fix(gatsby-remark-images): update to accept alt as title when no title present

[jamessimone]

Description

Some image sources only allow the setting of the alt image attribute when creating images in markdown. An example of this is Dropbox Paper. It would be nice to use the alt as the title for the image when no title is present and the showCaptions config value is enabled.

Related Issues

Fixes #13448

[sidharthachatterjee]

Thank you @jamessimone and congratulations on your first contribution to Gatsby! Enjoy the swag 😉

[sidharthachatterjee]

Went ahead and cleaned up some code I noticed after approving! Hope you don't mind @jamessimone 🙂

[jamessimone]

@sidharthachatterjee not at all! Thank you for making this even better 😄

[sidharthachatterjee]

Looking at the snapshots, it looks like we introduced a bug where title (used in other places) is always being set to alt as opposed to only the showCaption case!

Also maybe a good time to add tests for the showCaption behaviour

[jamessimone]

@sidharthachatterjee I went to add a test for this, but I wanted to ask first if there is a way to only run the tests for gatsby-remark-images. When I ran yarn jest packages/gatsby-remark-images/src/ I got the following error: Cannot find module 'gatsby-plugin-sharp' from 'index.js'

I saw that plugin-sharp is listed as a peer dependency to the project but perhaps the tests can only be run from the root gatsby level due to that? Just wanted to ensure I wasn't doing something wrong there. Thank you to you and @pieh for jumping on this!

[pieh]

@jamessimone Sid had exact same issue with jest. I'm not sure why both of you have this issue and I don't ...

But to move things forward - I added some basic tests for showCaptions to not block the PR because of those jest problems.

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[sidharthachatterjee]

Thank you so much @jamessimone and @pieh 🚀