Summary - Promises

Combine then, Promise.all and finally to write a parallel downloader.

Summary - Promises

In this group you learned the basics of Promises. A Promise represents the future result of an async operation and has three states: pending, fulfilled and rejected. To get the result you use then, to catch errors catch, and for cleanup finally.

For parallel work, Promise.all returns a result array when all finish; Promise.race gives the result of the first to finish. These are the core tools for managing several async jobs in a single flow. In the next lesson we will write the same logic more readably with async/await.

Exercise Your turn: build a chain

Start with Promise.resolve(5). Use .then(n => n * 2) to double the value. Add .then(n => console.log(n)) to log it. Finally add .finally(() => console.log("done")).

Show example solution

Promise.resolve(5)
  .then(n => n * 2)
  .then(n => console.log(n))
  .finally(() => console.log("done"));

Exercise Your turn: race with Promise.race

Create const a = new Promise(res => setTimeout(() => res("first"), 10)) and const b = new Promise(res => setTimeout(() => res("second"), 30)). Call Promise.race([a, b]) and log the winner with .then(winner => console.log(winner)).

Show example solution

const a = new Promise(res => setTimeout(() => res("first"), 10));
const b = new Promise(res => setTimeout(() => res("second"), 30));
Promise.race([a, b]).then(winner => console.log(winner));

Test yourself

What does a finally call not change in a Promise chain?
- The error state of the chain
- The speed of the async work
- The order of the chain
- The previous Promise's value
Show answer

The previous Promise's value — finally only causes side effects; it keeps the value.
Match the Promise methods

then

runs on success
catch

catches errors
finally

runs always
Promise.all

resolves when all done

Next lesson: async function

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