Compare Code Online, Any Language

Developers frequently need to compare code outside the context of a full version control workflow. You might receive a code snippet by email and need to compare it to the version in your repository. You might need to compare a deployed file against a local copy to investigate a production incident. You might want to review a contractor's changes without granting repository access, or share a visual diff with a non-technical colleague who cannot use Git. In all of these situations, a browser-based code diff tool provides an immediate solution: paste two versions and get a clear, colour-coded diff in seconds without installing any software or configuring any repository.

FixTools Diff Checker is language-agnostic. It does not parse or interpret the code it receives. Instead, it applies the Myers diff algorithm to the raw text, computing the longest common subsequence of lines and marking all departures from that sequence as insertions or deletions. This approach works equally well for Python, JavaScript, TypeScript, Go, Rust, SQL, shell scripts, YAML, Terraform, and any other text-based language or format. The diff output mirrors the unified diff format used by Git: changed lines are colour-coded and, within changed lines, the specific modified tokens or characters are highlighted at a finer granularity for precise identification.

When reviewing code diffs, pay particular attention to lines that changed in ways that are not obviously a refactor. A changed condition inside an if statement, a flipped boolean, a different default parameter value, or a changed comparison operator can be easy to overlook in a large diff. Use the inline view for compact scanning when you need to move quickly through many changes, and switch to side-by-side view when you need to read the full context around a change to understand its intent.

A practical question that comes up often is whitespace handling. The Myers algorithm operates on the lines it is given, so leading and trailing whitespace on each line is significant by default. Two functions that differ only in indentation will show every line as changed, which is rarely what a code reviewer wants to see. The fix is to normalise indentation before pasting: run both versions through a formatter such as Prettier, gofmt, black, or rustfmt and paste the formatted outputs. After normalisation, only real logic changes appear in the output. Performance is reliable for typical source files of a few thousand lines; even on machine-generated code such as compiled output or bundled JavaScript, the comparison completes in well under a second on modern hardware. Because nothing leaves the browser, the tool is safe to use for proprietary source code, internal libraries, and unreleased features that should never reach an external service.

Code review benefits from understanding the difference between a naive textual diff, a lexer-based diff, and an AST-level diff. The naive textual approach used by FixTools and by default git diff treats the source as lines of text and reports any line-level difference, which is fast and language-agnostic but reports cosmetic reformatting and meaningful logic changes with the same visual weight. A lexer-based diff tokenises both sides first and compares token streams rather than raw characters, which makes the output insensitive to whitespace and comment changes between identical token sequences. Tools like difftastic and the syntax-aware diff modes in IDEs implement this approach. An AST-level diff goes further by parsing both sides into an abstract syntax tree and comparing tree structures, which can recognise that a variable rename or a function-level move is a refactor rather than a wholesale rewrite. Each level of sophistication trades language-agnostic simplicity for higher review accuracy on specific languages the tool understands. For pragmatic day-to-day code review, the textual diff with a formatter applied first to both inputs is fast, works for every language, and produces output that any reviewer can read without learning new conventions. Reserve lexer-based and AST-level diffs for refactor-heavy reviews where reformatting noise would otherwise swamp the substantive changes you need to assess.