node.js 一个感叹号

// 错误的例子
if (!userInput) {
    console.log("User input is empty");
}

// 正确的例子
if (userInput === "") {
    console.log("User input is empty");
}

// 错误的例子
if (!someObject) {
    console.log("Object is empty");
}

// 正确的例子
if (!Boolean(someObject)) {
    console.log("Object is empty");
}

function validateInput(input) {
    if (!input) {
        return "Input is required";
    }
    if (typeof input !== "string") {
        return "Input must be a string";
    }
    if (input.length < 5) {
        return "Input must be at least 5 characters long";
    }
    return "Input is valid";
}

console.log(validateInput(""));       // "Input is required"
console.log(validateInput(123));      // "Input must be a string"
console.log(validateInput("abc"));    // "Input must be at least 5 characters long"
console.log(validateInput("hello"));  // "Input is valid"