Difference between concat and push?

Question

Why does a return of the push method cause

Uncaught TypeError: acc.push is not a function

But a return concat results in the correct solution?

[1, 2, 3, 4].reduce(function name(acc, curr) {
  if (even(curr)) {
    return acc.push(curr);
  }
  return acc;
}, []);


function even(number) {
  if (number % 2 === 0) {
    return true;
  }
  return false;
}

[1, 2, 3, 4].reduce(function name(acc, curr) {
  if (even(curr)) {
    return acc.concat(curr);
  }
  return acc;
}, []);


function even(number) {
  if (number % 2 === 0) {
    return true;
  }
  return false;
}

Answer 1

The push() adds elements to the end of an array and returns the new length of the array. Thus your return here is invalid.

The concat() method is used to merge arrays. Concat does not change the existing arrays, but instead returns a new array.

Better to filter, if you want a NEW array like so:

var arr = [1, 2, 3, 4];
var filtered = arr.filter(function(element, index, array) {
  return (index % 2 === 0);
});

Note that assumes the array arr is complete with no gaps - all even indexed values. If you need each individual, use the element instead of index

var arr = [1, 2, 3, 4];
var filtered = arr.filter(function(element, index, array) {
  return (element% 2 === 0);
});