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A rejected Promise stays rejected through each chained .then() that has no rejection handler. A .catch() handles the rejection at its own position: if its handler returns normally, the Promise created by that catch fulfills; if it throws or returns a rejected Promise, the rejection continues downstream. Each link creates a new Promise, so it helps to track the state of each one rather than think of an error changing the original Promise.
Each .then() creates a new Promise
Calling .then(onFulfilled, onRejected) does not change the Promise it is called on. It returns a derived Promise, and that new Promise’s outcome depends on which handler runs and what the handler does. This is the key to tracing rejection through a chain. MDN’s then() reference describes the returned Promise and its behavior.
If the source Promise is rejected and the .then() call has no callable rejection handler, the derived Promise is rejected with the same reason. A subsequent fulfillment-only .then() is skipped too, and its derived Promise remains rejected. The rejection is not moving backward or modifying earlier Promises; each link is producing a new result.
const p0 = Promise.reject(new Error("original");
const p1 = p0.then(value => value); // rejected with the same reason
const p2 = p1.then(value => value); // still rejected with that reason
const p3 = p2.catch(error => {
console.error(error);
return "fallback";
});
p3.then(value => console.log(value)); // logs "fallback"Here the fulfillment callbacks in the first two links do not run. The catch handles the rejection reaching p2 and returns a regular value, so p3 fulfills. The original p0, and the intermediate Promises, remain rejected.
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What a rejection handler returns determines what follows
.catch(handler) behaves like .then(undefined, handler): it installs a rejection handler and returns another Promise. A handler can recover, or it can leave the derived Promise rejected. MDN’s catch() reference covers this returned-Promise behavior.
| What the rejection handler does | Outcome of the Promise returned by that link | What a later .then() sees |
|---|---|---|
| Returns a regular value | Fulfilled with that value | The fulfillment handler can run with the value |
| Returns nothing | Fulfilled with undefined |
The fulfillment handler can run with undefined |
| Throws an error or other value | Rejected with the thrown value | A fulfillment-only handler is skipped; a later rejection handler can catch it |
| Returns a Promise or thenable | Adopts that result’s eventual state and value or reason | Depends on whether the adopted result fulfills or rejects |
For example, to log a failure but keep it as a failure, throw it again:
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fetchData()
.catch(error => {
console.error("Could not load data", error);
throw error;
})
.catch(showError);The first catch’s returned Promise is rejected with the thrown value, so the later catch can handle it. Returning Promise.reject(error) has the same broad effect on the chain: the Promise returned by the catch adopts a rejected result. By contrast, returning a fallback value is recovery, even if the handler’s purpose was primarily to log or report the problem.
Return nested asynchronous work to connect it to the chain
When a handler starts asynchronous work, return its Promise if later steps should wait for it or handle its rejection. A returned Promise or thenable is adopted by the Promise created for that handler. MDN’s promise guide explains why an unreturned Promise can become “floating” work.
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fetchData()
.then(data => {
return saveData(data); // downstream waits for saveData
})
.catch(handleError);If the callback instead calls saveData(data) without returning it, the callback completes normally with undefined. The outer chain can then fulfill and continue before saveData settles. A rejection from that unreturned operation is not conveyed by the outer chain, so the catch shown here is not connected to it.
Separate calls on one Promise create independent branches
Chaining from a prior result and attaching multiple handlers to the same source are different structures. Each call to .then() or .catch() returns its own derived Promise. A handler on one branch does not handle the derived Promise in another branch.
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const source = Promise.reject(new Error("failure"));
const recoveredBranch = source.catch(() => "fallback");
const stillRejectedBranch = source.then(value => value);recoveredBranch fulfills with "fallback". stillRejectedBranch remains rejected because its fulfillment-only handler does not handle the source’s rejection. Handling one branch does not automatically handle another branch’s rejection.
A reliable way to trace a nested chain
Name the original Promise and each derived Promise, then follow the selected callback one link at a time. This makes it clear which catch is connected to which result.
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- Record the current state. For each Promise, note whether it is fulfilled or rejected and its value or rejection reason.
- Identify the selected handler. A fulfilled Promise selects the fulfillment handler; a rejected Promise selects a callable rejection handler. If the relevant handler is missing or not callable, that state and value or reason pass to the derived Promise.
- Record the callback’s result. A normal return fulfills the derived Promise with the returned value, including
undefined. A throw rejects it. A returned Promise or thenable is adopted. - Follow only the connected chain. Check whether asynchronous work was returned and whether a handler is chained from the preceding derived Promise or attached as a separate branch.
This method reflects the rules documented in MDN’s Promise reference and then() reference: handler completion determines the outcome of the Promise returned by that call, not the state of the source Promise.
Unhandled-rejection reports are runtime notifications
Promise propagation rules are distinct from how an environment reports a rejection that remains without a handler at its relevant check. In browsers, MDN documents the unhandledrejection event and the rejectionhandled event for a handler attached after an unhandled notification. Node.js uses a process-level unhandledRejection event. Their names and runtime contracts differ, so check the documentation for the specific browser or Node.js version in use. These notifications can help with diagnosis, but they do not change which Promise a catch handles or connect an unreturned Promise to a chain.
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