What does catch() do in promises?

JavaScriptJunior~2 min read

1. What `.catch()` does

.catch() is a promise method that is called if the promise moves into the "rejected" state. That is, it catches every error that occurred:

inside the promise,

inside a .then() chain,

or from an explicit reject().

2. Syntax

javascript

promise.catch(onRejected)

where:

onRejected is a function that receives the error object.

3. Example: explicitly rejecting a promise

javascript

const promise = new Promise((resolve, reject) => {
  reject("Error loading data");
});

promise.catch(error => {
  console.log("Caught:", error);
});

Output:

javascript

Caught: Error loading data

.catch() fired because the promise was rejected (reject).

4. Example: an error in the code automatically triggers `.catch()`

javascript

const promise = new Promise(() => {
  throw new Error("Something went wrong");
});

promise.catch(err => {
  console.log("Caught the error:", err.message);
});

Output:

javascript

Caught the error: Something went wrong

Any error inside the executor function automatically triggers reject.

5. Example: handling errors in a `.then()` chain

javascript

Promise.resolve()
  .then(() => {
    throw new Error("Error in then");
  })
  .catch(error => {
    console.log("Caught:", error.message);
  });

Output:

javascript

Caught: Error in then

.catch() intercepts errors even if they occurred in one of the earlier .then() calls.

6. `.catch()` = `.then(null, onRejected)`

These two variants are fully equivalent:

javascript

promise.catch(onRejected);
promise.then(null, onRejected);

Both handle a rejected promise. But .catch() reads more simply and is used more often.

7. Example: returning a value from `.catch()`

If .catch() returns a value, it "recovers" the chain, and the next .then() receives that value as its result.

javascript

Promise.reject("Error")
  .catch(err => {
    console.log("Caught:", err);
    return "Continuing the work";
  })
  .then(result => {
    console.log("Result:", result);
  });

Output:

javascript

Caught: Error
Result: Continuing the work

.catch() can not only catch errors, but also "heal" the chain.

8. Example: re-throwing an error

If .catch() throws again (throw), the error is passed on to the next .catch() in the chain.

javascript

Promise.reject("Error 1")
  .catch(err => {
    console.log("Caught:", err);
    throw new Error("Error 2");
  })
  .catch(err => {
    console.log("Caught again:", err.message);
  });

Output:

javascript

Caught: Error 1
Caught again: Error 2

9. Example: `.catch()` in async functions

.catch() works the same as try...catch, but as a promise chain.

javascript

async function getData() {
  throw new Error("Error in async");
}

getData().catch(err => {
  console.log("Caught:", err.message);
});

Output:

javascript

Caught: Error in async

10. Example with `try...catch` for comparison

javascript

async function example() {
  try {
    await Promise.reject(new Error("Error in the promise"));
  } catch (err) {
    console.log("Caught via try...catch:", err.message);
  }
}
example();

The equivalent of .catch(), but in async/await syntax.

Summary

Feature	Description
When it fires	When the promise is rejected (`reject` or `throw`)
Equivalent to	`then(null, onRejected)`
Returns	A new promise
Can recover the chain	If a value is returned from `.catch()`
Can pass the error on	If `throw` is used again inside `.catch()`
Works with async/await	The equivalent of `try...catch`

Quick comparison:

Situation	What to use
Handling an error in a promise chain	`.catch()`
Handling errors in `async/await`	`try...catch`
Need to continue the chain after an error	return a value from `.catch()`
Need to pass the error further	`throw` inside `.catch()`

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