# Loops by Recursive and Tail Recursive Functions

As you already know, for the sake of immutability you can’t process data using for loops and while loops. So we have recursive functions to rescue.

## non-recursive (where immutability isn’t a concern) #

``````function sum(numbers) {
var total = 0;
for (var i = numbers.length - 1; i >= 0; i--) {
total += numbers[i];
}
}
``````

It’s a procedural code with mutations (over `total`).

## recursive to rescue #

``````function sum(numbers) {
if(numbers.length == 0) {
return 0;
}
return numbers[0] + sum(numbers.slice(1));
}
``````

this is the recursive version. there’s no mutation, but we are making a call stack as below which uses extra memory.

sum([10, 5, 6, 7]);

10 + sum([5, 6, 7]);

10 + 5 + sum([6, 7]);

10 + 5 + 6 + sum([7]);

10 + 5 + 6 + 7 + sum([]);

10 + 5 + 6 + 7 + 0;

## tail recursive to optimize #

``````function sum(numbers) {
return tail_sum(numbers, 0);
}

function tail_sum(numbers, acc) {
if(numbers.length == 0) {
return acc;
}
return tail_sum(numbers.slice(1), acc + numbers[0]);
}
``````

in the tail recursive version, function return value does not need to wait till the end to do it its calculations, so there’s no huge stack here; only two levels.

sum([10, 5, 6, 7]);

tail_sum([10, 5, 6, 7], 0);

tail_sum([5, 6, 7], 10);

tail_sum([6, 7], 15);

tail_sum([7], 21);

tail_sum([], 28);

28;