How to reverse an array in ES6

In ES6, you can use the reverse() method of the array object to achieve array reversal. This method is used to reverse the order of the elements in the array, putting the last element first, and the first The element is placed at the end, the syntax is "array.reverse()". The reverse() method will modify the original array. If you do not want to modify it, you need to use it with the expansion operator "...", and the syntax is "[...array].reverse()".

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

Reversing an array with specific constraints is one of the most common challenges in job interviews and coding tests.

In ES6, you can use the reverse() method of the array object to reverse the array. The reverse() method reverses the order of the array elements so that the first element becomes the last element and the last element becomes the first element.

How to reverse an array in JavaScript using the Reverse method

let numbers = [1, 2, 3, 4, 5];
let reversedNumbers = numbers.reverse();

console.log(reversedNumbers);
// [5, 4, 3, 2, 1]

But remember, the reverse method modifies the original array:

let numbers = [1, 2, 3, 4, 5];
let reversedNumbers = numbers.reverse();

console.log(reversedNumbers);
// [5, 4, 3, 2, 1]

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

Some coding challenges may want you to keep the original array, so let's look at how to reverse an array without changing the original array.

How to use the Spread operator to reverse an array in JavaScript

You can use the spread operator (Spread) in combination with the reverse method to reverse Convert an array without changing the original array.

First, place the elements returned by the spread operator into a new array by surrounding the spread syntax with square brackets []:

[...numbers]

Then, you add Call the reverse method on. This way, the reverse method will be executed on the new array instead of the original array:

let numbers = [1, 2, 3, 4, 5];
let reversedNumbers = [...numbers].reverse();

console.log(reversedNumbers);
// [5, 4, 3, 2, 1]

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

Note: The spread method is ES6 syntax , when you need to support older browsers or want to use ES5 syntax, you can use the slice and reverse methods together. Let's see now.

How to reverse an array in JavaScript using Slice and Reverse methods

slice method is used to get the selected elements as a new array Returns, when you call the method without any parameters, it will return a new array that is identical to the original array (from first element to last element).

Next, you call the reverse method on the newly returned array, which is why the original array is not reversed:

let numbers = [1, 2, 3, 4, 5];
let reversedNumbers = numbers.slice().reverse();

console.log(reversedNumbers);
// [5, 4, 3, 2, 1]

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

How to do it without Reverse array in JavaScript in case of Reverse method

Sometimes interviews challenge you to reverse the array without using reverse method. no problem! You can use a combination of a for loop and an array's push method, like in the example below.

let numbers = [1, 2, 3, 4, 5];
let reversedNumbers = [];

for(let i = numbers.length -1; i >= 0; i--) {
  reversedNumbers.push(numbers[i]);
}

console.log(reversedNumbers);

How to write your own reverse function in JS

Finally, let's say you are tasked with writing your own reverse function that needs to reverse an array without creating a copy. At first glance, this may seem complicated, but don't worry because it's actually quite simple.

What you need to do here is swap the first and last elements of the array, then the second and second-to-last elements, and so on until all elements are swapped.

Let's write a function to do this.

Write function customReverse and use array.length - 1 as a variable, while storing the first index as 0 and the last index .

function customReverse(originalArray) {
  let leftIndex = 0;
  let rightIndex = originalArray.length - 1;
}

Next, create a while loop that will run as long as leftIndex is less than rightIndex.

Within this loop, swap the values ​​of leftIndex and rightIndex, you can temporarily store one of the values ​​in a temporary variable:

while (leftIndex < rightIndex) {
  // 交换元素
  let temp = originalArray[leftIndex];
  originalArray[leftIndex] = originalArray[rightIndex];
  originalArray[rightIndex] = temp;
}

Finally, leftIndex is moved up, rightIndex is moved down, and when the while loop repeats, it swaps the second-to-last element, and so on:

function customReverse(originalArray) {

  let leftIndex = 0;
  let rightIndex = originalArray.length - 1;

  while (leftIndex < rightIndex) {

    // 用temp变量交换元素
    let temp = originalArray[leftIndex];
    originalArray[leftIndex] = originalArray[rightIndex];
    originalArray[rightIndex] = temp;

    // 将索引移到中间
    leftIndex++;
    rightIndex--;
  }
}

The loop will stop immediately when there are no more elements to reverse. For odd-numbered arrays, the values ​​of leftIndex and rightIndex will be equal, so there is no need to exchange them. For an array of even numbers leftIndex will be greater than rightIndex.

You can test the feature to see if it works properly, as shown below:

let myArray = [1, 2, 3, 4, 5];

customReverse(myArray);

console.log(myArray);

// output is [5, 4, 3, 2, 1]

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