incorrect formatting of JSDoc type assertion by JS parsers other than babel-*

[bradzacher]

Prettier 2.0.5
Playground link

# Options (if any):
--parser typescript

Input:

declare const foo: any;

const bar1 = /** @type {string} */(foo);

Output:

declare const foo: any;

const bar1 = /** @type {string} */ foo;

Expected behavior:

declare const foo: any;

const bar1 = /** @type {string} */(foo);
// or
const bar1 = /** @type {string} */ (foo);

What's wrong?

In a JS Codebase, dropping the parens breaks the JSDoc type assertion, which will break type checking.

declare const foo: any;

const bar1 = /** @type {string} */(foo);
// typeof bar1 === string

const bar2 = /** @type {string} */ foo;
// typeof bar2 === any

See deeper explanation in typescript-eslint/typescript-eslint#1943

It looks like this only occurs with the typescript parser, as the babel parser works correctly (though note that the flow parser exhibits the same paren scrubbing behaviour).

Related issues (not exactly the same, but pertaining to JSDoc in TS): #8045, #5850

[thorn0]

It's a known issue. See the release notes for 2.0. TypeScript, the language, doesn't support such type casts. TypeScript, the compiler, supports them only in JS.

Check this playground:

declare const foo: any;

const bar1 = /** @type {string} */(foo);

bar1 is any.

[thorn0]

People might have reasons to use --parser typescript to format JS, but still I wouldn't call this a bug. If the parser supported ParenthesizedExpression, it'd work for TS, but it doesn't seem to be a big deal that it doesn't.

[bradzacher]

No problem! I was just raising it because of the attached issue that someone raised - in testing I realised that prettier munges it.

I think we might need to add support for at least this case to the parser, which would fix the issue here as well.

[thorn0]

I didn't know the @typescript-eslint/* rules are used for JS.

BTW, it'd be interesting if the parser exposed the jsDoc property or even converted the types in it to ESTree-like nodes.

[bradzacher]

Yeah (almost) all of our rules work fine on JS code. If you're good with your jsdoc typing, ones that use type information will even work pretty well!

Unfortunately it looks like JSDoc will never be an official part of the ESTree spec (estree/estree#201), but there's no reason we couldn't consider exposing it, given that it's parsed as an AST by TS itself.
Would be an interesting experiment

[thorn0]

Related issue in the parser repos:

• Flow: Feature request: mark parenthesized expressions in AST facebook/flow#8311
• Meriyah: Feature request: new value for the preserveParens option meriyah/meriyah#183