How to understand trampoline in JavaScript?

Question

Here is the code:

function repeat(operation, num) {
  return function() {
    if (num <= 0) return
    operation()
    return repeat(operation, --num)
  }         


        

Answer 1

The trampoline is just a technique to optimize recursion and prevent stack-overflow exceptions in languages that don't support tail call optimization like Javascript ES5 implementation and C#. However, ES6 will probably have support for tail call optimization.

The problem with regular recursion is that every recursive call adds a stack frame to the call stack, which you can visualize as a pyramid of calls. Here is a visualization of calling a factorial function recursively:

(factorial 3)
(* 3 (factorial 2))
(* 3 (* 2 (factorial 1)))
(* 3 (* 2 (* 1 (factorial 0)))) 
(* 3 (* 2 (* 1 1)))
(* 3 (* 2 1))
(* 3 2)
6

Here is a visualization of the stack where each vertical dash is a stack frame:

         ---|---
      ---|     |---
   ---|            |--- 
---                    ---

The problem is that the stack has limited size, and stacking up these stack frames can overflow the stack. Depending on the stack size, a calculation of a larger factorial would overflow the stack. That is why regular recursion in C#, Javascript etc. could be considered dangerous.

An optimal execution model would be something like a trampoline instead of a pyramid, where each recursive call is executed in place, and does not stack up on the call stack. That execution in languages supporting tail call optimization could look like:

(fact 3)
(fact-tail 3 1)
(fact-tail 2 3)
(fact-tail 1 6)
(fact-tail 0 6)
6

You can visualize the stack like a bouncing trampoline:

   ---|---   ---|---   ---|---
---      ---       ---       

This is clearly better since the stack has always only one frame, and from the visualization you can also see why it is called a trampoline. This prevents the stack from overflowing.

Since we don't have the luxury of tail call optimization in Javascript, we need to figure out a way to turn regular recursion into an optimized version that will execute in trampoline-fashion.

One obvious way is to get rid of recursion, and rewrite the code to be iterative.

When that is not possible we need a bit more complex code where instead of executing directly the recursive steps, we will utilize higher order functions to return a wrapper function instead of executing the recursive step directly, and let another function control the execution.

In your example, the repeat function wraps the regular recursive call with a function, and it returns that function instead of executing the recursive call:

function repeat(operation, num) {
    return function() {
       if (num <= 0) return
       operation()
       return repeat(operation, --num)
    }
}

The returned function is the next step of recursive execution and the trampoline is a mechanism to execute these steps in a controlled and iterative fashion in the while loop:

function trampoline(fn) {
    while(fn && typeof fn === 'function') {
        fn = fn()
    }
}

So the sole purpose of the trampoline function is to control the execution in an iterative way, and that ensures the stack to have only a single stack frame on the stack at any given time.

Using a trampoline is obviously less performant than simple recursion, since you are "blocking" the normal recursive flow, but it is much safer.

http://en.wikipedia.org/wiki/Tail_call

http://en.wikipedia.org/wiki/Trampoline_%28computing%29