JavaScript 箭头函数

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1) Current Task & Steps

Current task: Step 1 - 基础箭头函数
实现 doubleNumber(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: doubleNumber(n)
Self-check: doubleNumber(6) => 12

2) Beginner Explanation (Current Step)

Step task: 实现 doubleNumber(n)，返回 n * 2
Define function(s): doubleNumber, 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 doubleNumber(n) {
  return null;
}
// Self-check: doubleNumber(6) should return 12
console.log(doubleNumber(6));

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 doubleNumber
doubleNumber(6) = 12

🔐 学习提示

📖 快速参考

箭头函数常用于回调场景，可读性更高。

About This Exercise

箭头函数是 function 的简写形式，尤其适合短逻辑和数组回调。

这一关的目标不是背语法，而是学会“从旧写法改成新写法”。

Beginner⏱ 15-20 min

Learning Objectives

掌握箭头函数基本语法
使用隐式返回
在数组方法中应用箭头函数

Scenario

你在优化用户行为统计代码，需要把旧式 function 写法改为更简洁的箭头函数版本。

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