No error in JavaScript on optional binding pattern parameter

[jespertheend]

Bug Report

🔎 Search Terms

TS2463 javascript

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about default

⏯ Playground Link

• TypeScript link
• JavaScript link

💻 Code

/**
 * @param [options]
 */
function test({ x = '' }) { x }

🙁 Actual behavior

No error on { x = '' }.

🙂 Expected behavior

Error 2463: "A binding pattern parameter cannot be optional in an implementation signature."

The equivalent Typescript has the correct error:

function test({ x = '' }?) { x }

Note that in the playground link I have checked all options that are supposed to make types more strict. Also note that when this code is run, you'll end up with a runtime error. The same is true for the TypeScript equivalent, but in that case you get an error in advance warning you that the code you wrote will cause issues.

[sandersn]

@param {Options} [options] declares the parameter as optional. The equivalent in typescript is { maybeUndefined = 'str' }?: options, which also gets rid of the error. (Although it adds an error at the declaration site since this combination isn't legal -- and should probably be illegal in javascript too.)

Instead, use @param {Options} options

[jespertheend]

Thanks for looking into this @sandersn!
I'm not really sure why this got closed as working as intended, since you mentioned

and should probably be illegal in javascript too.

Should I create a feature request / new issue for this?

[sandersn]

Oh, I misread the whole thing. I'll re-open.

[JE-lee]

I supposed that it's working as intended.
function test({ x = '' }) { x } means that the first argument is required.
You'll get the error of missing arguments when you attempt to call this test().

[jespertheend]

That's true, but with

/**
 * @param [options]
 */

added to the JavaScript function you can suppress type errors that would otherwise warn you about the potential runtime error.
Have a look at the two playground links in the first comment. The emitted code from both JavaScript and TypeScript are the same, and both trigger a runtime error, but the JavaScript equivalent doesn't warn you in advance.