Using the reduce function to return an array

push returns the new length of the array.

What you need is the initially provided array.

So change the code as below.

var store = [0, 1, 2, 3, 4];var stored = store.reduce(function(pV, cV, cI){  console.log("pv: ", pV);  pV.push(cV);  return pV; // *********  Important ******}, []);

concat returns the new array combining the elements of the provided array and concatenated elements. so it works.

Just for completeness, and for the next person who happens on this question, what you're doing is typically achieved with map which, as stated in the docs

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results

Contrast that with the description of reduce:

The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

(Emphasis mine) So you see, although you can manipulate reduce to return a new array, it's general usage is to reduce an array to a single value.

So for your code this would be:

var store = [0,1,2,3,4];var stored = store.map(function(pV){  console.log("pv: ", pV);  return pV;});

Much simpler than trying to reconstruct a new array using either push or concat within a reduce function.

I know this is the same answer, but I just want to show that using reduce (), the syntax can also be reduced to a single line of code using ES6:

var store = [0,1,2,3,4];var stored = store.reduce((pV,cV) => [...pV, cV], []);console.log(stored);