How do I keep setState() from stringifying my array?

I have a function, eventQuery which fetches data from a Firebase DB. I am able to see the data is successfully pulled and returns what I understand to be an array of arrays each composed of a string and an object. First, my code:

  const [activeDate, setActiveDate] = useState(new Date())
  const [existingEvents, setExistingEvents] = useState({})

  useEffect(() => {
    eventQuery(activeDate)
      .then(data => {
        setExistingEvents(data);
        console.log(data)
      })
      .then(setExistingEvents(''))
      .catch(err => console.log('no events'))
  }, [activeDate])

  useEffect(() => {
    console.log('existingEvents equals: ' + existingEvents)
  }, [existingEvents])

The output of console.log(data) within the promise yields something in the form of:

[["String1.1", {"String1.2": "String1.3", "String1.4": "String1.5", "String1.6": "String1.7"], ["String2.1", {etc.}], ["String3.1", {etc.}]]

However, when I use setExistingEvents(data) to pass that array of arrays to existingEvents, and then I subsequently console log the new value, the result is:

existingEvents equals: String1.1,[object Object],String2.1,[object Object],String3.1,[object Object]

As far as I can tell, setState() is essentially running toString() on my array of arrays. Am I doing something wrong or is this an inherent part of the function? Thanks so much for your help.

Your console.log statement is actually the reason why the output looks like existingEvents was stringified (setState doesn't stringify the argument).

Instead of:

  useEffect(() => {
    // `existingEvents` is coerced to a string
    console.log('existingEvents equals: ' + existingEvents)
  }, [existingEvents])

It should be:

  useEffect(() => {
    console.log('existingEvents equals:', existingEvents)
  }, [existingEvents])

Also, your second .then() function should be:

.then(() => setExistingEvents('')) // not .then(setExistingEvents('')) <- this causes setExistingEvents('') to be called immediately

Your problem is you're using a string append on this

console.log('existingEvents equals: ' + existingEvents)

If you want to have entire data in a string format, you can use JSON.stringify

console.log('existingEvents equals: ' + JSON.stringify(existingEvents))

Or use a separator , in console.log

console.log('existingEvents equals: ', existingEvents)