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filter , since the method must keep track of the current count. This method will pass the
following to our function for each member in the array:
1. The current result.
2. The value of the current member of the array.
3. The index that member has in the array (starting at 0).
4. The array itself.
The methods filter , map and reduce are not unique to JavaScript. These are im-
portant operators for dealing with data sets in many languages, and were pop-
ularised by Google's map/reduce algorithm for indexing web pages.
In order to see this working, execute the following:
[1,2,3,4,5].reduce(function(total, currentValue, index, array) {
console.log('Current value is ' + currentValue);
console.log('Total is ' + total);
return total += currentValue;
This will print the following:
Current value is 2
Total is 1
Current value is 3
Total is 3
Current value is 4
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