After heavily searching the web to find out why my Bootstrap dotnet core hamburger menu stopped working, I gave up. Every solution failed. So I took a closer look at the tag [script async src]. Then I saw the problem in the word async in the tag [script async src]. You see async loads the javascript files asynchronously. In short, the jquery.js file was not always loaded before the bootstrap.js file. This is a problem because for bootstrap to work, jquery must load first.

I removed async from my javascript tags and bingo, it worked. The problem did my head in because using a CDN for my javascript worked but local files failed whatever version I used until I removed async. The clue was there. I used the developer tools in the browser and a null error kept showing up in bootstrap. If jquery was not loaded before bootstrap then the object would be null. I’ll have to keep an eye out for that one in future.