TypeError in this array addition recursive problem

Question

This function has to add all the constants in the array recursively using JavaScript. Any idea why I'm getting a type error in this recursion function?

const slowAdd = (array) => {
  if (array.length === 0) return 0;

  const restOfArray = array.slice(1);

  return array[0] + slowAdd(array[restOfArray]);
}

const dataArray = [2, 5, 7]
slowAdd(dataArray);

Same Problem with using helper function:

const fastSum = (array) => {
  return _fastSum(array, 0);
};

const _fastSum = (array, start) => {
  if (start === array.length) return 0;

  return array[start] + _fastSum(array, start + 1);
}

const dataArray = [2, 5, 7]
console.log(fastSum(dataArray));

Why does the fastSum recursion work, but the first doesn't.

array[restOfArray] is a single number, not a different array. EDIT: OK, actually restOfArray is the rest of the array. I misread it and thought it was just an index. At any rate array[restOfArray] certainly doesn't produce another array and slowAdd does expect an array — VLAZ
– VLAZ, Commented Dec 1, 2020 at 13:45
@VLAZ It will not even give a number. It will return undefined — Maheer Ali
– Maheer Ali, Commented Dec 1, 2020 at 13:49
@MaheerAli indeed. I initially misread the code and thought restOfArray would be an index. However, when it's an array, then most of the time it's going to produce undefined as you said. — VLAZ
– VLAZ, Commented Dec 1, 2020 at 13:51

Maheer Ali · Accepted Answer · 2020-12-01 13:48:58Z

2

restOfArray is itself an array and you are trying to use it as index of another array. array[restOfArray] will always return undefined.

you should pass restOfArray instead of array[restOfArray]

const slowAdd = (array)=>{
    
    if(array.length === 0) return 0;

    const restOfArray = array.slice(1);

    return array[0] + slowAdd(restOfArray);

}

const dataArray = [2, 5, 7]
console.log(slowAdd(dataArray));

//Same Problem with this one.

const fastSum = (array)=>{
    return _fastSum(array, 0);
};
const _fastSum = (array, start)=>{
    if(start === array.length) return 0;

    return array[start] + _fastSum(array, start + 1);

}

console.log(fastSum(dataArray));

answered Dec 1, 2020 at 13:48

Maheer Ali

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Boris Adamyan · Accepted Answer · 2020-12-01 14:00:42Z

0

restOfArray is array

const slowAdd = (array) => {
  if (array.length === 0) return 0;

  const restOfArray = array.slice(1);

  return array[0] + slowAdd(restOfArray);
}

const dataArray = [2, 5, 7]
slowAdd(dataArray);

answered Dec 1, 2020 at 14:00

Boris Adamyan

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