<body>
<form action="">
请输入要转换的数字：
<input type="text" id="num1" >
<input type="button" class="btn" value="数字转换">
转换后的数字：
<input type="text" id="num2">
</form>
</body>
<script>
let num1 = document.getElementById('num1'),
num2 = document.getElementById('num2'),
btn = document.querySelector('.btn');
btn.onclick = function(){
let info=num1.value;
let str=String(Number(info));
if(str.length<=3){
num2.value=str;
}else{
var tempStr1='';
for(let i=0;i<3;i++){
tempStr1+=str[i];
}
tempStr1+='.';
for(let j=3,k=str.length;j<k;j++){
tempStr1+=str[j];
}
num2.value=tempStr1;
}
num1.value='';
}
</script>