Trampoline factory to use with Non Tail Recursion

trampolineFactory.js

let st = "helloworld";

//normal recursive (non tail recursion) function
function recursiveTransformString (source){
    if (source.length === 0)
        return "";
    if (source.length === 1)
        return source.toUpperCase() + source;

    
    let curItem = source[0];
    let transformed = recursiveTransformString(source.substring(1));
    return curItem.toUpperCase() + transformed + curItem;
}

console.log(recursiveTransformString(st));
//HELLOWORLDdlrowolleh

//the above function adapted to be used in a trampoline
function transformStringAdaptedForTrampoline (source){
    if (source.length === 0)
        return [""];
    if (source.length === 1)
        return [source.toUpperCase() + source];
    
    let curItem = source[0];
    let nextRecursion = () => transformStringAdaptedForTrampoline(source.substring(1));
    let postAction = (result) => curItem.toUpperCase() + result + curItem;
    return [undefined, nextRecursion, postAction];
}

//trampoline factory
function createTrampolineWithPostAction(fn){
    return (...args) => {
        let postActions = [];
        let [result, nextRecursion, postAction] = fn(...args);
        while (result === undefined){
            postActions.push(postAction);
            [result, nextRecursion, postAction] = nextRecursion();
        }

        while(postActions.length){
            result = postActions.pop()(result);
        }
        return result;
    }
}

let trampolined = createTrampolineWithPostAction(transformStringAdaptedForTrampoline);

console.log(trampolined(st));
//HELLOWORLDdlrowolleh