isNaN(i.value)//Used to determine whether i.value is not a number//NaN not a number is not a number
i represents a text input box
1. i.value gets a string, and when the string is -*/ (subtraction, multiplication and division), it will be automatically converted into a numerical value for operation
2. If it is +, the string concatenation "5" is performed *"5"-> 5*5, "5"+"5" is spliced into "55"
//d.innerText = "result:"+i.value+i.value;
3. Convert the string into a numeric value Ways:
//3.1. parseInt to integer or parseFloat to decimal
//d.innerText = "Result:"+(parseInt(i.value)+parseInt(i.value)); //3.2
. Let string *1 Force the string to be converted to the value "5"*1-> 5*1 = 5
d.innerText = "result:"+(i.value*1+i.value*1);
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="fn()">
<div>结果:0</div>
<script>
function fn() {
//得到文本框和div
let d = document.querySelector("div");
let i = document.querySelector("#i1");
//判断用户输入的内容是否是数值
if (isNaN(i.value)){//满足条件说明是NaN 不是数
d.innerText="输入格式错误!";
return;
}
//把文本框的值 相乘 后的结果给到div
//d.innerText = "结果:"+i.value*i.value;
//i.value得到的是字符串,字符串进行-*/时会自动转成数值进行运算
//如果是+ 则进行的是字符串拼接 "5"*"5"-> 5*5, "5"+"5"拼接成"55"
//d.innerText = "结果:"+i.value+i.value;
//将字符串转成数值两种方式:
//1. parseInt转整数或parseFloat转小数
//d.innerText = "结果:"+(parseInt(i.value)+parseInt(i.value));
//2. 让字符串*1 强制字符串转成数值 "5"*1-> 5*1 = 5
d.innerText = "结果:"+(i.value*1+i.value*1);
}
</script>
</body>
</html>