断言对象属性存在却得到意外结果的问题排查

Hey there! Let's break down why your code always lands in the else branch and get it working as you expect.

The Core Issue #

Your current condition checks if user.fName === true and user.lName === true—but those properties hold string values ('john' and 'doe'), not the boolean true. JavaScript's strict equality (===) only matches values that are identical in both type and content, so these checks will never pass, no matter what valid names you use.

How to Fix It #

Instead of comparing to true, we need to verify that these properties exist and have a meaningful value. Here are two reliable approaches:

1. Simple Truthy Check (Great for Most Scenarios)

Non-empty strings, positive numbers, and defined values are "truthy" in JavaScript—they evaluate to true when used in a boolean context. You can simplify your condition to:

if(user.fName && user.lName && user.age >= 18) {
    // Success case
}

This works because:

• user.fName is 'john' (truthy)
• user.lName is 'doe' (truthy)
• user.age is 18 (meets the >=18 requirement)

2. Strict Existence + Value Check (More Precise)

If you want extra safeguards—like ensuring properties exist on the object and aren't empty strings—you can use this stricter version:

if('fName' in user && 'lName' in user && user.fName.trim() !== '' && user.lName.trim() !== '' && user.age >= 18) {
    // Success case
}

This guards against edge cases where fName might be an empty string or the property doesn't exist at all.

Corrected Full Code #

Here's your code updated with the simple truthy check:

let user = {
    fName: 'john',
    lName: 'doe',
    uName: 'jDoe23',
    age: 18,
};

function helloUser(user) {
    if(user.fName && user.lName && user.age >= 18) {
        console.log(`Hello ${user.uName}. Welcome to the world of wonders.`);
    } else {
        console.log("Please get a parent's permission before playing this game.");
    }
};

helloUser(user);

Now when you run this, it'll trigger the welcome message just like you wanted!

内容的提问来源于stack exchange，提问作者Mr Neptune