How to e.startDefault when e.preventDefault is set?

function loaded() {
    document.addEventListener('touchmove', fu开发者_如何学运维nction(e){ e.preventDefault(); });
}

loaded();

At certain times I want to allow default. How can I do that?

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Depending on what you want, something like:

function loaded() {
    document.addEventListener('touchmove', function(e){ 
         if (conditionToPreventDefault) { e.preventDefault(); } });
}

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Based on your comment, I think you need removeEventListener - see here for explanation:

http://www.quirksmode.org/js/events_advanced.html