JavaScript 异步循环模式

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1) Current Task & Steps

Current task: Step 1 - 第一步：写单项异步转换
实现 async doubleAsync(n)，返回 n * 2

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: doubleAsync(n)
Self-check: doubleAsync(3) => 6

2) Beginner Explanation (Current Step)

Step task: 实现 async doubleAsync(n)，返回 n * 2
Define function(s): doubleAsync, 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 doubleAsync(n) {
  return null;
}
// Self-check: doubleAsync(3) should return 6
console.log(doubleAsync(3));

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 doubleAsync
doubleAsync(3)=6

🔐 学习提示

📖 快速参考

不要在 map 回调里忘记 return；否则 Promise.all 得到的结果会不对。

About This Exercise

当你有一组数据要“异步处理”时，常见流程是：

先写单项处理函数（一个输入 -> 一个输出）
再把整组数据并发处理（Promise.all）
最后对结果做拼接或求和

这就是前端批量请求和批量转换的基础模板。

Intermediate⏱ 25-30 min

Learning Objectives

写出基础 async 函数
掌握 Promise.all + map 的批处理写法
能对异步结果做字符串或数值汇总

Scenario

你在处理一批学习数据：先把每个分数做异步转换，再批量拿到结果，最后输出展示字符串和总分。

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