A promise can be explicitly created using the Promise constructor. The constructor takes only one argument, a callback function taking 2 arguments, resolve() and reject().
Inside the callback, you can perform any operations (both synchronous and asynchronous) you meed. Then, if everything went well, you call resolve() as a signal of success. If anything goes wrong inside the callback, you call reject() to indicate that the promise was not kept.
Read more at: https://imelgrat.me/javascript/javascript-promises-improvement-developers