What does double question mark mean in javascript / typescript?

Question

I saw this code but did not understand what this symbole means in Javascript or typescript:

const _apolloClient = apolloClient ?? createApolloClient(context);

Answer 1

Short answer for lazy people like me:

If firstValue is null or undefined

console.log(firstValue ?? secondValue) // secondValue

If firstValue isn't null or undefined

console.log(firstValue ?? secondValue) // firstValue