sync_promise.js

class SyncPromise {
    constructor( callback ) {
        try{
            this.returnValue = callback();
        }
        catch(err) {
            throw err;
        }
    }

    then(callback) {
        callback(this.returnValue);
    }
}
undefined
new SyncPromise((resolve, reject) => { return 'hello'}).then(value => console.log(value.toUpperCase()))