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Calling indexOf and includes in a readonly array results in a "not assignable" error message #54422
Description
Bug Report
I sometimes need to define a readonly array with a fixed set of values, and check if an unknown value matches one of those fixed values – for example, checking if an HTTP status code is one of the null body status codes (101, 103, 204, 205, 304) or redirect status codes (301, 302, 303, 307, 308).
But, after I put these status codes in a readonly array, and then call the indexOf method or the includes method on it, the compiler would generate an error message that the argument of the indexOf method or the includes method "is not assignable to parameter of type …" followed by the constants in the array.
🔎 Search Terms
- indexOf
- includes
- readonly
- array
- not assignable
🕗 Version & Regression Information
typescriptNPM package version: 5.0.4
⏯ Playground Link
💻 Code
const httpStatusCodes = {
nullBody: [ 101, 103, 204, 205, 304 ],
redirect: [ 301, 302, 303, 307, 308 ],
} as const;
function isRedirect(statusCode: number) {
return httpStatusCodes.redirect.includes(statusCode);
}🙁 Actual behavior
An error is marked under the statusCode argument in the includes call.
The error message is
Argument of type 'number' is not assignable to parameter of type '301 | 302 | 303 | 307 | 308'. ts(2345)
If I use indexOf instead, the error is the same.
🙂 Expected behavior
The code should compile normally without error, because includes and indexOf are both used to check if a user-supplied argument matches one of the values in the array. In pure JavaScript this operation is totally valid for variables of any types.
Activity
jcalz commentedon May 27, 2023
Duplicate of #14520 by way of #26255 by way of #42351
fatcerberus commentedon May 27, 2023
To summarize past discussion around this: Ideally it should be an error to pass e.g. a string here, because that’s likely to be a mistake, so that’s why it doesn’t just take
any. However there’s currently no mechanism to allow “any type that overlaps with the type of the array” (that’s what #14520 is for); the best TS can do is thus “value must be assignable to the type of the array”fatcerberus commentedon May 27, 2023
btw it’s not actually the
readonlytyping that’s the issue here but rather the fact thatas constalso infers literal types.guillaumebrunerie commentedon May 27, 2023
Would it make sense to have overloads for the special case "
Array.includeson an array of literal numbers should accept an arbitrary number" (and the same one with "string" instead of "number"). I feel like the vast majority of complaints aboutArray.includesis when the array contains only literal strings or only literal numbers.fatcerberus commentedon May 27, 2023
TS doesn’t have anything resembling template specialization for generics so I don’t think that’s possible.
It also doesn’t solve the equally common case where someone has e.g.
string[]and wants to check that array for a value typedstring | number.whzx5byb commentedon May 27, 2023
Guillaume Brunerie (@guillaumebrunerie)
Try this workaround:
ytxmobile98 commentedon May 27, 2023
Bruce Pascoe (@fatcerberus)
I don't think this is a real issue within a scope of readonly array with a set of compile-time fixed values.
Consider the use case as you have mentioned: I have a string variable
xand passes it into an arrayarrwith a fixed set of integers.arrcontainsx, then do something withx":arrdoes not contain the numberx, then do something withx":xis supposed to be a number, but is actually a string. If you call methods ofxthat are only present on number variables, or if you passxto a different function which expects a number, such mistakes will still be caught in the corresponding contexts.So this type mismatch is not surely an issue that
Array.prototype.includesorArray.prototype.indexOfshould take a notice of. The mismatch itself could indicate a problem at a larger scope within the software project, which could mostly be caught in higher-level components in the codebase.fatcerberus commentedon May 27, 2023
You’re right, and the “higher-level component” that’s taking notice of it is TypeScript—which is specifically designed to catch potential mistakes in your code at compile time based on the types used. Unfortunately that sometimes means this particular method is too narrow, due to inherent constraints in the type system.
Anyway, I don’t make the rules, I’m only telling you how this discussion has gone in the past based on responses from team members. Don’t shoot the messenger.
fatcerberus commentedon May 27, 2023
See for example #53904 (comment)
fatcerberus commentedon May 27, 2023
Just wanted to address this - like I said above, TS doesn’t have template specialization, so this isn’t a distinction that can be made. The fixed readonly arrays necessarily have to share the same
includetyping with the mutable ones.typescript-bot commentedon Jun 1, 2023
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.