simple generator function runner with requestAnimationFrame in javascript !

generator-runner.js

let sampleGenerator = function*() {};
let noop = () => {};

function *sleep(timer) {
    return new Promise((res, rej) => {
        setTimeout(() => {
            res();
        }, timer);
    });
}

function *fetchData(url) {
    return new Promise((res, rej) => {
        fetch(url).then((data) => res(data));
    });
}

function runMe() {
    console.log('Run !');
}


function *test() {
    yield runMe;
    yield sleep(5000);
    yield fetchData('https://jsonplaceholder.typicode.com/todos/1');
    yield sleep(2000);
    yield runMe;
    yield sleep(8000);
    yield fetchData('https://jsonplaceholder.typicode.com/posts/1');
}


function genRunner(genFunc) {
    const userGeneratorFunction = genFunc();
    let nextYield = userGeneratorFunction.next();
    let inPromise = false;
    let requestId;
    return function runner(){
        if (!inPromise && !nextYield.done) {
            if (nextYield.value.constructor == noop.constructor) {
                nextYield.value();
            }
            if (nextYield.value.constructor == sampleGenerator.prototype.constructor) {
                nextYield = nextYield.value.next();
            }
            if (nextYield.value instanceof Promise) {
                inPromise = true;
                if (requestId) {
                    cancelAnimationFrame(requestId);
                    requestId = void 0;
                }
                nextYield.value.then((data) => {
                    if (data) console.log(data);
                    if (!nextYield.done) {
                        inPromise = false;
                        requestId = window.requestAnimationFrame(runner);
                    }
                });
            } 
            nextYield = userGeneratorFunction.next();
        }
        if (!nextYield.done && !inPromise) {
            if (requestId) {
                cancelAnimationFrame(requestId);
                requestId = void 0;
            }
            requestId = window.requestAnimationFrame(runner);
        }
    }
}

const testRunner = genRunner(test);
testRunner();