Centralize message handling to allow for multiple async listeners.

[Osmose]

This is the messaging refactor extracted from #168.

[biancadanforth]

R+WC. Very clean! Extraction doesn't work right now due to a super minor change needed. Be sure to make that change before merging.

[biancadanforth]

Extraction doesn't work right now for a very minor reason:

• The handleExtractedProductData method needs to be updated, since you are now sending message instead of message.extractedProduct as the first argument.

Currently:

export async function handleExtractedProductData(contentExtractedProduct, sender) {
  // Fetch the favicon, which the content script cannot do itself.
  const extractedProduct = {
    ...contentExtractedProduct,
    vendorFaviconUrl: sender.tab.favIconUrl,
  };
// ...

Change that makes it work:

export async function handleExtractedProductData(message, sender) {
  // Fetch the favicon, which the content script cannot do itself.
  const extractedProduct = {
    ...message.extractedProduct,
    vendorFaviconUrl: sender.tab.favIconUrl,
  };
// ...

[Osmose]

Nice catch, my bad.

[biancadanforth]

Can you explain a little more why we want to return undefined here? I'm not sure I'm totally following.

Would we not want to console.warn or something that the message.type is not recognized?

[Osmose]

ESLint doesn't like if we have a return statement in some branches but not others. Since the if has a return, we need one outside too in case we don't return there. undefined is the default return value for functions that don't return, so I'm just using that here as well.

[Osmose]

Thank you!