JavaScript 可选链与空值合并

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Complete current Step 1 first

1) Current Task & Steps

Current task: Step 1 - 第一步：安全读取昵称
实现 getNickname(user)，读取 user.profile.nickname，缺失时返回 "guest"

Write your code in `script.js` for the current Step.
Click "Run Validation" to check if rules pass.
Once current Step passes, the next step unlocks automatically.
You need to pass Step 1 first to unlock Step 2.

Step Deliverables (Check these)

Define function: getNickname(user)
Self-check: getNickname({"profile":{"nickname":"amy"}}) => "amy"
Self-check: getNickname({}) => "guest"

2) Beginner Explanation (Current Step)

Step task: 实现 getNickname(user)，读取 user.profile.nickname，缺失时返回 "guest"
Define function(s): getNickname, then verify results with test inputs.
This step has a specific expected result. Focus on making the function return the correct value.
A function is a reusable block of code. Define it first, then call it.
`return` sends a result back. Without return, you usually can't get the function's result.

3) Starter Template (Type along)

function getNickname(user) {
  return null;
}
// Self-check: getNickname({"profile":{"nickname":"amy"}}) should return "amy"
console.log(getNickname({"profile":{"nickname":"amy"}}));

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 getNickname
有昵称时返回昵称
缺失时返回 guest

🔐 学习提示

📖 快速参考

?? 只在 null/undefined 时生效；0、空字符串不会触发默认值。

About This Exercise

很多新手第一次接接口数据会遇到报错：

Cannot read properties of undefined

这通常是因为某层字段不存在。

本关两把“安全钥匙”：

?.：中途断了就返回 undefined，不报错
??：当值是 null/undefined 时给默认值

Beginner⏱ 20-25 min

Learning Objectives

掌握 ?. 的安全访问语法
掌握 ?? 的默认值兜底语法
能写出对缺失字段更稳健的展示逻辑

Scenario

你要渲染用户资料卡，但后端字段经常不完整：有的人没有昵称，有的人没有城市。你要让页面“不报错、可显示、可兜底”。

Why Do This

Frontend JS is not just about knowing the syntax — the key is reliably manipulating the DOM and managing interaction state.
Break down the event flow (trigger -> handle -> update UI) before coding, and you will make significantly fewer errors.
Rule-based validation helps you develop a testable frontend mindset.

Hands-on Practice

Write out the event flow in comments first, then implement the functions.
Add an edge case branch for invalid input or empty data.
Explain why each validation rule is needed.

Common Mistakes

Only testing the happy path without verifying edge inputs or repeated clicks.
Mixing event binding and state updates together, making future maintenance difficult.
Console output looks correct but the UI is not actually updated.

Deliverable

A reusable interaction component scaffold (event flow description + key functions + edge case handling).

My Progress

Completed Steps0 / 3

Total Attempts0

Best Score0%

Completion Criteria (Checkable)

Confirm before completing this Lab

All rules for the current step passCan explain each line of your codeCan name a real-world business scenario for this

Reflection Questions (Recommended)

Describe the event flow of this Lab in 2-3 sentences (trigger -> handle -> update).

What edge case did you add? Why is it easy to miss?

How confident are you with this interaction pattern now?

Not confident yet, need more practiceCan complete similar tasks independentlyCan teach others how to do it