字符串的使用
如何查看数据类型 类型判断
<script>
let hd="3";
let arr=[1,2,3];
let obj={};
console.log(typeof hd);
console.log(typeof arr);
console.log(typeof obj);
</script>
log需要在控制台中查看。chrome浏览器按F12快捷键。
<script>
let hd="3";
let arr=[1,2,3];
let obj={};
console.log( hd instanceof String);
console.log( arr instanceof Array);
console.log( obj instanceof Object);
</script>
instanceof用于判断声明的变量是否为某一类型的对象。
字符串
**转义符 空格符\t 换行符\n **
<script>
let hd="hsujsof\"sdsfsf";
console.log(hd);
let str="hhhh\t\t\tsss\nddddd";
console.log(str);
</script>
连接多个字符串
<script>
let year="2020 nian";
let month="2 month";
let day="19 day";
console.log(year + month + day);
console.log(`${year} ${month} ${day}`);
console.log(year + "的" + month + "的" + day);
console.log(`${year}的${month}的${day}`);
</script>
dollar符旁边的小撇，在键盘上的1左边，Tab上边。
字符串一些常用函数
<body>
<script>
let name="abcdefghiklmn";
console.log(name.length);
console.log(name.toUpperCase());
console.log(name.toLowerCase());
let size = " abcdf ";
console.log(size.trim().length);//去除字符串
</script>
</body>
实例
设计一个表单输入密码，密码不能输入空格，小于5位时提示。大于5位提示消失。
<body>
<input type="password" name="password" />
<span></span>
<script>
let ps = document.querySelector("[name='password']");
ps.addEventListener("keyup",function(){
this.value = this.value.trim();
let span = document.getElementsByTagName("span");
let error="";
if(this.value.length<5){
error = "密码不能小于5位";
}
span[0].innerHTML = error;
});
</script>
</body>
字符串截取
<body>
<script>
let hd = "summervocation";
console.log(hd.slice(1));
console.log(hd.slice(1,6));
console.log(hd.slice(-6));
console.log(hd.substring(1));
console.log(hd.substring(1,5));
console.log(hd.substring(-3));
console.log(hd.substr(1));
console.log(hd.substr(1,7));
console.log(hd.substr(-3));
</script>
</body>
截取函数如上代码，substring函数会把负参数当0；
字符串检索
<body>
<script>
let hd = "summervocation";
console.log(hd.indexOf("s")); //检查是否存在字符"s" 存在则返回位置，不存在返回-1
console.log(hd.indexOf("o",5));// 从第五位开始检索
console.log(hd.indexOf("1")); //1 不存在，返回-1
console.log(hd.includes("o",3)); //检索 o 是否存在 返回boo类型
console.log(hd.lastIndexOf("m",10)); // 从第10位倒着检索
console.log(hd.startsWith("s")); //判断是否以s开头
console.log(hd.toLowerCase().startsWith("s"));//不区分大小写判断
console.log(hd.endsWith("n")); //判断是否以n结尾
</script>
</body>
实例： 电话号码模糊处理
<body>
<script>
function phone(mobile, len=3){
return String(mobile).slice(0,len*-1)+"*".repeat(len);
}
console.log(phone(16452735689,4));
</script>
</body>
字符串类型转换
<body>
<script>
const str ="99";
console.log(typeof str);
console.log(str*1+778); //隐式转换
console.log(Number(str)); //调用构造函数显示转换
console.log(typeof Number(str));
const num = 99;
console.log(typeof num);
const str1 = num + ""; //隐式转换位字符串
console.log(typeof str1);
console.log(typeof String(num)); //先手转换
const str2 = "99hhhhhh";
console.log(parseInt(str2)); //转换位整数类型，后面的字符自动省略，但必须以数字开头
const arr="abcdefgh";
console.log(arr.slice(""));
console.log(arr.slice("").length); //将字符串转换为数组
const arr2 = ["aaa","bbb","ccc"];
console.log(arr2.join(" ")); //将数组连为字符串，中间以空格相连
console.log(arr2.join("|")); //将数组连为字符串，中间以 | 相连
</script>
</body>
