"Cannot convert undefined or null to object" when using getServerSideProps

[Vista1nik]

NextJS throws "Cannot convert undefined or null to object" error when getServerSideProps used.
But console.log works and returns "stuff" array

export function getServerSideProps() {
    firebase.firestore().collection('stuff').get()
    .then(docs => {
      let stuff = [];
      docs.forEach(doc => {
        stuff.push({
          id: doc.id,
          ...doc.data()
        })
        if (docs.size === stuff.length) {
          console.log(stuff)
          return {
            props: {
              stuff: stuff
            }
          }
        }
      })
    })
}

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. Paste code from description into a page.

Expected behavior

getServerSideProps will pass "stuff" array to page props.

System information

• OS: Fedora
• Version of Next.js: 9.3.6
• Version of Node.js: 12.16

[justincy]

The problem here is that you're not actually returning anything from getServerSideProps(). The only return in the code snippet is inside of forEach(). You want something like this:

export async function getServerSideProps() {
    return firebase.firestore().collection('stuff').get()
    .then(docs => {
      let stuff = [];
      docs.forEach(doc => {
        stuff.push({
          id: doc.id,
          ...doc.data()
        })
      })
      return { props:{ stuff }};
    })
}

or

export async function getServerSideProps() {
    const docs = await firebase.firestore().collection('stuff').get()
    let stuff = [];
    docs.forEach(doc => {
      stuff.push({
        id: doc.id,
        ...doc.data()
      })
    })
    return { props:{ stuff }};
}

[Vista1nik]

Thanks!