Question

What is a Proper Tail Call in Javascript?

Answer 1

First, we should know about tail call before talking about "Proper Tail Call". A tail call is a subroutine or function call performed as the final action of a calling function. Whereas Proper tail call(PTC) is a technique where the program or code will not create additional stack frames for a recursion when the function call is a tail call.

For example, the below classic or head recursion of factorial function relies on stack for each step. Each step need to be processed upto n * factorial(n - 1)

function factorial(n) {
  if (n === 0) {
    return 1;
  }
  return n * factorial(n - 1);
}
console.log(factorial(5)); //120

But if you use Tail recursion functions, they keep passing all the necessary data it needs down the recursion without relying on the stack.

function factorial(n, acc = 1) {
  if (n === 0) {
    return acc;
  }
  return factorial(n - 1, n * acc);
}
console.log(factorial(5)); //120

The above pattern returns the same output as the first one. But the accumulator keeps track of total as an argument without using stack memory on recursive calls.