then and catch
- Node.js course
- Beginner
- Lesson 89/116
- 5 min
Learn promise chaining with .then and .catch.
Get the result with then
To obtain a Promise's result you use the then method. then(value => {...}) runs when the Promise is fulfilled and processes the incoming value. Since then itself returns a Promise, you can chain calls: the value returned by one then becomes the input of the next.
p.then(value => console.log(value));If you want to handle errors separately you can pass a second function: then(value => {...}, err => {...}). But the more readable way is to catch errors with catch.
Catch errors with catch
To catch a rejected Promise's error you use catch. catch(err => {...}) catches errors from anywhere in the chain, so it is commonly placed at the end.
p
.then(value => value.toUpperCase())
.catch(err => console.error(err));catch is actually a shortcut for then(null, err => {...}). After an error is caught, catch can return a normal value, so the chain recovers and the next then runs.
Practice Your turn: a then and catch chain
Create a Promise with new Promise((resolve, reject) => resolve("hello")). Add .then(value => console.log(value)) to log the result. Add .catch(err => console.error(err)) to catch any error.
Starter code
// Create a Promise and resolve it
// Log the result with .then, catch errors with .catchSample solution
new Promise((resolve, reject) => resolve("hello"))
.then(value => console.log(value))
.catch(err => console.error(err));Expected output
hello
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Fill in the blank: catch the error
p.then(value => console.log(value))
.____(err => console.error(err));p.then(value => console.log(value))
.catch(err => console.error(err));Explanation: catch runs when a Promise is rejected.
2 When does catch run in a Promise chain?
- When any Promise in the chain is rejected Correct answer
- When the Promise is fulfilled
- Only when then throws
- In every case
Explanation: catch runs after the first rejected Promise in the chain.
3 Match each method with its job
| Term | Meaning |
|---|---|
then |
runs on success |
catch |
catches errors |
resolve |
signals success |