // [2, 3]

• splice(): Modifies an array by adding, removing, or replacing elements. (Also listed under adding/removing elements.)

  const numbers = [1, 2, 3, 4];
  numbers.splice(2, 1, 'a', 'b'); // [1, 2, 'a', 'b', 4]

6. String Conversion

• toString(): Converts an array to a string, with elements separated by commas.

  const numbers = [1, 2, 3];
  const str = numbers.toString(); // '1,2,3'

• join(): Joins all elements of an array into a string with a specified separator.

  const numbers = [1, 2, 3];
  const str = numbers.join('-'); // '1-2-3'

JavaScript Array Search

JavaScript arrays come with various methods for searching and locating elements. These methods can be used to find specific values, check for the presence of elements, or retrieve indexes. Here's a detailed overview of the key array search methods:

1. indexOf()

The indexOf() method returns the first index at which a given element can be found, or -1 if the element is not found.

const fruits = ['apple', 'banana', 'cherry'];
const index = fruits.indexOf('banana'); // 1
const notFound = fruits.indexOf('orange'); // -1

• Syntax: array.indexOf(searchElement, fromIndex)

  • searchElement: The element to search for.
  • fromIndex (optional): The index to start the search from.

2. includes()

The includes() method determines whether an array contains a certain element and returns true or false.

const fruits = ['apple', 'banana', 'cherry'];
const hasBanana = fruits.includes('banana'); // true
const hasOrange = fruits.includes('orange'); // false

• Syntax: array.includes(searchElement, fromIndex)

  • searchElement: The element to search for.
  • fromIndex (optional): The index to start the search from.

3. find()

The find() method returns the first element in the array that satisfies a provided testing function. If no elements satisfy the testing function, it returns undefined.

const numbers = [4, 9, 16, 25];
const firstEven = numbers.find(num => num % 2 === 0); // 4
const noMatch = numbers.find(num => num > 30); // undefined

• Syntax: array.find(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

4. findIndex()

The findIndex() method returns the index of the first element in the array that satisfies a provided testing function. If no elements satisfy the testing function, it returns -1.

const numbers = [4, 9, 16, 25];
const index = numbers.findIndex(num => num % 2 === 0); // 0
const noMatchIndex = numbers.findIndex(num => num > 30); // -1

• Syntax: array.findIndex(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

5. some()

The some() method tests whether at least one element in the array passes the provided testing function. It returns true if any elements pass the test, otherwise false.

const numbers = [1, 2, 3, 4];
const hasEven = numbers.some(num => num % 2 === 0); // true
const allEven = numbers.some(num => num % 2 === 0 && num > 5); // false

• Syntax: array.some(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

6. every()

The every() method tests whether all elements in the array pass the provided testing function. It returns true if all elements pass the test, otherwise false.

const numbers = [2, 4, 6];
const allEven = numbers.every(num => num % 2 === 0); // true
const notAllEven = numbers.every(num => num > 3); // false

• Syntax: array.every(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

7. filter()

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

const numbers = [1, 2, 3, 4, 5];
const evens = numbers.filter(num => num % 2 === 0); // [2, 4]

• Syntax: array.filter(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

8. findLast() (Experimental)

The findLast() method returns the last element in the array that satisfies a provided testing function. If no elements satisfy the testing function, it returns undefined. Note that this method is experimental and may not be supported in all environments.

const numbers = [4, 9, 16, 25];
const lastEven = numbers.findLast(num => num % 2 === 0); // 16

• Syntax: array.findLast(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

9. findLastIndex() (Experimental)

The findLastIndex() method returns the index of the last element in the array that satisfies a provided testing function. If no elements satisfy the testing function, it returns -1. Note that this method is experimental and may not be supported in all environments.

const numbers = [4, 9, 16, 25];
const lastEvenIndex = numbers.findLastIndex(num => num % 2 === 0); // 2

• Syntax: array.findLastIndex(callback(element, index, array), thisArg)

  • callback: A function that tests each element.
  • thisArg (optional): Value to use as this when executing callback.

JavaScript Sorting Arrays

Sorting arrays in JavaScript can be done using the sort() method. This method allows you to arrange elements in an array according to a specified order. By default, the sort() method sorts the elements as strings, but you can provide a custom comparison function to sort elements in different ways.

Basic Usage of sort()

The sort() method sorts the elements of an array in place and returns the sorted array.

const fruits = ['banana', 'apple', 'cherry'];
fruits.sort();
console.log(fruits); // ['apple', 'banana', 'cherry']

Custom Sorting with a Comparison Function

To sort elements in a specific order, you need to pass a comparison function to sort(). The comparison function takes two arguments (let's call them a and b) and returns:

• A negative value if a should come before b.
• Zero if a and b are equal in the sort order.
• A positive value if a should come after b.

