FieldValue.arrayUnion() called with invalid data. Nested arrays are not supported

When trying to update an array in Cloud Firestore you might run into this error: FieldValue.arrayUnion() called with invalid data. Nested arrays are not supported.

However, that error might be a bit misleading. The following code will crash:

firebase.firestore.FieldValue.arrayUnion(['javascript', 'typescript']);

You don't have nested but your code will fail anyway because Firestore requires you to pass those items individually. All you have to do is using the spread operator instead:

firebase.firestore.FieldValue.arrayUnion(...['javascript', 'typescript']);