Question

I’m writing an HTML5 canvas drawing app, and I need to be able to tell whether or not the left mouse button is pressed down during a mousemove event. On Chrome, this works:

if (event.which == 1) { <do stuff> }

But in FireFox, event.which always is 1, no matter whether or not the button is pressed.

How can I detect whether the mouse button is pressed during a mousemove event?

Answer 1

In short, you can’t. MDN, for example, explains:

When a mouse button event occurs, a number of additional properties
are available to determine which mouse buttons were pressed and the
location of the mouse pointer. The event’s button property can be used
to determine which button was pressed, where possible values are 0 for
the left button, 1 for the middle button and 2 for the right button.
If you’ve configured your mouse differently, these values may be
different.

…

The button and detail properties only apply to the mouse button
related events, not mouse movement events. For the mousemove event,
for example, both properties will be set to 0.

If you want, you can set global mousedown and mouseup handlers that set flags appropriately and then at any given point in time you can with relative degree of accuracy determine if/which mouse button is pressed.