What does Qt Quick Compiler do exactly?

Question

What does Qt Quick Compiler do exactly? My understanding was that it "compiles" QML/JS into C++ and integrates this into the final binary/executable. So, there is no JIT compilation or any other JS-related things during runtime.

However, I saw somewhere an article that claimed that it's not like this and actually it only "bundles" QML/JS into final binary/executable, but there is still some QML/JS-related overhead during runtime.

At the documentation page there is this explanation:

.qml files as well as accompanying .js files can be translated into intermediate C++ source code. After compilation with a traditional compiler, the code is linked into the application binary.

What is this "intermediate C++ source code"? Why not just "C++ source code"? That confuses me, but the last statement kinda promises that yes, it is a C++ code, and after compiling it with C++ compiler you will have a binary/executable without any additional compiling/interpretation during runtime.

Is it how it actually is?

Answer 1

The code is of an intermediate nature because it doesn't map Javascript directly to C++. E.g. var i = 1, j = 2, k = i+j is not translated to the C++ equivalent double i = 1., j = 2., k = i+j. Instead, the code is translated to a series of operations that directly manipulate the state of the JS virtual machine. JS semantics are not something you can get for free from C++: there will be runtime costs no matter how you implement it. There is no additional compiling nor interpretation, but the virtual machine that implements the JS state still has to exist.

That's not an overhead easy to get rid of without emitting a lot mostly dead code to cover all contexts in which a given piece of code might run, or doing just-in-time compilation that you wanted to avoid. That's the primary problem with JavaScript: its semantics are such that it's generally not possible to translate it to typical imperative statically typed code that gives rise to "standard" machine code.

Answer 2

Your question already contains the answer.

It compiles the code into C++, that is of intermediate nature as it is not enough to have C++-Code. You need binaries. So after the compilation to C++, the files are then compiled into binaries. Those are then linked.
The statement only says: We do not compile to binary, but to C++ instead. You need to compile it into a binary with your a C++-Compiler of your choice.

The bundeling happens, if you only put it into the resources (qrc-file). Putting it into the resources does not imply that you use the compiler.

Then there is the JIT compiler, that might (on supported platforms) do a Just-in-Time-Compilation. More on this here