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Session Plan

Session Materials

These are some examples of previously created materials by mentors that you can use yourself, or for inspiration.

Session Outline

Promises is notoriously difficult to teach! I teach consumption and creation of promises totally separate! And show them that it's just like with functions. There is a creation part and a consumption part.

First when they fully understand one part of promises, I move on! Don't over-complicate things. Only mention the resolve function to begin with. When they get that, say that there also is a reject function. Take as many baby steps as is possible! Don't mention that resolve and reject can take an argument to begin with, first later explain that.

Exercises

Exercise 1

Using async await

  1. fetch yes or no from this api: https://yesno.wtf/api. log out the answer

Exercise 2

Using promises

  1. fetch yes or no from this api: https://yesno.wtf/api. log out the answer
  2. Try fetching a url that rejects e.g. https://knajskdskj.jasdk. Log out the error message

Exercise 3

  1. Create a promise that resolves after 4 seconds. Use this promise to log out the text 'hello' after 4 seconds.
  2. Now make the promise fail by rejecting it with an error message instead of resolving it, and log the error message to the console.

Exercise 4

Create a function that returns a promise, that you can use like this:

// YesNoFail4Seconds should wait 4 seconds before it does one of the following 3 things:
// resolves with a yes
// resolves with a no
// or rejects
// Look into Math.random()
YesNoFail4Seconds()
  .then((data) => {
    console.log(`The answer is ${data}`);
  })
  .catch((error) => {
    console.log(error);
  });

The above example show how to consume the promise using promises. Now try consume the YesNoFail4Seconds using async/await

Exercise 5

Using async await

  1. Fetch the astronauts
  2. After the astronauts has been fetched, fetch movies using this api
  3. Log out the movies

Exercise 6

Get the astronauts and the movies at the same time. Log out the movies and the battery status when both promises has been resolved.

Code inspiration

Async/await - simple usage

// Warm up exercise. The trainees has to say everything they can about a variable, ONLY from the variable name. e.g. the type, what it returns, what object we could expect etc.
// cars, car, title, getTitle, addTitle, isTitle, hasTitle, hasItem, users, year, yearIndex, user, review.
// DON'T EXPLAIN WHAT ASYNC OR AWAIT DOES YET! Explain on a higher level:
// You have to write async before a function for await to work. No details for now
// await waits until we have fetched the data from the api. Or said in another way await waits until fetch has resolved with the data from the api

// write async before a function for await to work. What does it mean that something is asynchronous?
async function getAstronauts() {
  // await waits until we have data from fetch before it runs the next line. No need for callbacks 🤯
  console.log("Before we fetch data");
  const astronautsResponse = await fetch(
    "http://api.open-notify.org/astros.json",
  );
  console.log(
    "This is logged out after some time, even though the code looks synchronous! 🤯",
  );
  const astronauts = await astronautsResponse.json();
  console.log("This is logged out after some time! 🤯");
  console.log(astronauts);
  return astronauts;
}

getAstronauts();

Promise consumption

So how did the async/await example above actually work? Let's get into promises!

If you have a promise, you can call two functions on that promise. '.then' and '.catch'. When are these functions called? What does it mean that a promise is resolved or rejected?

The trainees should be able to answer these questions: // Question 1: What does it mean that a promise is resolved? Which method on a promise get called? // Question 2: What does it mean that a promise is rejected? Which method on a promise get called? // How would you explain your mom what resolved and rejected means?

fetch("http://api.open-notify.org/astros.json")
  .then((astronautsResponse) => astronautsResponse.json())
  .then((astronauts) => {
    console.log(astronauts);
  })
  .catch((error) => console.log(error));

// https://codesandbox.io/s/scrollto-promise-example-0gjp6
// If not working try chrome
scrollTo("section.features")
  .then(() => console.log("scrolling done"))
  .catch((error) => console.log(error));

// HAMMER in this point:
// When you have a promise you can call two functions on that promise (.then and .catch). '.then' is called when the promise is resolved. '.catch' is called when the promise is rejected.

Promise creation

When you create a new promise you give it a function that has two functions as parameters (resolve and reject). Resolve is called when everything in a promise goes well. Reject is called when something goes wrong.

// Start as simple as possible, no reject, just resolve!
const oneSecondTimeoutPromise = new Promise((resolve) => {
  setTimeout(() => {
    resolve();
  }, 1000);
});

// You can pass data in the resolve
const oneSecondTimeoutPromise = new Promise((resolve) => {
  setTimeout(() => {
    resolve("1 second has passed");
  }, 1000);
});

oneSecondTimeoutPromise.then((timeoutData) => {
  console.log(timeoutData); // '1 second has passed'
});

const orderPizzaPromise = new Promise((resolve, reject) => {
  const pizzaMakingTime = 10000;
  const didPizzaBakingSucceed = true;
  const pizza = "Macaroni pizza";
  setTimeout(() => {
    if (didPizzaBakingSucceed) {
      resolve(pizza);
    } else {
      reject("The pizza was a mess");
    }
  }, pizzaMakingTime);
});

orderPizzaPromise
  .then((pizza) => {
    console.log(`Let's eat the ${pizza}`);
  })
  .catch((error) => {
    console.log(`Let's eat the nothing`);
  });

// HAMMER in this point:
// When you create a new promise you give it a function that has two functions as parameters (resolve and reject). Resolve is called when everything in a promise goes well. Reject is called when something goes wrong.

// Compare function creation and consumption to promise creation and consumption
// function creation
function test() {}

// function usage
console.log(test());

Back to async/await

So writing async in front of a function makes it return a promise! The keyword await makes JavaScript wait until that promise resolved and returns its result.

async function getAstronauts() {
  try {
    const astronautsResponse = await fetch(
      "http://api.open-notify.org/astros.json",
    );
    const astronauts = await astronautsResponse.json();
    return astronauts;
  } catch (err) {
    throw "Fetching the astronauts went wrong";
  }
}

const astronauts = getAstronauts();

Function that returns a promise

// This example could definitely be more real world! Any ideas, make a pull request!
const promise = new Promise((resolve) => {
  setTimeout(() => {
    const tea = {
      color: "green",
      taste: "Bitter",
    };

    resolve(tea);
  }, 3000);
});

const isThereMoreTea = false;

// This example could definitely be more real world! Any ideas, make a pull request!
function makeTea() {
  console.log("Start making tea");

  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const tea = {
        color: "green",
        taste: "Bitter",
      };

      if (isThereMoreTea) {
        resolve(tea);
      } else {
        reject("We don't have more TEA!!");
      }
    }, 3000);
  });
}

console.log(makeTea());

makeTea()
  .then((returnedTeaObject) => {
    console.log(returnedTeaObject);
  })
  .catch((error) => {
    console.log(error);
  });