How does Promise chain work when one callback doesn't return any promise?

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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

When a .then() lacks the appropriate function that returns a Promise object, processing simply continues to the next link of the chain. Therefore, a chain can safely omit every handleRejection until the final .catch(). Similarly, .catch() is really just a .then() without a slot for handleFulfilled.

Considering:

The methods promise.then(), promise.catch(), and promise.finally() are used to associate further action with a promise that becomes settled. These methods also return a newly generated promise object, which can optionally be used for chaining;

How does Promise chain work when one callback doesn't return any promise?

promise node.js javascript
1 Answers

If the promise chain returns a promise, it will call the next then (or catch) with the resolved (or rejected) value of that promise. If it doesn't, then it'll just call the next then with the returned value (or undefined if nothing was returned):

const myPromise = myApiCall().then(response => {
   return anotherApiCallThatReturnsAPromise(response);
}).then(secondResponse => {
   // this is the resolved value of anotherApiCall...
   return secondResponse;
}).catch(err => {
 // this error could be because myApiCall failed or because anotherApiCall... failed
})

const myPromise = myApiCall().then(response => {
   return 42
}).then(value => {
   // value is 42
}).catch(err => {
 // this error, if present, is because myApiCall failed
})

Extrapolating, let's return nothing from the first callback:

const myPromise = myApiCall().then(response => {
   // notice we do not return anything here
   // we just call a function - i.e. return undefined
   doSomeWorkAndReturnNothing();
}).then(value => {
   // value is undefined because nothing was returned from the previous `then`
}).catch(err => {
 // this error, if present, is because myApiCall failed
})

Just for fun, here's what is looks like using await in an async function:

const myFunc = async () => {
   try {
     const response = await myApiCall();
     const secondResponse = await anotherApiCallThatReturnsAPromise(response);
     return secondResponse;
   } catch(err) {
     // this error could be because either of the previous two await-ed calls failed
   }

}
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