js if string

let str = "";

if (str === "") {
    console.log("字符串为空");
} else {
    console.log("字符串不为空");
}

let str = "Hello";

if (str.length > 5) {
    console.log("字符串长度大于5");
} else {
    console.log("字符串长度小于或等于5");
}

let str = "Hello, world!";

if (str.includes("world")) {
    console.log("字符串包含'world'");
} else {
    console.log("字符串不包含'world'");
}

let str = "JavaScript";

if (str === "JavaScript") {
    console.log("字符串是JavaScript");
} else {
    console.log("字符串不是JavaScript");
}

let str = "123-45-6789";

let regex = /^\d{3}-\d{2}-\d{4}$/; // 匹配社会安全号码格式

if (regex.test(str)) {
    console.log("字符串符合社会安全号码格式");
} else {
    console.log("字符串不符合社会安全号码格式");
}

let str1 = "JavaScript";
let str2 = "javascript";

if (str1.toLowerCase() === str2.toLowerCase()) {
    console.log("两个字符串相等（忽略大小写）");
} else {
    console.log("两个字符串不相等");
}

let str = "  JavaScript! ";

// 去除前后空格和特殊字符
str = str.trim().replace(/[^a-zA-Z0-9]/g, '');

console.log(str); // 输出: JavaScript

let str = ""; // 或者 let str = undefined;

if (typeof str === 'string' && str.trim() !== "") {
    console.log("字符串有效");
} else {
    console.log("字符串无效或为空");
}