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Showing content from https://webplatform.github.io/docs/javascript/Array/reduce below:

reduce · WebPlatform Docs

reduce Summary

Calls the specified callback function for all the elements in an array. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.

Syntax

reduce( callbackfn [, initialValue ])

callbackfn: Required. A function that accepts up to four arguments. The reduce method calls the callbackfn function one time for each element in the array.
initialValue: Optional. If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.

Return Value

The accumulated result from the last call to the callback function.

Examples

The following example concatenates array values into a string, separating the values with "::". Because no initial value is provided to the reduce method, the first call to the callback function has “abc” as the previousValue argument and “def” as the currentValue argument.


 function appendCurrent (previousValue, currentValue) {
     return previousValue + "::" + currentValue;
     }

 
 var elements = ["abc", "def", 123, 456];

 
 
 var result = elements.reduce(appendCurrent);

 document.write(result);

The following example adds the values of an array after they have been rounded. The reduce method is called with an initial value of 0.


 function addRounded (previousValue, currentValue) {
     return previousValue + Math.round(currentValue);
     }

 
 var numbers = [10.9, 15.4, 0.5];

 
 var result = numbers.reduce(addRounded, 0);

 document.write (result);

The following example adds the values in an array. The currentIndex and array1 parameters are used in the callback function.

function addDigitValue(previousValue, currentDigit, currentIndex, array) {
     var exponent = (array.length - 1) - currentIndex;
     var digitValue = currentDigit * Math.pow(10, exponent);
     return previousValue + digitValue;
     }

 var digits = [4, 1, 2, 5];

 
 var result = digits.reduce(addDigitValue, 0);

 document.write (result);

The following example gets an array that contains only those values that are between 1 and 10 in another array. The initial value provided to the reduce method is an empty array.

function Process(previousArray, currentValue) {
     
     
     var nextArray;
     if (currentValue >= 1 && currentValue <= 10)
         nextArray = previousArray.concat(currentValue);
     else
         nextArray = previousArray;

     
     
     
     
     return nextArray;
 }

 
 var numbers = [20, 1, -5, 6, 50, 3];

 
 var emptyArray = new Array();
 var resultArray = numbers.reduce(Process, emptyArray);

 document.write("result array=" + resultArray);

Remarks

If an initialValue is provided, the reduce method calls the callbackfn function one time for each element present in the array, in ascending index order. If an initialValue is not provided, the reduce method calls the callbackfn function on each element, starting with the second element.

The return value of the callback function is provided as the previousValue argument on the next call to the callback function. The return value of the last call to the callback function is the return value of the reduce method.

The callback function is not called for missing elements of the array.

Note – The reduceRight Method (Array) processes the elements in descending index order.

The syntax of the callback function is as follows:

function callbackfn(previousValue, currentValue, currentIndex, array1)

You can declare the callback function by using up to four parameters.

The following table lists the callback function parameters.

Callback argument Definition previousValue The value from the previous call to the callback function. If an initialValue is provided to the reduce method, the previousValue is initialValue the first time the function is called. currentValue The value of the current array element. currentIndex The numeric index of the current array element. array1 The array object that contains the element.

The first time the callback function is called, the values provided as arguments depend on whether the reduce method has an initialValue argument.

If an initialValue is provided to the reduce method:

The previousValue argument is initialValue.
The currentValue argument is the value of the first element present in the array.

If an initialValue is not provided:

The previousValue argument is the value of the first element present in the array.
The currentValue argument is the value of the second element present in the array.

The array object can be modified by the callback function.

The following table describes the results of modifying the array object after the reduce method starts.

Condition after the reduce method starts Element passed to callback function? Element is added beyond the original length of the array. No. Element is added to fill in a missing element of the array. Yes, if that index has not yet been passed to the callback function. Element is changed. Yes, if that element has not yet been passed to the callback function. Element is deleted from the array. No, unless that element has already been passed to the callback function. Exceptions

A TypeError exception is thrown when either of the following conditions is true:

The callbackfn argument is not a function object.
The array contains no elements and initialValue is not provided.