I didn't manage to reproduce this problem. Here are my steps.
1. Created a js file in the root of the project with a simple function in it
2. Went to templates and add js widnget on the template, then linked it to the js file I created in step 1.
3. Opened a page and added another js widget where I called my function
4. Edited the js file created in step 1 and requested the page again where I saw the new data.
Page caching was disabled.
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