JavaScript 事件处理
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About This Exercise
事件就是“用户做了什么”：点击按钮、输入内容、提交表单。
前端代码大部分不是一直跑，而是“等事件发生再执行”。
你将学到：
- 用
addEventListener绑定监听 - 处理 click / input / submit 三种常见事件
- 从
event.target读取输入值 - 用
preventDefault阻止表单默认行为
Beginner⏱ 25-30 min
Learning Objectives
- 理解事件驱动编程的基本模型
- 掌握 addEventListener 的基础写法
- 能实现点击计数与实时输入同步
- 能安全处理表单提交
Scenario
你在做一个交互演示页：
用户点击按钮时计数增加，输入文本时实时显示，提交表单时输出日志但不刷新页面。
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
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?
Tags
JavaScriptEventsaddEventListenerInteraction