Cancellable Function

Submission https://leetcode.com/problems/design-cancellable-function/submissions/1479735064/

/**
 * @param {Generator} generator
 * @return {[Function, Promise]}
 */
//IMPORTANT DETAIL: This is SYNC generator. And not ASYNC.
var cancellable = function(generator) {
    let canceled = false;
    const cancel = () => {
        canceled = true;
    }
    const promise = new Promise((resolve, reject) => {
        const next = async (err, pastValue) => {
            const handleIfDone = async (done, value) => {
                if(done){
                    try {
                        const resolvedValue = await value;
                        return resolve(resolvedValue);
                    } catch(err){
                        return reject(err);
                    }
                }
                Promise.resolve(value)
                    .then((resolvedValue) => setImmediate(next.bind(this, canceled ? 'Cancelled' : null, resolvedValue)))
                    .catch((err) => setImmediate(next.bind(this, canceled ? 'Cancelled' : err)));
            }
            try {
                const { done, value } = (err || canceled) ? generator.throw(err || 'Cancelled') : generator.next(pastValue);
                await handleIfDone(done, value);
            } catch(err){
                return reject(err);
            }
        }
        setImmediate(next);
    })
    return [cancel, promise];
}

module.exports.cancellable = cancellable;