检查字符串是否包含在 TypeScript 中的只读字符串数组中

Question

const ReadOnlyStrings = ['a', 'b', 'c'] as const;
const fn = (key: string)=>{
  ReadOnlyStrings.includes(key);
}

Errors in code:
Argument of type 'string' is not assignable to parameter of type '"a" | "b" | "c"'.

How to advoids the error?

Answer 1

ReadOnlyStrings isn't just a readonly array of strings, it's a readonly tuple that says, in its type, that it contains exactly "a" in the first element, "b" in the second, and "c" in the third. From a pure types perspective, it doesn't make sense to ask if it contains any random string; you know it contains exactly those strings in exactly that order. But pragmatically, we sometimes need to do things like that. :-)

You can either make it just a readonly array of strings ( playground ):

const ReadOnlyStrings: readonly string[] = ["a", "b", "c"] as const;
//                   ^^^^^^^^^^^^^^^^^^^
const fn = (key: string) => {
    console.log(ReadOnlyStrings.includes(key));
};

...or tell TypeScript to treat it as one for just this operation by using a type assertion to widen its type ( playground ):

const ReadOnlyStrings = ["a", "b", "c"] as const;
const fn = (key: string) => {
    console.log((ReadOnlyStrings as readonly string[]).includes(key));
    //          ^               ^^^^^^^^^^^^^^^^^^^^^^
};