Paths & optionality

• . : read a value out of the input
• ?. : required for optional or nullable segments
• ?? : supply a fallback for null/undefined, strips optionality
• Playground

A path reads a value out of the input: user.address.city. Because the input is typed, every segment is validated at compile time — a typo is a compile error, not a silent undefined.

Operator	Effect
.	read a value out of the input
?.	required for optional or nullable segments
??	supply a fallback for null/undefined, strips optionality

. : read a value out of the input

Every segment is validated — a typo is TF2002 with a "did you mean" suggestion, not a silent undefined.

city: user.address.city

?. : required for optional or nullable segments

If a segment may be missing (contact?:) or nullable, plain . access is refused (TF2003). Without a default, a possibly-undefined value becomes an optional field of the output type — the field is simply absent from the output JSON when its value is undefined.

email: user.contact?.email

?? : supply a fallback for null/undefined, strips optionality

The compiler also flags the opposite mistake: using ?? on a value that is never nullish is warning TF2013 — dead code.

email: user.contact?.email ?? "unknown"

Playground

Mapping✔Playground ↗

input user: {
  name: string,
  contact?: { email?: string },
}

map {
  name: user.name,
  email: user.contact?.email ?? "unknown",
}

input user: { name: string, contact?: { email?: string }, } map { name: user.name, email: user.contact?.email ?? "unknown", }

Input JSON

{ "name": "Ada", "contact": {} }

{ "name": "Ada", "contact": {} }

Output

{
  "name": "Ada",
  "email": "unknown"
}

inferred type{ name: string; email: string }

Next: Arithmetic & strings.