Sorting Numbers

By default, the sort() method converts numbers to strings and sorts them lexicographically. To sort numbers correctly, provide a comparison function that performs numerical comparisons.

const numbers = [10, 5, 100, 1];
numbers.sort((a, b) => a - b); // Ascending order
console.log(numbers); // [1, 5, 10, 100]

numbers.sort((a, b) => b - a); // Descending order
console.log(numbers); // [100, 10, 5, 1]

Sorting Strings

Strings are sorted lexicographically (dictionary order) by default. For case-insensitive sorting, you can convert strings to the same case (e.g., lowercase) in the comparison function.

const words = ['banana', 'Apple', 'cherry'];
words.sort((a, b) => a.localeCompare(b)); // Case-sensitive
console.log(words); // ['Apple', 'banana', 'cherry']

words.sort((a, b) => a.toLowerCase().localeCompare(b.toLowerCase())); // Case-insensitive
console.log(words); // ['Apple', 'banana', 'cherry']

Sorting Objects

To sort an array of objects, use a comparison function that compares the desired properties of the objects.

const people = [
  { name: 'John', age: 30 },
  { name: 'Jane', age: 25 },
  { name: 'Mike', age: 35 }
];

// Sort by age
people.sort((a, b) => a.age - b.age);
console.log(people);
// [ { name: 'Jane', age: 25 }, { name: 'John', age: 30 }, { name: 'Mike', age: 35 } ]

// Sort by name
people.sort((a, b) => a.name.localeCompare(b.name));
console.log(people);
// [ { name: 'Jane', age: 25 }, { name: 'John', age: 30 }, { name: 'Mike', age: 35 } ]

Sorting Multi-Dimensional Arrays

When sorting multi-dimensional arrays (arrays of arrays), provide a comparison function that compares the relevant elements.

const matrix = [
  [1, 4],
  [3, 2],
  [5, 0]
];

// Sort by the first element of each sub-array
matrix.sort((a, b) => a[0] - b[0]);
console.log(matrix); // [ [1, 4], [3, 2], [5, 0] ]

// Sort by the second element of each sub-array
matrix.sort((a, b) => a[1] - b[1]);
console.log(matrix); // [ [5, 0], [3, 2], [1, 4] ]

Stable Sorting

JavaScript's sort() method is stable in modern environments, meaning that elements with equal values retain their relative order. However, this is not guaranteed in all JavaScript engines, so if stability is crucial, consider using a custom stable sorting algorithm or library.

Sorting with localeCompare()

The localeCompare() method can be useful for sorting strings in a locale-aware manner, accounting for different cultural sorting rules.

const words = ['résumé', 'resume', 'apple'];
words.sort((a, b) => a.localeCompare(b, 'en', { sensitivity: 'base' }));
console.log(words); // ['apple', 'resume', 'résumé']

JavaScript Array Iteration

JavaScript provides several methods for iterating over arrays, allowing you to execute a function on each element or transform the array in various ways. Here’s a comprehensive overview of the array iteration methods available in JavaScript:

1. forEach()

The forEach() method executes a provided function once for each array element. It does not return a value and cannot be stopped or broken out of early.

const fruits = ['apple', 'banana', 'cherry'];
fruits.forEach(fruit => console.log(fruit));
// Output:
// apple
// banana
// cherry

• Syntax: array.forEach(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

2. map()

The map() method creates a new array with the results of calling a provided function on every element in the original array. It’s used for transforming elements.

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6, 8]

• Syntax: array.map(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

3. filter()

The filter() method creates a new array with all elements that pass the test implemented by the provided function. It is used for selecting elements that meet certain criteria.

const numbers = [1, 2, 3, 4, 5];
const evens = numbers.filter(num => num % 2 === 0);
console.log(evens); // [2, 4]

• Syntax: array.filter(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

4. reduce()

The reduce() method applies a function against an accumulator and each element in the array to reduce it to a single value, such as a sum or a concatenated string.

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((acc, num) => acc + num, 0);
console.log(sum); // 10

• Syntax: array.reduce(callback(accumulator, currentValue, index, array), initialValue)

  • callback: Function that is executed for each element.
  • initialValue (optional): Value to use as the first argument to the first call of the callback.

5. reduceRight()

The reduceRight() method is similar to reduce(), but it processes the array from right to left.

const numbers = [1, 2, 3, 4];
const product = numbers.reduceRight((acc, num) => acc * num, 1);
console.log(product); // 24

• Syntax: array.reduceRight(callback(accumulator, currentValue, index, array), initialValue)

  • callback: Function that is executed for each element.
  • initialValue (optional): Value to use as the first argument to the first call of the callback.

