JSDoc template constraint ignored when noImplicitAny

[mhofman]

Bug Report

🔎 Search Terms

jsdoc template constraint unknown

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ

Tested all recent versions and nightly (4.6)

⏯ Playground Link

Playground Link: Provided

💻 Code

/**
 * @template {object} T
 * @param {string} detail
 * @param {T} obj
 * @returns {T}
 */
function foo(detail, obj =/** @type {T} */ ({})) {
  if ("bar" in obj) {
    console.log(obj.bar);
  }
  return obj;
}

Compiler Options

{
  "compilerOptions": {
    "strictNullChecks": true,
    "downlevelIteration": true,
    "checkJs": true,
    "allowJs": true,
    "declaration": true,
    "target": "ES2017",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

🙁 Actual / 🙂Expected behavior

T should extend object, but when noImplicitAny: false, type T extends unknown (toggle in config to see correct extends object)

This seem to be a JSDoc only behavior as the equivalent TS code with noImplicitAny: false

function foo<T extends object>(detail: string, obj: T = ({} as T)) {
  if ("bar" in obj) {
    console.log(obj.bar);
  }
  return obj;
}

Output

function foo(detail, obj = {}) {
    if ("bar" in obj) {
        console.log(obj.bar);
    }
    return obj;
}

Compiler Options

{
  "compilerOptions": {
    "strictNullChecks": true,
    "downlevelIteration": true,
    "declaration": true,
    "target": "ES2017",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

Playground Link: Provided

[sandersn]

object in JSDoc means any when noImplicitAny: false. That's the setting that we expect people will use when first adding old JS to a compilation, and object is frequently used to mean "basically any type" in non-checked JSDoc. Setting noImplicitAny: true is a signal that you want stricter TS semantics applied to JS.

You'll get the same T extends unknown in TS when you write T extends any. That's because any doesn't behave soundly in assignability checks that use the constraint of a type parameter.

[mhofman]

object in JSDoc means any when noImplicitAny: false

That's a surprising and fairly significant limitation I haven't seen documented anywhere! For code that cannot transition to noImplicitAny yet, is there any way to get the behavior of object so that code can be gradually made stricter?

[sandersn]

... I haven't seen documented anywhere!

Hm, I thought that it was, but it's not. Historically, the object type was introduced about the same time as checkJs, and it was a niche, new feature for a couple of years.

The workaround I have suggested before is to alias object in Typescript and refer to the alias in Javacript file.