All Promise instances accept a method as an argument called the executor. This executor takes two methods as arguments: resolve and reject. Within the executor, if resolve is called, the Promise instance becomes fulfilled. If an exception is thrown, reject is called instead, and the Promise instance becomes rejected.

const executor = (resolve, reject) => {
  setTimeout(() => resolve("I'm done"), 1000);

new Promise(executor).then(result => {
  // Output after 1000ms: I'm done