Detect left mouse button press

Updated answer. The following will detect if the left and only the left mouse button is pressed:

function detectLeftButton(evt) {
    evt = evt || window.event;
    if ("buttons" in evt) {
        return evt.buttons == 1;
    var button = evt.which || evt.button;
    return button == 1;

For much more information about handling mouse events in JavaScript, try

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