JavaScript: In a promise chain, what happens if one function in the middle returns nothing but still performs a async operation?

Question

When chaining promises in Javascript, what happens if one function in the middle of the chain does not return anything, but still performs a async operation that needs to be completed before moving on?

Does the next 'then()' wait for the previous async function to resolve even though it is not passed any values?

EDIT: here's a application example:

connectDatabase()
  .then(() => createTables())
  .then(() => fillTables())
  .then(() => selectTables)
  .then(tables => showTables());

This code simply represents the creation of a SQLite database and some further operations to finally display the table values.

connectDatabase() is a async function. Once the database is created, the function doesn't need to return anything since the database is stored in some global function (for use outside the promise chain). The next step doesn't need any arguments although it needs the database to be created to insert tables in it.

The createTables() function creates some tables asynchronously and once it's done, it returns nothing and the next function can be called.

selectTables() just uses the sql SELECT command to store the table values in a variable and returns it to the next function.

showTables() uses the table values that's passed on to it and displays on the screen.

Depends how it is written. Please provide an example as per [mcve] — charlietfl, Dec 15 '18 at 18:59
It moves on to the next `then` in the chain right away, it doesn't wait for your internal async operation. — Lennholm, Dec 15 '18 at 19:01
@Lennholm but if it just returns `return`, and not `return response`, is the behavior the same as pointed by you? — Rodrigo Silva, Dec 15 '18 at 19:16
@RodrigoSilva If the `then` callback returns a promise, the next `then` in the chain will wait, otherwise it won't wait. But it doesn't sound like you're actually talking about what gets returned by the `then` callback but rather about what gets resolved from the async operation initiated inside the `then` callback. That doesn't affect the chain in that way, it's all dependent on what the `then` callback **itself** returns. — Lennholm, Dec 15 '18 at 19:30
All the callback function in your example *do* return something? — Bergi, Dec 15 '18 at 20:03
Just think of callbacks: Does the application of a callback depends on arguments passed? No. You can invoke a callback without any. Its exactly the same with promises. — , Dec 15 '18 at 20:45

corecase · Accepted Answer · 2018-12-15T19:48:46.000

If a function in the promise chain resolves a value, then the following function in the chain will have access to the resolved value. For example, in your example if createTables resolved a value, then that resolved value would be available to the next function in the chain as an argument (in this case, (resolvedValue) => fillTables()).

If it doesn't resolve a value (just calls resolve() with no arguments, or simply doesn't return a value in the case of an async function), then the following promise in the chain will simply not receive any arguments.

So, yes the next then will always wait for the previous promise to resolve (whether it resolves with a value or not).

EDIT: Removed/corrected the first portion of my answer based on Lennholm's feedback (thank you!). then will by default return a promise, so that will always be then-able. If the callback function passed to then returns a promise, then then's promise will resolve to the value that the callback resolves; if the callback doesn't return a promise, then then will simply resolve the value that the callback returns.

For more info on this, checkout the following answer: https://stackoverflow.com/a/46142909/815086

Not entirely accurate, you **can** return something *non-then-able* from a callback in a promise chain, in which case it will simply pass this value to the next `then` right away. It's used frequently to do some kind of post-processing of the resolved value before returning the final value. — Lennholm, Dec 15 '18 at 19:36

JavaScript: In a promise chain, what happens if one function in the middle returns nothing but still performs a async operation?

1 Answers1