Listen to textarea input through Keyup to change button style

When making a comment box , Imagine setting the send Comment button to ： When no characters are entered （ The initial state ）, Button not available , The color is white ; When characters are entered in the comment box , Button available , The color is orange .

The effect is as shown in the picture ：

The initial state , No input

After entering comments

Because the button style changes depending on textarea Whether there is any input in , namely $(textarea).val() Is it empty

The code is as follows ：

//comment_content_2 by textarea Of id
 $("#comment_content_2").keydown(function(e)
		 {
    
        	var text = $(this).val(),
               	  len = text.length;
            if(len == 0)
            {
    
            	$("#send_btn button").removeClass('send_btn_active');
            	$("#send_btn button").addClass('send_btn_unactive');
            	$("#send_btn button").attr('disabled', 'true');
            }
            else
            {
    
            	$("#send_btn button").removeClass('send_btn_unactive');
            	$("#send_btn button").addClass('send_btn_active');
            	$("#send_btn button").removeAttr('disabled');  	
            }
     }); 

When the code actually runs, it finds , When characters are added in the input box , Button changes normally , But when characters are reduced , When there are no characters in the input box （ The number of characters is 0）, The button style has not changed , Continue to press the backspace key , Button style changes . namely The characters in the input box are reduced to 0 when , You need to continue to backspace to trigger the button style change .

After consulting relevant materials , Find out keydown and keyup Can monitor textarea The input , So, in the above code keydown Change it to keyup, The expected effect has been achieved , The characters in the input box are reduced to 0 when , Trigger button style change .