JavaScript Array reduce() Method

Last Updated : 01 Nov, 2021

Below is the example of Array reduce() method.

Example:

<!DOCTYPE html>
<html>
      
<head>
    <title>
        JavaScript Array reduce() Method
    </title>
</head>
  
<body style="text-align:center;">
      
    <h1 style="color: green;">GeeksforGeeks</h1>
      
    <p>
        Click here to get the Subtract
        of array elements from the left side
    </p>
      
    <button onclick="myGeeks()">
        Click Here!
    </button>
      
    <br><br>
      
    Subtract: <span id="GFG"></span>
      
    <!-- Script to use reduce method -->
    <script>
        var arr = [175, 50, 25];
  
        function subofArray(total, num) {
            return total - num;
        }
        function myGeeks(item) {
            document.getElementById("GFG").innerHTML
                    = arr.reduce(subofArray);
        }
    </script>
</body>
  
</html>          

Output:

The arr.reduce() method in JavaScript is used to reduce the array to a single value and executes a provided function for each value of the array (from left-to-right) and the return value of the function is stored in an accumulator.

Syntax:

array.reduce( function(total, currentValue, currentIndex, arr), 
initialValue )

Parameter: This method accepts five parameters as mentioned above and described below:

function(total, currentValue, index, arr): It is the required parameter and used to run for each element of array. It contains four parameter which are listed below:
- total: It is required parameter and used to specify the initialValue, or the previously returned value of the function.
- currentValue: It is required parameter and used to specify the value of the current element.
- currentIndex: It is optional parameter and used to specify the array index of the current element.
- arr: It is optional parameter and used to specify the array object the current element belongs to.
initialValue: It is optional parameter and used to specify the value to be passed to the function as the initial value.

Example 1: This example use reduce() method to return the sum of all array elements.

<!DOCTYPE html>
<html>
      
<head>
    <title>
        JavaScript Array reduce() Method
    </title>
</head>
  
<body style="text-align:center;">
      
    <h1 style="color: green;">GeeksforGeeks</h1>
      
    <p>
        Click here to get the sum
        of array elements
    </p>
      
    <button onclick="myGeeks()">
        Click Here!
    </button>
      
    <br><br>
      
    Sum: <span id="GFG"></span>
      
    <!-- Script to use reduce method -->
    <script>
        var arr = [10, 20, 30, 40, 50, 60];
   
        function sumofArray(sum, num) {
            return sum + num;
        }
        function myGeeks(item) {
            document.getElementById("GFG").innerHTML
                    = arr.reduce(sumofArray);
        }
    </script>
</body>
  
</html>                    

Output:

Example 2: This example use reduce() method to return the round sum of all array elements.

<!DOCTYPE html>
<html>
      
<head>
    <title>
        JavaScript Array reduce() Method
    </title>
</head>
  
<body style="text-align:center;">
      
    <h1 style="color: green;">GeeksforGeeks</h1>
      
    <p>
        Click here to get the sum
        of array elements
    </p>
      
    <button onclick="myGeeks()">
        Click Here!
    </button>
      
    <br><br>
      
    Sum: <span id="GFG"></span>
      
    <!-- Script to use reduce method -->
    <script>
        var arr = [1.5, 20.3, 11.1, 40.7];
   
        function sumofArray(sum, num) {
            return sum + Math.round(num);
        }
        function myGeeks(item) {
            document.getElementById("GFG").innerHTML
                    = arr.reduce(sumofArray, 0);
        }
    </script>
</body>
  
</html>                    