How to Use Javascipt to disable multiple HTML Text fields with a HTML checkbox

THIS IS THE CHECKBOX 

<input type="checkbox" id="checkboxA"> click to disable<br>
<input type="text" id="textbox_A"> 
<input type="text" id="textbox_B">

THIS IS THE JAVASCRIPT

<script type="text/javascript"> 
function disablefields(){ 
if (document.getElementById('checkboxA').checked == 1){ 
document.getElementById('textbox_A').disabled='disabled'; 
document.getElementById('textbox_A').value='disabled';
document.getElementById('textbox_B').disabled='disabled'; 
document.getElementById('textbox_B').value='disabled';
}else{ 
document.getElementById('textbox_A').disabled=''; 
document.getElementById('textbox_A').value='Allowed'; 
document.getElementById('textbox_B').disabled=''; 
document.getElementById('textbox_B').value='Allowed'; 
} 
} 
</script>