JavaScript .reduce, previousValue is always undefined

Question

I am curious as to why previousValue is always undefined in the following code when using .reduce on an array:

Code:

[2,2,2,3,4].reduce(function(previousValue, currentValue){
    console.log("Previous Value: " + previousValue);
    console.log("Current Value: " + currentValue);
},0)

Output:

Previous Value: 0 (index):
Current Value: 2 (index):
Previous Value: undefined (index):
Current Value: 2 (index):
Previous Value: undefined (index):
Current Value: 2 (index):
Previous Value: undefined (index):
Current Value: 3 (index):24
Previous Value: undefined (index):23
Current Value: 4 

A fiddle can be found here: http://jsfiddle.net/LzpxE/

Answer 1

You need to return a value in order to use reduce correctly. The returned value will be used in the next step.

Like:

[0,1,2,3,4].reduce(function(previousValue, currentValue) {
     return previousValue + currentValue;
});
// returns 10 in total, because
// 0 + 1 = 1 -> 1 + 2 = 3 -> 3 + 3 = 6 -> 6 + 4 = 10