Exhaustive Typechecking in Typescript
Written 1 week ago
Consider an enum:
enum Color {
Red,
Blue,
Green,
}
that has a switch case:
function whichColor(c: Color): string {
switch (c) {
case Color.Red:
return "Red"
case Color.Blue:
return "Blue"
default:
return "Unknown color"
}
}
where we missed Color.Green, and yet no errors!
A simple solution (Typescript 4.9+):
function whichColor(c: Color): string {
switch (c) {
case Color.Red:
return "Red"
case Color.Blue:
return "Blue"
default:
c satisfies never // <-- Magic
return "Unknown color"
}
}
Which leads to:
Error: Type 'Color.Green' does not satisfy the expected type 'never'.
Easy peasy!
…Okay, but how does it work exactly?
“When narrowing, you can reduce the options of a union to a point where you have removed all possibilities and have nothing left. In those cases, TypeScript will use a never type to represent a state which shouldn’t exist.”
See the typescript handbook - never type for authoritative info and more use cases for never.
Satisfies is like as const without readonly and : Type without type widening.
See learningtypescript - the satisfies operator for what the satisfies operator does.
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