JavaScript フォーマッター

A JavaScript formatter takes a script — minified, copied from a stack trace, or written in a hurry — and rewrites it with consistent indentation, line breaks, and bracket placement. The script does not change behaviour; it just becomes possible to read. That difference is what makes review, debugging, and onboarding tractable.

Why formatting deserves its own moment, not a side-effect of writing

Most JavaScript regressions are not algorithmic — they are structural: a missing brace, a callback nested one level too deep, a return that drops out of an arrow function unexpectedly. Those problems are almost invisible in a one-line dump and become obvious once the file is properly indented. Treating formatting as a separate step ensures every reader sees the same shape, regardless of how the code arrived.

What a sensible formatter changes — and what it must not touch

Formatting is allowed to change the visual shape of the file: indentation, where lines wrap, where braces sit, how chained calls are stacked. Formatting must not change identifiers, the order of statements, the contents of strings, regex literals, or template expressions, and it must not insert or delete semicolons in a way that alters ASI behaviour.

• Indent each block consistently, using the same character (spaces or tabs) and the same depth as the surrounding project.
• Place one statement per line so that a one-character logical change is also a one-line diff.
• Stack long function call chains so the dot operators line up and the call tree is obvious at a glance.
• Preserve every comment in place — comments are part of the reviewable record, not optional decoration.
• Keep declaration order, expression order, and semicolon placement byte-for-byte equivalent to the source.

Rule of thumb: after formatting, the parsed AST of the file should be identical to the AST before formatting. Anything that changes that is no longer formatting — it is refactoring.

このツールの使い方

1. Prepare representative JavaScript snippets and copied code that need readable structure for review in JavaScript フォーマッター instead of starting with the largest or most sensitive real input.
2. Run the workflow, generate formatted JavaScript that is easier to diff, annotate, and inspect, and review strings, comments, semicolon boundaries, regex literals, nested blocks, and whether the result still matches the intended runtime behavior before deciding the result is ready.
3. Only copy or download the result after it fits code review, debugging, docs examples, and support investigation and no longer conflicts with this constraint: Formatting should be treated as a readability step, not as proof that the code is lint-clean or runtime-safe.

JavaScript フォーマッター の例

この例は、JavaScript フォーマッター が想定している入力の形と、自分の作業に使う前に確認しておきたい結果の見え方を示しています。

入力例

function add(a, b) { return a + b; }

期待される出力

function add(a,b){return a+b}

A minified or carelessly written snippet, reformatted

// before — copied out of a bundle
function loadUsers(ids){return Promise.all(ids.map(id=>fetch("/api/users/"+id).then(r=>r.json())))}

// after — readable for review and diffs
function loadUsers(ids) {
  return Promise.all(
    ids.map((id) =>
      fetch("/api/users/" + id).then((r) => r.json())
    )
  );
}

When reformatting JavaScript directly saves time

Formatting pays off whenever a piece of JavaScript will be read by more than one person, more than once. The situations below are where reformatting before reading is almost always worth it.

• Debugging a production stack trace that points into a minified bundle, after retrieving the original or source-mapped snippet.
• Reviewing third-party snippets, gists, or community examples before pasting them into a project.
• Cleaning code copied from chat or a ticket, where whitespace was lost or partially preserved by the transport.
• Onboarding teammates — a reformatted file gives every reader the same starting point, regardless of which editor they use.
• Authoring documentation, blog posts, or slides — readers cannot follow logic that wraps awkwardly or hides braces.

What formatting cannot tell you about the code

A reformatted file is easier to read, but readability is not the same as correctness. A few categories of problem stay hidden even after the indentation is fixed; they need linting, type checking, or actually running the code.

• Logical bugs: an off-by-one in a loop or a wrong conditional looks just as tidy after formatting as before. Tests, not formatting, catch those.
• Type mismatches in plain JavaScript: a formatter has no type information, so silent string/number coercions slide through untouched.
• Async pitfalls: missing await, unhandled promise rejections, or fire-and-forget background tasks. Indentation will not flag them.
• Performance regressions: a clean nested loop is still O(n²); formatting only makes the shape visible, not faster.

Formatter vs the steps it is often confused with

実用上の注意

• 元の JavaScript 内容が本番向けだったり長文だったりする場合は、まず小さな断片で試してから全体に適用してください。
• 整形や圧縮は見た目とサイズを変える処理であり、lint、構文解析、実行確認の代わりにはなりません。
• SQL やスクリプトのような実行可能な出力は、実環境で動かす前に必ず内容を見直してください。

JavaScript フォーマッター の参考情報

JavaScript フォーマッター は、整形 によって JavaScript の断片がどう変わるか、そして別環境へ貼り付ける前に何を確認すべきかを説明します。

• 整形は読みやすさを高め、圧縮は payload を小さくし埋め込みやすくすることを重視します。
• 処理後は、引用符、コメント、セミコロン、複数行の内容を優先的に確認してください。
• SQL やスクリプトのような実行可能な出力は、本番環境で動かす前に必ず見直してください。

参考資料

• ECMAScript Language Specification
• MDN: JavaScript reference