<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>入门</title> </head> <body> <script> //弹出框 //alert("hello js") //写入到页面 //document.write("hello javascript") //浏览器控制台 推荐 console.log("浏览器控制台输出") console.log("浏览器控制台输出") //变量 全局变量 var a = 12; console.log( typeof a); console.log("变量:"+a); { var b = 22; } console.log("变量:"+b); // { // let c = 30; // } // console.log("变量:"+c); const d = 10; console.log("变量:"+d); //数据类型 /* 5种 number String null boolean undefind */ let n = 20; let s = "hello js"; let m = null; let t = true; let f ; console.log(typeof n) console.log(typeof s) console.log(typeof m) console.log(typeof t) console.log(typeof f) //转换 var s1 = "20"; var s2 = "30abc"; var a = +"10"+10; console.log(s1+a); console.log(s2+a); console.log(a); //方法 let number = parseInt("a123abc"); console.log(number) //布尔 true 1 false 0 let b2 = true+1; console.log(b2) //数字 booelan 0 false 其余 true if (3){ console.log(3) } //字符串 if (""){ console.log("空窜") // false 其余 true } </script> </body> </html>