Lesson 05 · 12 min
Unions and narrowing
The main TypeScript skill. Everything else derives from it.
12 minLesson 5 of 10Practice includedAsk AI on any point
A union means "one of". After string | number TS won't let you call .toUpperCase() until you prove it's a string. That's narrowing: narrowing a type with a check. A method has to exist on every member of the union — otherwise branch first.
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