Is it necessary to put scripts at the bottom of a page when using the "defer" attribute?

The current best practice? Use deferred scripts in order in the head, unless you need to support older browsers (IE < 10, Opera Mini, etc.) - 97.45% browser usage (ref)

Why? With defer, parsing finishes just like when we put the script at the end of the body tag, but overall the script execution finishes well before, because the script has been downloaded in parallel with the HTML parsing. This scenario will trigger the faster domInteractive event that is used for page loading speed. With async, the order in which your script will execute varies based on how fast the script is fetched, so order can be compromised. Futhermore, async scripts are executed inline and pause the parsing of the HTML.

Best practices have shifted since these answers were written, because support for the defer attribute has grown to 98% globally.

Unless you need to optimize speed for older browsers, you should put the script in the head and mark as defer. This 1) keeps all your script references in one place (more maintainable) and 2) makes the browser aware of the script sooner, which lets it start prioritizing resources earlier.

The performance difference difference should be negligible for most pages, because the browser's pre-loader probably isn't going to start downloading a deferred script until the whole document is parsed anyway. But, it shouldn't hurt, and it leaves more work for the browser, which is generally best.

First of all, the defer attribute is not supported by all browsers (and some that do support it just ignore it). Putting the script at the bottom of the page ensures that all HTML elements above it have been loaded into the DOM before the script executes. An alternative is using the onload method or using jQuery's DOM ready function.