Using the reduce function to return an array

javascript arrays reduce
2K01B5 picture 2K01B5 · Feb 16, 2016 · Viewed 50k times · Source

Why is it that when I want to use the push function inside the reduce function to return a new array I get an error. However, when I use the concat method inside the reduce function, it returns a new array with no problem.

All I'm trying to do is pass an array to the reduce function and return the same array.

var store = [0,1,2,3,4];

var stored = store.reduce(function(pV,cV,cI){
  console.log("pv: ", pV);
  return pV.push(cV);
},[]);

This returns an error. But when I use concat: 

var store = [0,1,2,3,4];

var stored = store.reduce(function(pV,cV,cI){
  console.log("pv: ", pV);
  return pV.concat(cV);
},[]);

It returns the same array.

Any ideas why?

Answer

Venkata Raju picture Venkata Raju · Feb 16, 2016

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.

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