ts-r
  • Error
  • Reported by the lexer and parser

TS8004

Type parameter declarations can only be used in TypeScript files.

Type parameters (`<T>`) appear on a function or class in a JavaScript file. Use JSDoc `@template` or rename the file to .ts.

What it means

Generic type parameter lists such as function f<T>(x) are TypeScript syntax and can't appear in a .js file.

ts-r's parser reports this code when it reads .js files with tsc's rules for TypeScript syntax in JavaScript. ts-r-check doesn't report it yet: today it parses .js files with the plain JavaScript grammar and reports a syntax error at the same place instead.

Example

index.js
function first<T>(xs) {
  return xs[0];
}
cargo run -p ts_r_parser --example dump -- index.js --ts-in-js report ts-r output
index.js(1,16): error TS8004: Type parameter declarations can only be used in TypeScript files.

Recorded by node scripts/verify-examples.mjs with ts-r built from commit 6deaf01. The example uses the default tsconfig: strict, target: es2022, module: esnext, moduleResolution: bundler.

How to fix

  • Remove the type parameters and describe them in JSDoc: /** @template T @param {T[]} xs */.
  • Rename the file to .ts if it's meant to be TypeScript.