`Parameters` does not work in JS files unless a `@param` has been explicitly defined

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🔎 Search Terms paramaters param 🕗 Version & Regression Information This is the behavior in every version I tried, and I reviewed the FAQ for entries about Parameters ⏯ Playground Link https://www.typescriptlang.org/play/?strictNullChecks=true&filetype=js#code/GYVwdgxgLglg9mABAJwKYAdUEMoAoBQAkAPQBUpiAAlAJ6aIDeAzlMjGAOYC+ipxiAW1RMmWDqgA0RMhWp1UjMCAEAjVMh59EsIUykBKRkTRQQyJAEFkyLDVw7h+gHTAYAGze5do8c4BWcOy4AAaIwfoA3Phc+PgyVLT0DAAKWDZCUOpMADyJqHDAKBjYUAB8ANoADAC6msT4EAgsgsI+CgC8YQAWqB5wwVFx5AnyjKnpqJnIOXkFRZg4FQCMtbz1jWDNDkyInQDMUUA 💻 Code function repeat( /** @type {string} */ message, /** @type {number} */ times, ) { return Array(times).fill(message).join(` `); } /** @type {Parameters<typeof repeat>[0]} */ const message = `hello`; // Error: Type '"hello"' is not assignable to type 'never'. (ts2322) /** @type {Parameters<typeof repeat>[1]} */ const times = 3; // Error: Type '3' is not assignable to type 'never'. (ts2322) 🙁 Actual behavior Parameters<typeof repeat> coalesces into type never. 🙂 Expected behavior Parameters<typeof repeat> coalesces into type [message: string, times: number]. Additional information about the issue This error occurs if a function's arguments are defined with @type, but not if they're explicitly defined with @param. For example, this works as expected: /** * @param {string} message * @param {number} times */ function repeat(message, times) { return Array(times).fill(message).join(` `); } /** @type {Parameters<typeof repeat>[0]} */ export const myMessage = `hello`; // No error /** @type {Parameters<typeof repeat>[1]} */ export const myTimes = 3; // No error However, it is unreasonable to always require that @params be explicitly written for every function, e.g. when writing a callback that already has a defined shape. This utility type serves as a workaround: export type Params<Target extends (...args: any) => any> = Target extends (arg0: infer Arg0, arg1: infer Arg1, arg2: infer Arg2) => any // Can add more args if we need them ? [Arg0, Arg1, Arg2] : never; function repeat( /** @type {string} */message, /** @type {number} */times, ) { return Array(times).fill(message).join(` `); } /** @type {Params<typeof repeat>[0]} */ export const myMessage = `hello`; // No error /** @type {Params<typeof repeat>[1]} */ export const myTimes = 3; // No error