Is there a way to create a union type that strictly limits values to 'a', 'b', 'c' when using a list like
const list: string[] = ['a', 'b', 'c']I know one method is:
const list = ['a', 'b', 'c'] as const;
type allowedValues = typeof list[number]
However, this approach makes the list read-only and cannot be typed with string[]. Is there a technique to constrain the values of list while still maintaining its typing flexibility?
Here is a method I frequently utilize:
const items = ['x', 'y', 'z'] as const;
type ItemType = typeof items;
type PermittedValues = ItemType[number];
const specifiedVariable:PermittedValues[] = ["x"]; // works fine
I choose this approach because it allows for sorting data based on the defined order of values in items. This can be achieved as shown below:
someArrayofPermittedValues.sort(
(a, b) => allTypes.indexOf(a) - allTypes.indexOf(b)
);
If you want to calculate a Union Type, you need to specify your variable using Literal Types within the array. It's impossible for the compiler to determine that your array has values of "a", "b", and "c" if it was initially declared with a broader type like string[].
const list: ["a", "b", "c"] = ["a", "b", "c"];
type AllowedValues = (typeof list)[number];
// type AllowedValues = "a" | "b" | "c"
To convert back to an arbitrary type of string[], you can use Type Assertion.
(list as string[]).push("d");
The issue with const assertions is that TypeScript requires you to assert to unknown first. Therefore, you must do the following:
(list as unknown as string[]).push("d");
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