v3.7.2 not compiling valid Javascript

// test.js
function something({one} = {}) {}

$ tsc -v; tsc test.js --allowJS --checkJS --noEmit
Version 3.7.2
test.js:2:21 - error TS2525: Initializer provides no value for this binding element and the binding element has no default value.

2 function something({one} = {}) {}
                      ~~~


Found 1 error.

[kroleg]

Your javascript is not exactly "valid". If you remove --checkJS param it will comile without errors

What's not valid about it? It runs in node and chrome perfectly. I want to keep checkJS because I have some files with jsdocs I'd like to validate. I don't want to have to manually put each file I want to validate in include, I just want to validate all files in a folder and I thought typescript works with javascript, seems not to be the case.

[fatcerberus]

checkJs means you’re type-checking it exactly like a .ts file, just without type annotations. Any type inference semantics and errors that apply to TS code also apply to checkJs’d JavaScript. This is, in fact, a type error because you’re trying to destructure a one property out of a type ({}) which is known statically not to have it.

So Typescript does not compile valid Javascript then? Why isn't it an implicit any? I thought the promise of Typescript is that it works out of the box with JS.

[AnyhowStep]

It worked out of the box and told you there was something suspicious it sniffed from your code.

there's nothing suspicious, something({one} = {}) is a valid and common javascript pattern. why would I want an error on it? I thought the whole point of TS is I don't have to rewrite my entire JS codebase.

[milesj]

@fatcerberus @kroleg answered your question above. something({one} = {}) is valid JS but not valid TS, and when you pass --checkJS, it validates your code as if it was TS.

Are you purposefully ignoring everyones answer here?

[kroleg]

@dave-dm there is something suspicious. By default TS treats all object properties as required, so in case of something({one} = {}) one is missing in {}.
One solution will be to add jsdoc to tell TS that one is optional:

/**
 * @param {object} param0
 * @param {string} [param0.one]
 */
function something({one} = {}) {}

Another is to add ts-ignore directive

// @ts-ignore
function something({one} = {}) {}

[apalaniuk]

I doubt it strictly has to do with type checking via inferred types (if it was, I think you'd see Property 'one' does not exist on type '{}'. ts(2339)) instead):

// error TS2525: Initializer provides no value for this binding element
// and the binding element has no default value.
function something({ one } = {}) {}

// no error!
function something2({ one }) {}

// no error!
function something3(someObj = {}) { // inferred: `(parameter) someObj: {}`
    let { one } = someObj;
}

// no error!
const someObj = {}; // inferred: `const someObj: {}`
let { someProp } = someObj;

All equally suspicious/statically verifiable, with the same inferred types. In fact, checking the same file contents in a .ts file and the most permissive tsconfig results in ts(2339) for all the above cases (save for the no-default-value case), so it seems the claim that checkJs enforces the same checks as in .ts files using the same inferences isn't correct. It's not as though there haven't been similar apparent issues in the same ballpark before, which is a vaguely similar case, so I don't think it's unreasonable to be a bit surprised at this behaviour, or for it to be dismissed so readily.

[fatcerberus]

Maybe so, but the point remains: TS only promises it will compile JS out of the box (which it does—it even emits code in the presence of type errors!), it doesn’t promise it will typecheck. checkJs isn’t just a “Verify the JSDoc” mode, it also attempts to infer types and verifies those too. It’s not even clear in many cases where the explicit documentation ends and the inferences begin.

We’ll just have to wait to see what a TS team member has to say about this, but I still suspect this is by design.

[AnyhowStep]

I think (maybe talking out of my ass?) there's also some stuff that TS only does syntactically, and not semantically for checkJs.

So, some constructs give errors because TS can identity the problem syntactically (someone specifically coded a check for it, that looks at the syntax only).

While other constructs (that express the same thing) don't give errors because the check can't just be done syntactically (or no one coded the syntax-only check).

checkJs is an overall more relaxed mode that doesn't always perform all checks.

Well my use case is only to check jsdocs in all my .js files, and ignore all valid javascript. If that's not possible, that is unfortunate.

@kroleg adding comments doesn't work for my use case. I have a huge codebase and only want jsdocs in a few places. It has to let valid javascript pass through without any addition.

[RyanCavanaugh]

If you just want to validate that your JS Doc matches your parameter names, checkJs would be the wrong thing - it is for doing much more than that.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.