Google Firebase Error(Function returned undefined, expected Promise or value)

I'm developing Server with Firebase.

I had copied Google Developer's Video on Youtube.

It works well, but on log there is an error:

>Function returned undefined, expected Promise or value

It says function returned undefined, but I make function return a promise `set``

How can I solve this?

function sanitize(s) {
    var sanitizedText = s;
    console.log('sanitize params: ', sanitizedText);
    sanitizedText = sanitizedText.replace(/\bstupid\b/ig, "wonderful");
    return sanitizedText;
}
exports.sanitizePost = functions.database
    .ref('/posts/{pushId}')
    .onWrite(event => {
        const post = event.data.val();
        if (post.sanitized) return;

        console.log('Sanitizing new post', event.params.pushId);
        console.log(post);
        post.sanitized = true;
        post.title = sanitize(post.title);
        post.body = sanitize(post.body);
        return event.data.ref.set(post); 
    })

I'm beginner of Firebase, Nodejs.

As Frank indicates in his comment on your post, the return statement that is producing the warning is this one:

if (post.sanitized) return;

The warning can be silenced by returning a dummy value (e.g. null, false, 0). The value is not used.

Earlier versions of Cloud Functions did not complain when a function exited using a return statement with no value. That explains why you see return; in the video you linked and in the documentation. The comment on the question by Firebaser Frank van Pufeelen, explains why the change was made.

The simplest way to eliminate the warning is to add a return value, as Frank suggested:

if (post.sanitized) return 0;

Another option is to change the trigger from onWrite() to onCreate(). Then the function will not be invoked when the post is sanitized and the check that produces the warning is not needed:

exports.sanitizePost = functions.database
    .ref('/test/{pushId}')
    .onCreate(event => {  // <= changed from onWrite()
        const post = event.data.val();
        //if (post.sanitized) return; // <= no longer needed

        console.log('Sanitizing new post', event.params.pushId);
        console.log(post);
        //post.sanitized = true; // <= not needed when trigger is onCreate()
        post.title = sanitize(post.title);
        post.body = sanitize(post.body);
        return event.data.ref.set(post);
    });

I was getting this same error for attempting to read a document using .get() which returns a promise.

I found out in the official Firebase YouTube tutorial that to resolve this error, I needed to return the promise line of code. Check minute 4:18 in the tutorial linked video [https://youtu.be/d9GrysWH1Lc]

Also, as a side note, what lead me to the tutorial solution is when I noticed in the function logs did actually log valid values but only after the function is closed, even though the error says the result was undefined.

Adding to what @bob-snyder said, your problem is that your returning undefined under a condition.

if (post.sanitized) return;

My suggestion is to use a single exit point, which is a common tip when programming. Article.

Example

// code...
const SUCCESS_CODE = 0;

exports.sanitizePost = functions.database
    .ref('/posts/{pushId}')
    .onWrite(event => {
        const post = event.data.val();

        let response = Promise.resolve(SUCCESS_CODE);

        if (!post.sanitized) {
            console.log('Sanitizing new post', event.params.pushId);
            console.log(post);
            post.sanitized = true;
            post.title = sanitize(post.title);
            post.body = sanitize(post.body);
            response = event.data.ref.set(post); 
        }

        return response;
    })

I noticed that when the trigger handler is an async function, the warning isn't raised even if we don't have an explicit return statement.

When the trigger handler is a sync function, we'd need an explicit return statement to silence the warning, like like it was suggested in the accepted answer.

That is, if we rewrote

exports.updatePost = functions.firestore
.document("posts/{postId}")
.onUpdate(async (snap, context) => { /* async handler*/
  ...
});

to

exports.updatePost = functions.firestore
.document("posts/{postId}")
.onUpdate((snap, context) => { /* sync handler*/
  ...
});

We'd need to do return 0; or similar inside the sync handler.

In any case, for async, it's probably a good idea to explicitly return a promise on the async handler, as advised on the official docs:

> Resolve functions that perform asynchronous processing (also known as "background functions") by returning a JavaScript promise.