How to convert array of strings "variable" to typescript types?

Question

I'd like to know how to convert array of string variable to type script types.

Note: There are two SO answers already that deal with this but strings are passed hardcoded to keep values of strings as they are.

Both answers pass an array in-line to keep string values in array as literals. But when you pass an array as a variable, it fails to work.

Passing values in-line works.

But passing values as a variable doesn't work

Question

So how do you create a type to limit a list of allowable string values in domElements as a variable to asLiterals?

unional · Accepted Answer · 2019-01-27 08:06:34Z

1

When an array literal is assigned to a variable, it is widened to match the typical behavior.

Because of that, the variable domElements is of type string[] instead of ('a' | 'div' | ...)[]

The answer is in your own code. Use a similar function as your asLiterals() to create a literal array.

export function literalArray<T extends KeyTypes>(...entries: T[]): T[] {
    return entries
}

You can also find this function in the type-plus library.

edited Jan 27, 2019 at 8:06

answered Jan 27, 2019 at 7:54

unional

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3 Comments

dance2die Over a year ago

Thanks @unional about widened type behavior. I've tried it with literalArray after installing type-plus but the DOMType is still recognized as string. imgur.com/a/uwQcF8Y. (codesandbox.io/s/7j15z1o856) What could I be missing?

unional Over a year ago

After it is widened, there is no automatically way to narrow it down. I.e, you have to in-line it. Your domElements and arr are the same. Only type is different.

dance2die Over a year ago

Thank you for the clarification 👊. I will in-line the code as a last resort for my library.

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