6. some()

The some() method tests whether at least one element in the array passes the test implemented by the provided function. It returns true if at least one element passes the test, otherwise false.

const numbers = [1, 2, 3, 4];
const hasEven = numbers.some(num => num % 2 === 0);
console.log(hasEven); // true

• Syntax: array.some(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

7. every()

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns true if all elements pass the test, otherwise false.

const numbers = [2, 4, 6];
const allEven = numbers.every(num => num % 2 === 0);
console.log(allEven); // true

• Syntax: array.every(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

8. find()

The find() method returns the first element in the array that satisfies the provided testing function. If no elements satisfy the testing function, it returns undefined.

const numbers = [4, 9, 16, 25];
const firstEven = numbers.find(num => num % 2 === 0);
console.log(firstEven); // 4

• Syntax: array.find(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

9. findIndex()

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. If no elements satisfy the testing function, it returns -1.

const numbers = [4, 9, 16, 25];
const index = numbers.findIndex(num => num % 2 === 0);
console.log(index); // 0

• Syntax: array.findIndex(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

10. for...of Loop

The for...of loop provides a clean syntax for iterating over iterable objects like arrays. It is especially useful for looping through array values.

const fruits = ['apple', 'banana', 'cherry'];
for (const fruit of fruits) {
  console.log(fruit);
}
// Output:
// apple
// banana
// cherry

11. for...in Loop

The for...in loop iterates over the enumerable properties of an object. When used with arrays, it iterates over array indices, not values. It is generally not recommended for arrays, as it is intended for objects.

const fruits = ['apple', 'banana', 'cherry'];
for (const index in fruits) {
  console.log(index); // 0, 1, 2
}

12. flatMap()

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. This is useful for when you need to map and then flatten the results in one go.

const numbers = [1, 2, 3];
const flattened = numbers.flatMap(num => [num, num * 2]);
console.log(flattened); // [1, 2, 2, 4, 3, 6]

• Syntax: array.flatMap(callback(element, index, array), thisArg)

  • callback: Function that is executed for each element.
  • thisArg (optional): Value to use as this when executing callback.

JavaScript Array Const

In JavaScript, const is a keyword used to declare variables that are intended to remain constant—i.e., their references cannot be reassigned. However, this does not mean the value or object they point to is immutable. For arrays declared with const, the array itself cannot be reassigned, but its elements can still be modified.

Here's a more detailed look at using const with arrays:

1. Declaring Arrays with const

When you declare an array with const, you are creating a constant reference to that array. This means you cannot reassign the array to a different value or array, but you can still modify its elements or its structure (such as adding or removing elements).

const fruits = ['apple', 'banana', 'cherry'];

// Valid: modifying elements
fruits[0] = 'blueberry'; // ['blueberry', 'banana', 'cherry']
fruits.push('date');     // ['blueberry', 'banana', 'cherry', 'date']

console.log(fruits);

// Invalid: reassigning the array
fruits = ['kiwi', 'mango']; // TypeError: Assignment to constant variable.

2. Modifying Arrays

Even though you cannot reassign the const array, you can use array methods to modify its contents:

• Adding Elements: Use methods like push(), unshift(), or splice().

  const numbers = [1, 2, 3];
  numbers.push(4); // [1, 2, 3, 4]
  numbers.unshift(0); // [0, 1, 2, 3, 4]

• Removing Elements: Use methods like pop(), shift(), or splice().

  const colors = ['red', 'green', 'blue'];
  colors.pop(); // ['red', 'green']
  colors.shift(); // ['green']

• Modifying Elements: Directly access and change elements by index.

  const animals = ['cat', 'dog', 'bird'];
  animals[1] = 'fish'; // ['cat', 'fish', 'bird']

3. Array Methods

Methods that modify the array in place are allowed:

• sort(): Sorts the elements of the array.

  const numbers = [3, 1, 4, 1, 5];
  numbers.sort(); // [1, 1, 3, 4, 5]

• reverse(): Reverses the order of elements.

  const letters = ['a', 'b', 'c'];
  letters.reverse(); // ['c', 'b', 'a']

• splice(): Adds or removes elements.

  const fruits = ['apple', 'banana', 'cherry'];
  fruits.splice(1, 1, 'blueberry', 'date'); // ['apple', 'blueberry', 'date', 'cherry']

4. Immutability of Arrays

If you need an immutable array, where changes to the array are not allowed, you need to use additional techniques or libraries to achieve this. JavaScript itself does not provide immutable arrays directly.

