[Bug] Backing class JavaScript file not working if path contains underscore

[didoo]

Probably a bug. Did some experiments to understand why in some cases the backing class JavaScript file associated with a Markdown file was not picked up, and I've found a strange behaviour. If the path of the file (be it a parent folder, or the file itself) contains an underscore (_) then the JS file is ignored.

To test this I have forked this repo, and simply added a bunch of test files (direct copies of the custom-button.js + custom-button.md demo files) and checked what happened in different cases.

You can test yourself using this branch in my fork: https://github.com/didoo/field-guide-testing/tree/backing-class-path-underscore-bug/docs

Here's what I've found:

With a simple file name and a parent folder that doesn't contain an underscore - It works ✅

• markdown file: test-file.md
• javascript file: test-file.js
• parent folder: 01-testing-normal-folder

You can see that it works by the `0` zero visible after the `Count` text (and clicking the button increases the value).

Same file but with an underscore in the name - It doesn't work ❌

• markdown file: test-file_with_underscore.md
• javascript file: test-file_with_underscore.js
• parent folder: 01-testing-normal-folder

You can see that it works by the lack of`0` zero after the `Count` text and clicking the button triggers this error:

Same file with a 0x-- prefix - It works ✅

• markdown file: 01--test-file-with-number-prefix.md
• javascript file: 01--test-file-with-number-prefix.js
• parent folder: 01-testing-normal-folder

Same file with a 0x_ prefix in both the Markdown and the JS files - It doesn't work ❌

• markdown file: 02_test-file-with-number-prefix-and-underscore.md
• javascript file: 02_test-file-with-number-prefix-and-underscore.js
• parent folder: 01-testing-normal-folder

Same file with a 0x_ prefix in the Markdown, but a normal dash in the JS file - Strangely, it works 🤔

• markdown file: 03_test-file-with-number-prefix-and-mixed.md
• javascript file: 03-test-file-with-number-prefix-and-mixed.js
• parent folder: 01-testing-normal-folder

Same file with a parent folder that contains an underscore - It doesn't work ❌

• markdown file: test-file.md
• javascript file: test-file.js
• parent folder: 02_testing-folder_with_underscore