[10,1000].reduce( (finalValue, whatToAddToFinalValue) => finalValue + whatToAddToFinalValue );
The array has 2 items [10, 1000], which means we run 2 iterations.
On each iteration, reduces() takes 1 item ONLY!
For the FIRST iteration, since we did not declare the finalValue starting value, so it will always be 0 :
We have whatToAddToFinalValue variable, which will get its value from the array – but one item at a time.
// FIRST ITERATION finalValue = 0 whatToAddToFinalValue = 10 // the first item in the array
====== at the end of iteration, finalValue = 10
At the subsequent iteration, we will work with a new set of data :
// SECOND ITERATION finalValue = 10 whatToAddToFinalValue = 1000 // the second item in the array
====== at the end of the second iteration, finalValue = 1010
reduce() function operation stops right here, because no more item left in the array to deal with.
You can use reduce() for a more complicated calculation. This article is meant for a beginner who is still trying to make sense of the concept of iteration.
Ready to read more about reduce()? Visit JS docs here.