Lets REDUCE the JavaScript Arrays.

Shreyas Kumar

Understanding how javascript array method reduce works.

I have been a fan of map and filter but never bothered to look into how we can use reduce to simplify and minimise the lines of code. First let’s have a look at what the documentation says.

array.reduce( callback(accumulator, currentValue, currentIndex, arr), initialValue);
  • callback : to execute on each element in the array (except for the first, if no initialValue is supplied )
  • accumulator: It recieves the return value of the callback, its the value returned from the last invocation of callback or initialValue.
  • currentValue: Current value of the array being processed.
  • currentIndex: Index of the current value of the array.
  • arr: The entire array on which we are calling reduce.
  • initialValue: Value as a first argument to call the callback for the first time.

Here is a small snippet to reverse a string using reduce method. We will loop through the code and dig deeper to see how it works.

// Reverse a string

var stringToReverse= "fiction";
var arrList = stringToReverse.split('');
// ["f", "i", "c", "t", "i", "o", "n"]
var getReverse = function(total, currentValue, currentIndex, arr) {
let valueToBeReturned = currentValue+total;
return valueToBeReturned;
var b = arrList.reduce(getReverse, '#');

So according to the docs, here “callback” is getReverse() function, “initialValue” is ‘#’.

iteration 1

First iteration of the loop.
first iteration

As we run the code ,

accumulator gets a value: ‘#’ this is the initial value that I have passed to the reduce method.

currentValue : ‘f’ // first element of the array.

valueToBeReturned: Its just concatenated value of currentValue and accumulator. (‘f#’ ).

lets see what next.

second iteration
third iteration
fourth iteration
fifth iteration
sixth iteration
last iteration

Here is the shortened code.

var stringToReverse= "fiction";var arrList = stringToReverse.split(''); 
// ["f", "i", "c", "t", "i", "o", "n"]
var getReverse = (reverse, currentValue) => currentValue+reverse;var reversedStr = arrList.reduce(getReverse, '');

Thats it folks. How about you go further and share about what happens if we don’t pass the “initialValue” ? what does accumulator gets in that case ?

Also , feedback would be valuable.


Shreyas Kumar

Written by

I'm a JavaScript developer.

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