Why Do Recursive Functions Sometimes Return Undefined?

Recursive Function Returns Undefined

A difficulty arises when working with recursive functions: they can sometimes return an unexpected value of undefined. This issue can be encountered in scenarios like this:

function calculateTaxes(tax, taxWage) {
  const minWage = firstTier;

  if (taxWage > minWage) {
    tax += calculateDifference(taxWage) * calculateTaxStep(taxWage);
    const newSalary = taxWage - calculateDifference(taxWage);
    calculateTaxes(tax, newSalary);
  } else {
    return tax + taxWage * calculateTaxStep(taxWage);
  }
}

Here, it's evident that the function keeps calling itself without returning a value in the recursive branch. This omission results in the function returning undefined.

To resolve this issue, it's crucial to ensure that the recursive call returns a value or sets a return variable. The following code demonstrates the intended behavior:

function calculateTaxes(tax, taxWage) {
  const minWage = firstTier;

  if (taxWage > minWage) {
    tax += calculateDifference(taxWage) * calculateTaxStep(taxWage);
    const newSalary = taxWage - calculateDifference(taxWage);
    return calculateTaxes(tax, newSalary); 
  } else {
    return tax + taxWage * calculateTaxStep(taxWage);
  }
}

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