8 Common JavaScript Formatting Mistakes (and How to Fix Them)

None of these mistakes will crash your code. All of them make it slower to read — and slower-to-read code hides bugs longer. Here are the ones that show up most often in real codebases.

1. Mixing tabs and spaces

When some lines are indented with tabs and others with spaces, code can look aligned in one editor and jagged in another. Pick one and let a formatter enforce it everywhere.

2. Inconsistent quote style

Switching between 'single' and "double" quotes within the same file adds visual noise for no functional benefit. Most style guides pick one as the default and use the other only when needed to avoid escaping.

3. Inconsistent semicolon usage

JavaScript's automatic semicolon insertion means code can technically run without them, but mixing "sometimes there, sometimes not" is a common source of confusion and the occasional real bug around line breaks.

4. Deeply nested code with no visual break

Five levels of nested if statements with identical indentation are hard to trace by eye. This is more of a structure problem than a pure formatting one — restructuring with early returns often helps more than any formatter can.

5. Long lines that force horizontal scrolling

A single line with a long chain of .then().then().catch() or a giant object literal is hard to review in a diff. Formatters like Prettier will automatically wrap these at a sensible width.

6. No blank lines between logical sections

A wall of code with zero blank lines makes it hard to tell where one idea ends and the next begins. A blank line between setup, logic and return is a small habit with an outsized readability payoff.

7. Trailing whitespace and inconsistent trailing commas

Trailing whitespace is invisible until it shows up as a noisy line in a diff. Trailing commas in multi-line arrays and objects, on the other hand, make future additions produce cleaner one-line diffs.

8. Formatting by hand instead of automating it

The biggest mistake isn't any single style choice — it's relying on manual discipline to maintain consistency across a team. Every one of the issues above is solved permanently by running code through a formatter automatically.

The fastest fix

Paste any file with these issues into the JS formatter on this site and they're resolved in one click. For the bigger picture on why this matters, see what JavaScript formatting actually does, and for structural (not just cosmetic) improvements, read 10 best practices for writing clean JavaScript.