JavaScript let/const 作用域

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Complete current Step 1 first

1) Current Task & Steps

Current task: Step 1 - 块级作用域
实现 getBlockValue()，函数内 if 块中定义 let value = "inside"，返回该值

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: getBlockValue(value)
Self-check: getBlockValue() => "inside"

2) Beginner Explanation (Current Step)

Step task: 实现 getBlockValue()，函数内 if 块中定义 let value = "inside"，返回该值
Define function(s): getBlockValue, 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 getBlockValue(value) {
  return null;
}
// Self-check: getBlockValue() should return "inside"
console.log(getBlockValue());

4) Rule Validation & Hints

📋 当前任务 - Step 1

定义 getBlockValue
返回 inside

🔐 学习提示

📖 快速参考

let/const 能有效减少变量污染和隐藏 bug。

About This Exercise

这关专门解决新手常见困惑：为什么同一个变量名在不同位置会“看起来不一样”？

let 有块级作用域，const 表示这个引用不再重新赋值。

你不需要一次记住所有规则，先通过 3 个小函数建立直觉。

Beginner⏱ 15-20 min

Learning Objectives

理解块级作用域
用 const 表达不变引用
避免 var 带来的问题

Scenario

你在维护用户列表逻辑，旧代码大量使用 var，容易出现作用域和重名 bug。目标是改写为更安全、更可维护的 ES6 变量声明方式。

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