限制textarea字数

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js，用prototype写的
function countChar(textareaName,spanName) {var sup = $('t_count').innerHTML;var count = sup - $('fbmessage').value.length; if(count <= 0) { var text = $('fbmessage').value.substring(0, 43); $('fbmessage').setValue(text); count = sup - $('fbmessage').value.length; }  $('surplus').update(count);}var sup = $('t_count').innerHTML;var count = sup - $('fbmessage').value.length;$('surplus').update(count);
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