JavaScript 不可变数据更新

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Complete current Step 1 first

1) Current Task & Steps

Current task: Step 1 - 第一步：数组追加元素
实现 appendItem(list, item)，返回新数组

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: appendItem(list, item)
Self-check: appendItem([1,2], 3) => "1,2,3"

2) Beginner Explanation (Current Step)

Step task: 实现 appendItem(list, item)，返回新数组
Define function(s): appendItem, 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 appendItem(list, item) {
  return null;
}
// Self-check: appendItem([1,2], 3) should return "1,2,3"
console.log(appendItem([1,2], 3));

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 appendItem
appendItem([1,2],3) -> 1,2,3

🔐 学习提示

📖 快速参考

不可变更新更易调试与回溯。

About This Exercise

不可变更新就是：旧数据不动，新建一份再改。

在 React 状态管理里，这是最基础也最重要的习惯。

这样做可以避免“改了旧状态却没触发更新”这类隐蔽 bug。

Beginner⏱ 20-25 min

Learning Objectives

掌握数组的不可变追加写法
掌握对象字段不可变更新写法
掌握列表项按条件不可变更新写法

Scenario

你在做任务列表状态管理：需要追加任务、改用户名、切换任务完成状态，但不能直接改原数据。

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