How to use map() on an array in reverse order with JavaScript ?

Last Updated : 31 Dec, 2019

Given a JavaScript array and the task is to apply the map() method but on the reverse of the array efficiently. Here are a few approaches discussed. If you don’t want to change the original array then you can create a shallow copy of the array after that you can perform the task.

Approach 1: The idea is to use .reverse() method just after applying .slice() method. Then use the .map() method on the reversed array to perform the task.

Example: This example implements the above approach.

<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        How to use map() on an array in 
        reverse order with JavaScript ? 
    </title> 
  
    <style> 
        body { 
            text-align: center; 
        } 
        h1 { 
            color: green; 
        } 
        #geeks { 
            font-weight: bold; 
        } 
    </style> 
</head> 
  
<body> 
    <h1>GeeksforGeeks</h1> 
      
    <p> 
        Click on the button to use 
        <b>map()</b> on array in  
        <b>reverse</b> order<br> 
        Array = [1, 3, 5, 7, 9, 10]; 
    </p> 
      
    <button onclick="gfg_Run()"> 
        Click Here 
    </button> 
      
    <p id="geeks"></p> 
      
    <script> 
        var el_down = document.getElementById("geeks"); 
        var arr = [1, 3, 5, 7, 9, 10]; 
  
        /* Main function */ 
        function gfg_Run() { 
            newArr = arr.slice(0).reverse().map( 
                function(val, index) { 
                    return val * 2; 
                } 
            ); 
              
            el_down.innerHTML = "New Array = [" 
                                + newArr + "]"; 
        } 
    </script> 
</body> 
  
</html> 

Output:

Approach 2: In this approach we will use the .map() method and call a function inside this method with 2 arguments (value, index). Now we need to access the value, we will access it from reverse side (eg.. arr[arr.length – 1 – index]), this is immutable operation (It doesn’t change the original array).

Example: This example implements the above approach.

<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        How to use map() on an array in 
        reverse order with JavaScript ? 
    </title> 
      
    <style> 
        body { 
            text-align: center; 
        } 
        h1 { 
            color: green; 
        } 
        #geeks { 
            font-weight: bold; 
        } 
    </style> 
</head> 
  
<body> 
    <h1>GeeksforGeeks</h1> 
      
    <p> 
        Click on the button to use  
        <b>map()</b> on array in  
        <b>reverse</b> order<br> 
        Array = [8, 5, 15, 70, 9, 10]; 
    </p> 
      
    <button onclick="gfg_Run()"> 
        Click Here 
    </button> 
      
    <p id="geeks"></p> 
      
    <script> 
        var el_down = document.getElementById("geeks"); 
        var arr = [8, 5, 15, 70, 9, 10]; 
          
        /* Main function */ 
        function gfg_Run() { 
            newArr = arr.map((val, index, array) =>  
                    1/2*arr[arr.length - 1 - index]); 
              
            el_down.innerHTML = "New Array = [" 
                                + newArr + "]"; 
        } 
    </script> 
</body> 
  
</html> 

Output:

My Personal Notes

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