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

TS8012

Parameter modifiers can only be used in TypeScript files.

A constructor parameter in a JavaScript file has a modifier like `private` or `readonly`. Assign the field in the body instead.

What it means

Parameter properties (constructor(private name)) 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
class User {
  constructor(public name) {}
}
cargo run -p ts_r_parser --example dump -- index.js --ts-in-js report ts-r output
index.js(2,15): error TS8012: Parameter modifiers 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

  • Declare the field and assign it: constructor(name) { this.name = name; }.
  • Rename the file to .ts if it's meant to be TypeScript.