How to detect right click + left click

Question

I am building a game

And I need to do something when the user clicks on the right mouse button, holds it and then presses the left button

Answer 1

Use MouseEvent.buttons in your event handler.

.addEventListener("mousedown", function(event){
    if ((event.buttons & 3) === 3){
        //Do something here
    }
}, true);

It is kinda recent though, you may want to implement fallback method, recording state of mouse buttons.