For example, you could use libraries like Immutable.js for immutability:

import { List } from 'immutable';

const immutableList = List([1, 2, 3]);
const newList = immutableList.push(4); // Returns a new List: List [ 1, 2, 3, 4 ]
console.log(immutableList); // List [ 1, 2, 3 ]

Summary

JavaScript provides a wide range of operations and methods to handle arrays, including creation, manipulation, searching, and iteration. Here's a summary of the most common array operations:

1. Penciptaan dan Permulaan

• Sintaks Literal: tatasusunan const = [1, 2, 3];
• Menggunakan Array Constructor: const array = new Array(3); // Mencipta tatasusunan dengan 3 slot kosong

2. Mengakses dan Mengubah Suai Elemen

• Akses mengikut Indeks: const firstElement = tatasusunan[0];
• Ubah suai mengikut Indeks: tatasusunan[0] = 10;

3. Kaedah Tatasusunan

Penciptaan dan Transformasi

• concat(): Menggabungkan tatasusunan. const digabungkan = array1.concat(array2);
• flat(): Meratakan tatasusunan bersarang. const flatArray = nestedArray.flat();
• flatMap(): Peta dan kemudian diratakan. hasil const = array.flatMap(x => [x, x * 2]);

Isih

• isih(): Isih unsur. array.sort((a, b) => a - b);
• reverse(): Membalikkan tertib. array.reverse();

Menambah dan Mengalih Keluar Elemen

• push(): Menambah elemen pada penghujung. array.push(4);
• pop(): Mengalih keluar elemen terakhir. const last = array.pop();
• unshift(): Menambah elemen pada permulaan. array.unshift(0);
• shift(): Mengalih keluar elemen pertama. const pertama = array.shift();
• splice(): Menambah atau mengalih keluar elemen pada indeks yang ditentukan. array.splice(1, 1, 'newElement');

Lelaran

• forEach(): Melaksanakan fungsi pada setiap elemen. array.forEach(elemen => console.log(elemen));
• map(): Mengubah elemen dan mengembalikan tatasusunan baharu. const newArray = array.map(x => x * 2);
• filter(): Mengembalikan elemen yang lulus ujian. const ditapis = array.filter(x => x % 2 === 0);
• reduce(): Mengurangkan tatasusunan kepada satu nilai. const sum = array.reduce((acc, x) => acc + x, 0);
• reduceRight(): Mengurangkan tatasusunan dari kanan ke kiri. produk const = array.reduceRight((acc, x) => acc * x, 1);
• some(): Menguji jika mana-mana elemen lulus ujian. const hasEven = array.some(x => x % 2 === 0);
• every(): Menguji jika semua elemen lulus ujian. const allPositive = array.every(x => x > 0);
• find(): Mencari elemen pertama yang lulus ujian. const found = array.find(x => x > 3);
• findIndex(): Mencari indeks elemen pertama yang lulus ujian. const index = array.findIndex(x => x > 3);
• flatMap(): Peta dan meratakan hasil. const diratakan = array.flatMap(x => [x, x * 2]);

4. Cari dan Pengindeksan

• indexOf(): Mencari indeks pertama unsur. indeks const = array.indexOf(3);
• termasuk(): Menyemak sama ada unsur wujud. const wujud = array.includes(3);
• lastIndexOf(): Mencari indeks terakhir elemen. const lastIndex = array.lastIndexOf(3);

5. Pelbagai

• slice(): Mengembalikan salinan cetek sebahagian daripada tatasusunan. const subArray = array.slice(1, 3);
• join(): Mencantumkan elemen tatasusunan ke dalam rentetan. const str = array.join('-');
• toString(): Menukar tatasusunan kepada rentetan. const str = array.toString();

6. Susun atur dan konst

• const dengan Tatasusunan: Menghalang penugasan semula rujukan tatasusunan, tetapi membenarkan pengubahsuaian elemen dan struktur tatasusunan. tatasusunan const = [1, 2, 3]; array.push(4); // Dibenarkan

7. Lelaran Terperinci

• untuk...daripada: Berulang melebihi nilai. untuk (nilai const tatasusunan) { console.log(value); }
• untuk...dalam: Berulang ke atas indeks (kurang disyorkan untuk tatasusunan). untuk (indeks const dalam tatasusunan) { console.log(index); }

Pendek kata

• Penciptaan: Gunakan literal tatasusunan atau pembina.
• Akses/Ubah suai: Gunakan operasi berasaskan indeks.
• Kaedah: Untuk transformasi, menyusun, menambah/mengalih keluar dan lelaran.
• Cari/Indeks: Untuk mencari elemen dan indeks.
• const: Gunakan untuk rujukan berterusan, membenarkan pengubahsuaian.
• Lelaran: Gunakan forEach(), map(), filter(), dsb., untuk keperluan lelaran yang berbeza.

Memahami dan menggunakan operasi ini dengan berkesan akan membantu anda mengurus dan memanipulasi tatasusunan dalam JavaScript dengan kecekapan dan fleksibiliti yang lebih tinggi.

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