resolve() and reject()

new Promise(executor) calls the executor immediately and synchronously with two functions: resolve(value) fulfills the promise (or makes it follow another promise) and reject(reason) rejects it, as does a throw in the executor. Neither returns from the executor, only the first call counts, and then() handlers always run later.

The executor runs at once, and a promise settles only onceJavaScriptLive

const p = new Promise((resolve, reject) => {
  resolve("done");
  reject(new Error("ignored"));                  // only the first call counts
  console.log("1 the executor runs at once; resolve() did not return");
});
p.then(value => console.log("3 fulfilled:", value));
console.log("2 after the constructor");
new Promise(() => { throw new TypeError("bad input"); }).catch(err => console.log("4", err.name));

1 the executor runs at once; resolve() did not return
2 after the constructor
3 fulfilled: done
4 TypeError

Creating a promise starts the work, so wrap it in a function to start it later, like sleep() in Practical Asynchrony. Node's util.promisify(fn) wraps any error-first callback API.

MENU