Avoid to overfill the stack, part 2

recursiveTransformString.js

function transformString (source){
    if (source.length === 0)
        return "";
    if (source.length === 1)
        return source.toUpperCase() + source;

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

let st = "helloworld";

console.log(transformString(st));

function transformStringWithTimeout (source, counter = 0, postActions, resFn){
    let curItem;
    let postAction = transformed =>  curItem.toUpperCase() + transformed + curItem;
    
    //initial call	
	if (resFn === undefined){
        var pr = new Promise((res, rej) => resFn = res);
        postActions = [];
	}

    //end recursion
    if (source.length === 1){
        let initValue = source.toUpperCase() + source;
        //using reduceRight to run the stack of actions
        let result = postActions.reduceRight((accum, fn) => fn(accum), initValue);
        resFn(result);
    }
	else{
        curItem = source[0];
        postActions.push(postAction);
		if (counter < 5){
			transformStringWithTimeout(source.substring(1), ++counter, postActions, resFn);
		}
		else{
            //let's release the stack
            setTimeout(() => {
                console.log("timeout handling");
                transformStringWithTimeout(source.substring(1), 0, postActions, resFn);
            }, 0);
            
		}
	}
	return pr;
}

(async () => {
    console.log("-- with timeout");
    let result = await transformStringWithTimeout("helloworld");
    console.log(result);
})();