TypeScript: Create an union type of strings from a certain type

Question

I have a type IconName that looks like this:

type IconName = 'cars' | 'mars' | 'truck' | 'warning'

I want then to create another type VehicleIconName which contains all icons name related to a vehicle:

type VehicleIconName = 'cars' | 'truck'

But how can I tell to TypeScript that each strings in VehicleIconName have to be of type IconName too?

So that I couldn't write this:

type VehicleIconName = 'cars' | 'truck' | 'invalidIconName'

brunnerh · Accepted Answer · 2020-06-25 11:40:52Z

8

You can add a helper type:

type Subset<Parent, Sub extends Parent> = Extract<Parent, Sub>;

type IconName = 'cars' | 'mars' | 'truck' | 'warning'

type VehicleIconName = Subset<IconName, 'cars' | 'truck' | 'invalidName'>

Error on last line:

Type '"cars" | "truck" | "invalidName"' does not satisfy the constraint 'IconName'.
Type '"invalidName"' is not assignable to type 'IconName'.ts(2344)

edited Jun 25, 2020 at 11:40

answered Jun 25, 2020 at 11:00

brunnerh

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4 Comments

johannchopin Over a year ago

Yep works like a charm 👍 Do you think it's the only way to achieve this?

brunnerh Over a year ago

@johannchopin: Maybe; at the very least i could not think of another method.

ftm Over a year ago

You could improve the Subset type by having it equal to Extract<Parent, Sub> instead of just Sub. Otherwise although the definition of VehicleIconName throws an error, const x: VehicleIconName = "invalidName" would still be a valid assignment.

brunnerh Over a year ago

@ftm: Probably a good idea to report those errors right away, thanks for the suggestion.

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