Arrays in JavaScript

Detailed explanation of core JavaScript array methods

The following are some commonly used JavaScript array methods:

1. push(): Adds one or more elements to the end of the array and returns the length of the new array.

   <code class="language-javascript">let numbers = [1, 2, 3];
   console.log(numbers.push(4)); // 输出：4  numbers数组变为 [1, 2, 3, 4]</code>

2. slice(): Extract elements within the specified index range from the array and return a new array. The original array is not changed.

   <code class="language-javascript">let numbers = [1, 2, 3, 4, 5];
   console.log(numbers.slice(1, 4)); // 输出：[2, 3, 4]  (从索引1到3)</code>

3. splice(): Add or remove elements from the array. You can specify the starting index, the number of elements to delete, and the new elements to insert.

   <code class="language-javascript">let fruits = ['apple', 'mango', 'orange', 'pear'];
   fruits.splice(2, 0, 'pawpaw', 'strawberries'); // 从索引2开始，不删除任何元素，插入'pawpaw'和'strawberries'
   console.log(fruits); // 输出：['apple', 'mango', 'pawpaw', 'strawberries', 'orange', 'pear']
   
   fruits.splice(2, 1); // 从索引2开始，删除1个元素
   console.log(fruits); // 输出：['apple', 'mango', 'strawberries', 'orange', 'pear']</code>

4. concat(): Concatenates two or more arrays and returns a new array. The original array is not changed.

   <code class="language-javascript">let arr1 = [1, 2, 3, 4];
   let arr2 = [5, 6, 7];
   console.log(arr1.concat(arr2)); // 输出：[1, 2, 3, 4, 5, 6, 7]
   let arr3 = [8, 9];
   console.log(arr1.concat(arr2, arr3)); // 输出：[1, 2, 3, 4, 5, 6, 7, 8, 9]</code>

5. fill(): Replaces elements in the specified range in the array with the specified value.

   <code class="language-javascript">let arr4 = [1, 2, 3, 4];
   arr4.fill('Anurag', 2, 4); // 从索引2到3，用'Anurag'填充
   console.log(arr4); // 输出：[1, 2, 'Anurag', 'Anurag']</code>

6. shift(): Delete the first element of the array and return that element.

   <code class="language-javascript">let arr4 = [1, 2, 3, 4];
   arr4.shift();
   console.log(arr4); // 输出：[2, 3, 4]</code>

7. indexOf(): Returns the index of the first occurrence of the specified element in the array. If the element does not exist, returns -1.

   <code class="language-javascript">let arr4 = [1, 2, 3, 4];
   console.log(arr4.indexOf(3)); // 输出：2</code>

8. lastIndexOf(): Returns the index of the last occurrence of the specified element in the array. If the element does not exist, returns -1.

   <code class="language-javascript">let arr4 = [1, 2, 3, 4, 3];
   console.log(arr4.lastIndexOf(3)); // 输出：4</code>

9. includes(): Determines whether the array contains the specified element and returns a Boolean value.

   <code class="language-javascript">let arr4 = [1, 2, 3, 4];
   console.log(arr4.includes(4)); // 输出：true
   console.log(arr4.includes(5)); // 输出：false</code>

10. pop(): Delete the last element of the array and return that element.

    <code class="language-javascript"> let arr4 = [1, 2, 3, 4];
     arr4.pop();
     console.log(arr4); // 输出：[1, 2, 3]</code>

11. join(): Concatenates array elements into a string and returns the string. Delimiters can be specified.

    <code class="language-javascript"> let arr4 = [1, 2, 3];
     console.log(arr4.join('and')); // 输出：1and2and3</code>

12. unshift(): Adds one or more elements to the beginning of the array and returns the length of the new array.

    <code class="language-javascript"> let arr4 = [1, 2, 3];
     arr4.unshift(0);
     console.log(arr4); // 输出：[0, 1, 2, 3]</code>

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