js中字符串大写字母改成小写字母

<script>
	var chara = prompt('请输入一行字符串');
	var num = 0
	for(var i = 0; i < chara.length; i++){
    
    
		if(chara[i].charCodeAt() >= 65 && chara[i].charCodeAt() <= 90){
    
    
			document.write(chara[i].toLowerCase());
		} 
	}
</script>