Utility Parameters type cast tuple type elements to optional

[maksimr]

TypeScript 4.1.3

    /**
     * @template {function} T
     * @param {T} fn
     * @returns {(...args: Parameters<T>) => void}
     */
    function throttle(fn) {
      return (...args) => {
        fn(...args);
      };
    }

    const tfn = throttle((/**@type {string}*/x) => x);
    tfn(); // NO ERROR (WRONG)
    tfn("foo") // No error (Right!)
    tfn(1); // Error (Right!)
    tfn("foo", 2); // Error (Right!)

tsconfig.json

{
  "compilerOptions": {
    "target": "es6",
    "jsx": "react",
    "allowJs": true,
    "checkJs": true,
    "noEmit": true,
    "skipLibCheck": true,
    "strict": true,
    "noImplicitAny": false,
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "allowSyntheticDefaultImports": true,
    "typeRoots": [
      "./node_modules/@types",
      "./@types"
    ]
  },
  "include": [
    "@types/**/*",
    "lib"
  ],
  "exclude": [
    "node_modules"
  ]
}

[RyanCavanaugh]

Parameters in JS files are always considered optional, since there's no native syntax for denoting that they're not. This isn't dependent on Parameters<T> to observe; you just need a function of any sort:

    const tfn = (/**@type {string}*/x) => x;
    tfn(); // NO ERROR (INTENDED)
    tfn("foo") // No error (Right!)
    tfn(1); // Error (Right!)
    tfn("foo", 2); // Error (Right!)

[brendankenny]

@maksimr @type is a little weird in that position, and note that switching to @param (and bumping it out of the params parens) does correctly give an error:

/**
 * @template {function} T
 * @param {T} fn
 * @returns {(...args: Parameters<T>) => void}
 */
function throttle(fn) {
  return (...args) => {
    fn(...args);
  };
}

const tfn = throttle(/** @param {string} x */ (x) => x);
tfn(); // Now an error (Right!)
tfn("foo") // No error (Right!)
tfn(1); // Error (Right!)
tfn("foo", 2); // Error (Right!)

or from @RyanCavanaugh's example:

const tfn = /** @param {string} x */ (x) => x; // single line or @param above
tfn(); // Now an error (Right!)
tfn("foo") // No error (Right!)
tfn(1); // Error (Right!)
tfn("foo", 2); // Error (Right!)

[maksimr]

@brendankenny @RyanCavanaugh thanks. Didn’t know that parameters in JS treated as optional by default. Yes, param fix the problem, thanks 🙏