Javascript: .push is not a function

Array.push doesn't return an array. It returns the new length of the array it was called on.

So, your return array.push(b); returns an int. That int gets passed back as array... which is not an array so it doesn't have a .push() method.

You need to do:

array.push(b);
return array;

The problem here is that array.push(b) returns the new length of the array. So after calling combine(current, array[i]) for the first time, the length of your array will be returned and current becomes an integer, and since current is the passed to combine(current, array[i]) in the next iteration, JavaScript throws the TypeError. Your implementation for combine(current, array[i] should look like this:

function(array, b) {
    array.push(b);
    return array;
}

Return just the Array, see below:

http://jsfiddle.net/0en82r7t/1/

var arrays = [[1, 2, 3], [4, 5], [6]];
console.log(reduce(arrays,function(array,b){
  array.push(b);
  return array;
}));

function reduce(array,combine){
  var current = [];
  for(var i = 0;i<array.length;i += 1){
    current = combine(current,array[i]);
  }
  return current;
}
console.log(reduce([1, 2, 3, 4], function(array, b) {
  array.push(b)
  return array;
}));

array.push does not return an Array, but instead the new length

Also, I know this is just a test, but in the future and in real app development don't name an Array array. Use more verbose and clear naming, examples: numGroupArray, datesArray, timeArray, tagsArray...