If you’ve ever touched a more traditional Object-Oriented Language (C, C#, Java), you’ve probably seen an enum data type. While JavaScript does not have support for enums, Flow’s typechecking can make up for it.
Let’s say you’re writing a program that deals with poker chips. The color of a chip is a string (i.e. ‘red’, ‘blue’, et cetera). So your Chip
type would look something like:
…and a function that would create chips would be:
But you want to make sure that it only uses valid colors:
The simplest way to define an enum is as a union of string literals:
From here, you can use the type Color
in place of string
:
Poker chips have value depending on the color. So let’s say that you have an object to look up values somewhere in your project:
The Color
type is repeating the keys of chipValues
It’s not so bad when there are just three, but it’s still duplication and it’ll get annoying if more are added. Flow provides a way to derive an enum from the keys of an object with the $Keys
type:
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