Pull out `addMessage` from core.js into includes/message.ts

Is your proposal related to a problem?

addMessage should move from core.js to a new file client/src/includes/messages.ts

Wagtail’s JavaScript core.js is quite large and many items can be pulled out to shared utils or includes to help make the code more maintainable and supported with base unit tests.

Pulling some basic functions out lets us put these into TypeScript (type safety) but also document and unit test them better.

Describe the solution you’d like

The function addMessage should be moved from https://github.com/wagtail/wagtail/blob/aaea8710c21ba4d5368d3825781bc54e1869c561/client/src/entrypoints/admin/core.js#L4-L14

To a new file client/src/includes/messages.ts (note: TypeScript ts not JavaScript js)

From there it can be imported and set to the same global (window.addMessage) as per current state.

Requirements

• Move the function to the the new TypeScript file and set up correct types - https://www.typescriptlang.org/
• Add a JSDOC description to the function
• Add a basic set unit test (just one is fine) for this function in a new client/src/includes/messages.test.js file using Jest - look at client/src/includes/breadcrumbs.test.js for an example of mocking some HTML to then trigger some behaviour on that HTML
• Ensure the util is imported in core.js correctly so that window.addMessage and any other usage within that function still acts the same way.

Additional context

• This has been created as part of the Outreachy program to help new contributors that are interested in the Stimulus project.

[lb-]

@salty-ivy I think it may be best you look at some of those other ones I pinged you on yesterday.

Let’s let Ivy pursue this, it’s not a critical improvement and it’s very possible we will look at this code differently in the coming weeks with the Stimulus RFC 78 work.

[salty-ivy]

alright, got it.