JavaScript Set 与 Map

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1) Current Task & Steps

Current task: Step 1 - 第一步：去重后统计数量
实现 uniqueCount(list)，返回去重后的元素数量

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: uniqueCount(list)
Self-check: uniqueCount(["a","a","b","c"]) => 3

2) Beginner Explanation (Current Step)

Step task: 实现 uniqueCount(list)，返回去重后的元素数量
Define function(s): uniqueCount, 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 uniqueCount(list) {
  return null;
}
// Self-check: uniqueCount(["a","a","b","c"]) should return 3
console.log(uniqueCount(["a","a","b","c"]));

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 uniqueCount
uniqueCount 返回正确

🔐 学习提示

📖 快速参考

Map 的 key 可以是任意类型，且提供 clear/size/get/set 等明确 API。

About This Exercise

Set 适合“只保留唯一值”，天然去重。

Map 适合“键值查询表”，比普通对象更明确。

这一关会带你做 3 个常见任务：统计唯一数、字典查询、去重后排序输出。

Beginner⏱ 20-25 min

Learning Objectives

掌握 Set 的去重思路与 size 用法
掌握 Map 的 get 查询与默认值兜底
能把 Set 转数组后继续做排序/拼接

Scenario

你在做课程标签与语言包功能：一批标签有重复，需要统计唯一项；语言映射需要按 key 快速查值。

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