It's probably a combination of runtime analysis and code construction analysis. Submissions with similar runtimes will use the same algorithm. If most student

It's probably a combination of runtime analysis and code construction analysis. Submissions with similar runtimes will use the same algorithm. If most students use the proper way, but a group of five students all, by "chance," pick an obscure dumb way (or a really obscure fast way), then they may be copying. Manual investigation would be triggered. Look into how JavaScript minimizers/obfuscators work. They could be parsing your code down to a form where your variable names and comments don't matter at all. They're just analyzing and looking for coincident structure of programs. Now, your administration is smart enough to not flag code where there's one way to do it (write "find a member of a list"), but if 10 people copy the same wrong way to do it, it would be a flag to investigate further.