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.