Error: result.push is not a function

I've encountered an issue with two functions I have. Both of them generate a range from a starting integer to an ending integer, inclusive. Here's how they're defined:

function createRange(start, end) {
  var result = [];
  for (var i = start; i <= end; i++) {
    result.push(i);
  }
  return result;
}

console.log(createRange(1,5));
//Expected output [1, 2, 3, 4, 5]

function functionalCreateRange(start, end) {
  function helper(result, strt, nd) {
    if (strt > nd) return [];
    if (strt === nd) {
      return result;
    } else {
      return helper(result.concat([strt]), strt + 1, nd);
    }
  }
  return helper([], start, end);
}

console.log(functionalCreateRange(1,5));
//Expected output [1, 2, 3, 4, 5]

The first function works as expected without any issues. However, the second one throws an error saying "TypeError: result.push is not a function." This has left me puzzled because I'm passing an array as the first argument to the helper function and using the correct parameter name within the helper function. Since it is indeed an array, I am unsure why this error is occurring.

Answer №1

Array.prototype.push() actually returns the new length of the array, not the array itself. Therefore, in the line

functional_range_helper(result.push(start), start + 1, end)
, a number is being passed as the first argument instead of an array.

Answer №2

The issue at hand is that the .push() method does not return the array itself, but rather the length of the array. This means when you execute "result.push(strt)", it will give you an integer value instead of the expected array.

To resolve this, you should push the value into the array in a separate step like so:

function createRange(start, end) {
    function createRangeHelper(resultArray, startValue, endValue) {
        if (startValue > endValue) return [];
        if (startValue === endValue) {
            return resultArray;
       } else {
           resultArray.push(startValue);
           return createRangeHelper(resultArray, startValue + 1, endValue);
       }
   }
    return createRangeHelper([], start, end);
}

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