1、typeof:把一种数据类型的变量转换成另外一种数据类型。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var num = 10;
console.log(typeof num);
var str = 'Echo';
console.log(typeof str);
var flag = true;
console.log(typeof flag);
var vari = undefined;
console.log(typeof vari);
var timer = null;
console.log(typeof timer);
var age = prompt('请输入您的年龄');
console.log(age);
console.log(typeof age);
</script>
</head>
<body>
</body>
</html>
2、数据类型转换
2.1 转换为字符串类型=
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var num = 10;
var str = num.toString();
console.log(str);
console.log(typeof str);
console.log(String(num));
console.log(num + '');
</script>
</head>
<body>
</body>
</html>
2.2 转换为数字型
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
var age = prompt('请输入您的年龄');
console.log(parseInt(age));
console.log(parseInt('3.14'));
console.log(parseInt('3.94'));
console.log(parseInt('120px'));
console.log(parseInt('rem120px'));
console.log(parseFloat('3.14'));
console.log(parseFloat('120px'));
console.log(parseFloat('rem120px'));
var str = '123';
console.log(Number(str));
console.log(Number('12'));
console.log('12' - 0);
console.log('123' - '120');
console.log('123' * 1);
</script>
</head>
<body>
</body>
</html>
2.3 转换为布尔型:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script>
console.log(Boolean(''));
console.log(Boolean(0));
console.log(Boolean(NaN));
console.log(Boolean(null));
console.log(Boolean(undefined));
console.log('------------------------------');
console.log(Boolean('123'));
console.log(Boolean('你好吗'));
console.log(Boolean('我很好'));
</script>
</head>
<body>
</body>
</html>