jQuery: Detecting pressed mouse button during mousemove event

You can write a bit of code to keep track of the state of the left mouse button, and with a little function you can pre-process the event variable in the mousemove event.

To keep track of the state of the LMB, bind an event to document level for mousedown and mouseup and check for e.which to set or clear the flag.

The pre-processing is done by the tweakMouseMoveEvent() function in my code. To support IE versions < 9, you have to check if mouse buttons were released outside the window and clear the flag if so. Then you can change the passed event variable. If e.which was originally 1 (no button or LMB) and the current state of the left button is not pressed, just set e.which to 0, and use that in the rest of your mousemove event to check for no buttons pressed.

The mousemove handler in my example just calls the tweak function passing the current event variable through, then outputs the value of e.which.

$(function() {
    var leftButtonDown = false;
    $(document).mousedown(function(e){
        // Left mouse button was pressed, set flag
        if(e.which === 1) leftButtonDown = true;
    });
    $(document).mouseup(function(e){
        // Left mouse button was released, clear flag
        if(e.which === 1) leftButtonDown = false;
    });

    function tweakMouseMoveEvent(e){
        // Check from jQuery UI for IE versions < 9
        if ($.browser.msie && !e.button && !(document.documentMode >= 9)) {
            leftButtonDown = false;
        }

        // If left button is not set, set which to 0
        // This indicates no buttons pressed
        if(e.which === 1 && !leftButtonDown) e.which = 0;
    }

    $(document).mousemove(function(e) {
        // Call the tweak function to check for LMB and set correct e.which
        tweakMouseMoveEvent(e);

        $('body').text('which: ' + e.which);
    });
});

Try a live demo here: http://jsfiddle.net/G5Xr2/

Most browsers (except Safari) *) now support the MouseEvent.buttons property (note: plural "buttons"), which is 1 when the left mouse button is pressed:

$('#whichkey').bind('mousemove', function(e) { 
    $('#log').html('Pressed: ' + e.buttons);
});

https://jsfiddle.net/pdz2nzon/2

*) The world is moving forward:
https://webkit.org/blog/8016/release-notes-for-safari-technology-preview-43/
https://trac.webkit.org/changeset/223264/webkit/
https://bugs.webkit.org/show_bug.cgi?id=178214