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Practice creating your own promises

You may have used promises provided by libraries or built-in functions before. For example:

fetch("/test")
  .then((response) => console.log(response))
  .catch((error) => console.error(error));

A promise is an object with a .then method. This method takes a callback function that it will call with the result when the promise is finished (or "resolved"). You can imagine a promise object looks something like this:

// this isn't really how they work
// but it's a good mental model to start with
const promise = {
  then: (callback) => {
    // magically wait until result is ready somehow
    callback(result);
  }
  catch: (callback) => {
    // magically wait until an error happens somehow
    callback(error);
  }
}

But how do you create your own promise objects?

Creating promises

You can create your own promise objects with new Promise(). You have to pass in a function that defines when the promise will resolve or reject. This function is passed two arguments: resolve and reject. These are functions you call with the value you want to resolve/reject with.

For example:

function doSomethingAsync() {
  const promise = new Promise((resolve, reject) => {
    // do some async stuff that might error
    if (error) {
      reject(error);
    } else {
      resolve(result);
    }
  });
  return promise;
}

You could use the above just like any other promise-returning function:

doSomethingAsync()
  .then((result) => console.log(result))
  .catch((error) => console.error(error));

Challenge one

You're going to create a promisified version of setTimeout, called wait. It should take a number of millliseconds to wait as an argument, set a timeout for that long, then resolve the promise.

It should be usable like this:

wait(1000).then(() => console.log("done"));
// (after 1000ms) Logs: "done"

You can run the tests to check if your solution works:

npm run test:one

Challenge two

You're going to create your own promisified wrapper of Node's fs.readFile method. It usually takes a callback to be run when it finishes its asynchronous task.

Implement the readFilePromise function so that it returns a new promise. It should use fs.readFile to read whatever file path is passed in, then resolve with the result. It should reject with any error that occurred. For example:

readFilePromise("./test.txt")
  .then((contents) => console.log(contents))
  .catch((error) => console.error(error));

You can run the tests to check if your solution works:

npm run test:two