That's standard dead code elimination, and that's only true in static linking. Dynamic linking, which is far more popular, will load the entire library into mem

That's standard dead code elimination, and that's only true in static linking. Dynamic linking, which is far more popular, will load the entire library into memory. And then you get interesting hybrids like Java, where libraries are dynamically linked but only the classes that are actually used are loaded. [0] The equivalent for dynamic languages is tree shaking, and is a harder or impossible problem depending on the language. When you can do something like the following in JavaScript, how do you determine what's dead or alive? var fn = document.getElementById("function").value; var input = document.getElementById("input").value; var output = functions[fn](input); document.getElementById("output").value = output; [0] EDIT: But all the method and fields within a class are loaded, regardless of whether they are used. Which is why I called it a hybrid.