Migrate script removes JSDoc comments when there are default values

[rgieseke]

Describe the bug

When running npx sv migrate svelte-5 JSDoc comments of the following form in a Svelte 4 app

<script>
	/** @type {number} [default_value=1] */
	export let default_value = 1;

	/** @type {number} [comment_default_value=1] - This has a comment and an optional value. */
	export let comment_default_value = 1;
</script>

get converted to the following, the comments and default value are not included after the conversion.

<script>
	/**
	 * @typedef {Object} Props
	 * @property {number} [default_value]
	 * @property {number} [comment_default_value]
	 */

	/** @type {Props} */
	let { default_value = 1, comment_default_value = 1 } = $props();
</script>

From running npx vitest packages/svelte/tests/migrate -t="jsdoc-with-comments" and looking around in the migrate script it seems that cleaned_comment is empty here which I think it shouldn't?

svelte/packages/svelte/src/compiler/migrate/index.js

Line 1640 in 74917ae

let cleaned_comment = cleaned_comment_arr

Happy to help fix this but would need some further pointers how.

I tried updating Layer Cake components to Svelte 5 syntax.

Reproduction

I have failing test cases here: rgieseke@634ccc6

System Info

Node: 22.13.1 - ~/.local/bin/node
    npm: 10.9.2 - ~/.local/bin/npm
    pnpm: 9.4.0 - ~/.local/bin/pnpm
npmPackages:
    svelte: workspace:^ => 5.23.1

Severity

annoyance

[paoloricciuti]

Technically speaking the [comment_default_value = 1] is not a thing for the @type JSDoc comment so that will also be interpreted as a comment. We could fix it by also appending the rest of the sentence after the @type but this means your code will be converted to

<script>
	

	
	/**
	 * @typedef {Object} Props
	 * @property {number} [default_value] -  [default_value=1] 
	 * @property {number} [comment_default_value] -  [comment_default_value=1] - This has a comment and an optional value. 
	 */

	/** @type {Props} */
	let { default_value = 1, comment_default_value = 1 } = $props();
</script>

I guess there's no harm in fixing it if people use this still of comment for @type

[rgieseke]

Personally, I wouldn't mind not having the duplicated default value declaration, but I guess it's helpful to have it in the JSDoc declaration for API docs and editor hints.

On https://jsdoc.app/tags-type the definition with an equal sign is included:

An optional parameter named foo.

@param {number} [foo]
// or:
@param {number=} foo

An optional parameter foo with default value 1.

@param {number} [foo=1]

[paoloricciuti]

But that's for @param not for @type

[rgieseke]

True, but it's used as component property/parameter, so that's probably why it's used that way. I don't know if the usage is common.

[rgieseke]

Fixed in #15567 and released as 5.24.0. (The repeated default value declaration in the JSDoc comment is removed though.)