Type Guard ↔ Runtime Type-Check to Narrow Type of Variable
aka Runtime-Type-Check, Runtime-Check
Type Guard used when Type too general, requires Narrow
typeof & instanceof are JS Operator that act as Type Guard
Equality Operator can be used as Type Guard to check for specific value
can only use === & !== to exclude Union Type if component is Singleton
Narrow
to reduce static type T to a (narrowed down) subset of T
Assertion Function
Type Predicate
Union Type can be too general & require Type Guard for Narrow
nullish parameters type can be too generaltsxfunction f(arg: string|null) {}
function f(arg: string|undefined) {}