then and catch

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.

Exercise 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.

Show example solution

new Promise((resolve, reject) => resolve("hello"))
  .then(value => console.log(value))
  .catch(err => console.error(err));

Test yourself

Fill in the blank: catch the error

p.then(value => console.log(value))
 .____(err => console.error(err));

Show answer

p.then(value => console.log(value))
 .catch(err => console.error(err));

When does catch run in a Promise chain?
- When any Promise in the chain is rejected
- When the Promise is fulfilled
- Only when then throws
- In every case
Show answer

When any Promise in the chain is rejected — catch runs after the first rejected Promise in the chain.
Match each method with its job

then

runs on success
catch

catches errors
resolve

signals success

Next lesson: Promise.all

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