Skip to the content
Start

Promises

JavaScript · Unit 18: Asynchronous code

A setTimeout lets you know, but it hands you no data. For data that is on its way there's something better: a promise. A promise is a value that isn't there yet, with a then to say what to do when it lands.

fetch(address) asks for data and returns a promise. In the app the data is practice data: fetch answers a few fixed addresses the engine carries, always the same. That practice server is Mexican, so its paths and its keys are in Spanish: /clima is the weather and grados is the degrees.

const api = "https://api.bybecode.dev";
fetch(api + "/clima").then((r) => {
  console.log(r.status);
});
console.log("request sent");

Prints

request sent
200

The rest of the explanation is in the lesson, which is part of the full plan.

Exercises in this lesson

You do them in the app, which checks them on the spot and explains why.

  1. 1. Predict the output

    What does this code print?

  2. 2. Predict the output

    The prices are { cafe: 45, pan: 12, leche: 28 }. What does it print?

  3. 3. Predict the output

    The weather answers { ciudad: 'Puebla', grados: 21, llueve: false }. What does it print?

  4. 4. Complete the code

    Fill in to print the city from the weather.

  5. 5. Predict the output

    What does this code print?

  6. 6. Predict the output

    The address /secreto answers with status 401, which means "no permission". What does it print?

  7. 7. Find the bug

    It should print the object with the prices. Which line has the error?

  8. 8. Put the lines in order

    Put it in order to print how many days parcel 9 has left (dias).

  9. 9. Predict the output

    Parcel 9 is already delivered and answers fast; parcel 7 is on the way and takes longer. What does it print?

  10. 10. Multiple choice

    What does this code do?

Do this lesson

It opens in your browser. This lesson is part of the full plan; the first unit of each course is free.

See all lessons →