[codemod request] svelte 4 : stricter types for createEventDispatcher, Action , ActionReturn , and onMount

[arshcodemod]

svelte 4 : stricter types for createEventDispatcher, Action , ActionReturn , and onMount

Codemod description

createEventDispatcher now supports specifying that a payload is optional, required, or non-existent, and the call sites are checked accordingly (#7224)

Action and ActionReturn have a default parameter type of undefined now, which means you need to type the generic if you want to specify that this action receives a parameter. The migration script will migrate this automatically (#7442)

onMount now shows a type error if you return a function asynchronously from it, because this is likely a bug in your code where you expect the callback to be called on destroy, which it will only do for synchronously returned functions (#8136)

Examples

Note: The example section helps codemod champions identify all the transformation patterns that this codemod should handle. Repeat before/after snippets for every transformation example you can find.

Code before transformation

const action: Action<HTMLElement, string> = (node, params) => { .. } 
async () => {
     const something = await foo();

Code after transformation

const action: Action<HTMLElement, string> = (node, params) => { .. } 
() => {
foo().then(something =>  ..

Applicability Criteria

Migration for Svelte 3->4

Additional context or links

Migration Guide for Svelte