15. What is optional chaining?

Optional chaining is a syntax of JavaScript, useful in checking if left side of expression equals null or undefined. If yes, right side of expression is not executed and undefined is returned.

Optional chaining can be used to acces value by key, by index or to invoke a function.

Attempt to access field from null or undefined without optional chaining will end with an error of type Cannot read property 'x' of undefined/null

const val = {a: {}};
val.a.b.c.d // Cannot read property 'c' of undefined
val.a.b?.c.d // undefined

val.a.b.c() // Cannot read property 'c' of undefined
val.a.b.c?.() // undefined


val.a.b[1] // Cannot read property '1' of undefined
val.a.b?.[1] // undefined