JavaScript Promise 并发

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1) Current Task & Steps

Current task: Step 1 - 并发拿值并求和
实现 async sumAsync(a, b)，并发等待 Promise.resolve(a)、Promise.resolve(b) 后返回和

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: sumAsync(a, b)
Self-check: sumAsync(2, 3) => 5

2) Beginner Explanation (Current Step)

Step task: 实现 async sumAsync(a, b)，并发等待 Promise.resolve(a)、Promise.resolve(b) 后返回和
Define function(s): sumAsync, 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 sumAsync(a, b) {
  return 0;
}
// Self-check: sumAsync(2, 3) should return 5
console.log(sumAsync(2, 3));

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 sumAsync
sumAsync(2,3) 结果为 5

🔐 学习提示

📖 快速参考

任务互不依赖就优先并发；有前后依赖才用串行 await。

About This Exercise

当两个请求互不依赖时，不要一个一个等，可以一起发。

Promise.all 就是“同时等待多个任务”的标准写法。

这关你会做三次练习：数字汇总、字符串拼接、结果格式化。

Intermediate⏱ 25-30 min

Learning Objectives

理解串行等待和并发等待的差别
熟练使用 Promise.all + await
能把并发结果组装成业务需要的格式

Scenario

你在加载学习仪表盘：课程数据和统计数据可以同时请求，最后再统一显示给用户。

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