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Vibe Coding · レッスン

バグの原因を絞り込む

AIの助けを借りて問題を切り分けます。

レッスン 3/413 ステップ

「バグの原因を絞り込む」はCoddyKit上の無料Vibe Codingレッスンです。 これはレッスン3/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはVibe Coding学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Vibe Codingコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

When There's No Clear Error

Sometimes nothing crashes — the app just does the wrong thing. The button doesn't work, the total is wrong, the data never shows up. No red text to paste.

This is where real debugging begins: narrowing down where the problem actually lives. The good news? AI is an incredible partner for this — if you know how to drive the investigation.

The Core Idea: Divide and Conquer

A bug hides somewhere between your input and your output. Your job is to cut the search space in half, over and over, until you've cornered it.

Think of it like finding a broken light in a string of bulbs: you check the middle first. Is the first half fine? Then the break is in the second half. This is the single most powerful debugging mindset there is.

Your Best Friend: console.log

The simplest tool to narrow a bug is printing values at different points. Run this — the logs show you exactly what each variable holds along the way.

When the printed value isn't what you expected, you've found the rough location of the bug.

function getDiscountedPrice(price, percent) {
  console.log("input price:", price);
  console.log("input percent:", percent);
  const discount = price * percent;
  console.log("discount:", discount);
  return price - discount;
}

console.log("final:", getDiscountedPrice(100, 20));

Read the Trail

Look at the output you just saw. The final price came out negative! The logs reveal why: percent was 20, but the math expected 0.20. The discount log showed 2000 — an obvious giveaway.

That's narrowing in action: the logs pointed at the exact line where reality diverged from your expectation. You don't fix blindly — you locate, then fix.

Ask AI to Add the Logs

You don't have to place logs by hand. Ask your AI tool to instrument the code for you. This is a perfect job for Cursor, Claude Code, or Copilot.

"This function returns the wrong total and I can't tell
why. Add console.log statements at each step so I can
see how the value changes. Don't change the logic yet —
just help me see what's happening."

Comment Out to Isolate

Another narrowing trick: temporarily disable chunks of code. If you suspect a section, comment it out (or return early) and see if the problem disappears.

If the bug goes away, it lives in the part you removed. If it stays, look elsewhere. You're shrinking the haystack with every test.

function renderPage() {
  loadHeader();
  // loadSidebar();   // <- commented out to test
  loadContent();
}

// Bug gone? The problem was in loadSidebar().
// Bug still here? Look at loadHeader or loadContent.

Check Your Assumptions

Most stubborn bugs come from a wrong assumption: 'I assumed this variable held a number, but it's actually text.' Narrowing means testing those assumptions one by one.

A great AI prompt for this:

"Here's my function and what I expect each variable to
hold. Tell me which of my assumptions might be wrong,
and suggest one console.log I could add to confirm
the most likely one."

Type Confusion Is Sneaky

A super common hidden bug: a value is text when you think it's a number. Run this — the result looks crazy until you realize "5" is a string, so + glues instead of adds.

Logging the type (not just the value) instantly reveals these.

const quantity = "5"; // came from a form input as text
const price = 10;
console.log("sum looks like:", quantity + price);
console.log("type of quantity:", typeof quantity);
console.log("fixed:", Number(quantity) + price);

Narrow the Steps to Reproduce

Narrowing isn't only about code — it's about actions. Find the smallest sequence that triggers the bug.

Does it happen every time, or only with an empty field? Only on the second click? Only with certain data? Each answer shrinks the suspect list. Hand these findings to AI:

"I narrowed it down: the crash only happens when the
search box is EMPTY and I press Enter. With any text
it works fine. Given this, where should I look in
this handler function? [paste function]"

Use the Debugger for Hard Cases

For trickier bugs, browser DevTools and editors like Cursor let you set a breakpoint — the code pauses on a line so you can inspect every variable at that moment.

You don't need it for everything; console.log handles most cases. But know it exists. You can even ask AI: 'Where should I put a breakpoint to catch this?' and it'll point you to the right line.

A Narrowing Workflow

Put it together into a repeatable loop:

  • 1. Describe expected vs actual output
  • 2. Log or disable a midpoint to split the problem in half
  • 3. Check which half the bug is in
  • 4. Repeat until you've cornered the exact line
  • 5. Hand AI the narrowed location + your findings for a precise fix

This turns 'somewhere in my 500 lines' into 'this one line on line 42'.

Quick Check

Your app shows a wrong total but throws no error. What's the smartest first move to narrow it down?

対話的なレッスンで答える

Recap: You Can Corner Any Bug

You learned to debug even when there's no error to paste. Your toolkit now includes:

  • Divide and conquer — cut the search space in half repeatedly
  • console.log values and types to see reality
  • Comment out sections to isolate the culprit
  • Test your assumptions and minimize the steps to reproduce
  • Let AI add logs, suggest breakpoints, and confirm assumptions

But what if you and the AI both get stuck in a loop? That's the final lesson: when AI gets stuck.

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よくある質問

「バグの原因を絞り込む」レッスンは無料ですか？

はい。「バグの原因を絞り込む」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Vibe Codingコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Vibe Codingコースには全4レッスンが含まれています。

「バグの原因を絞り込む」で何を学びますか？

AIの助けを借りて問題を切り分けます。 ブラウザで直接実行するハンズオンコードでVibe Codingを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Vibe Codingを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのVibe Codingは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/4です。

「バグの原因を絞り込む」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このVibe Codingレッスンでコードを書いて実行できますか？

はい。すべてのVibe Codingレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. エラーメッセージを理解する
  2. エラーを効果的にAIへ貼り付ける
  3. バグの原因を絞り込む
  4. AIが行き詰まったとき
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