![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
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<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Title</title></head><body><label for="info">年龄：</label><input type="text" id="info"><button onclick="trans();">提交</button><script>/********1、javascript变量、函数的定义**************/var a=70;var b=1.75;var bmi = function (){return a/(b*b);};var c = bmi();console.log(c);/********2、javascript流程控制if else switch**************/if(c<18.5){console.log('偏瘦');}else if(c>=18.5 && c<=24.9) {console.log('正常');}else if(c>24.9){console.log('超重');}//流程控制 switchswitch (true) {//注意此处传的参数case c < 18.5:console.log('偏瘦');break;case c >= 18.5 && c <= 24.9:console.log('正常');break;default:console.log('超重');}/********3、javascript三种循环**************///while循环while(a<80){a++;console.log(a);}//do...while循环do{a++;console.log(a);}while(a<80);//for循环for(a;a<80;a++){console.log(a);}//4、数据类型转换：parseInt、isNaN函数的使用function trans(){var age = document.getElementById('info').value;if(isNaN(age)){console.log('获取方法有误');}console.log(typeof(age));var age_num = parseInt(age);console.log(typeof(age_num));// var num = parseInt(age);// console.log(age_num+'年');}</script></body></html>
js也为动态弱语言，定义时没有具体数据类型声明，只有关键字var或let.
函数，流程控制（ifelse,switch case），循环语句（for,while,do while）与php类似，php多了foreach循环数组这种循环形式。
typeof()的返回值为string.
输入数字判断isNaN(),转换为数字parseInt().