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 = 0for(var i = 0; i < chara.length; i++){if(chara[i].charCodeAt() >= 65 && chara[i].charCodeAt() <= 90){document.write(chara[i].toLowerCase());}}</script>