Why use side-by-side diff view?

Side-by-side diff view displays the old and new file versions in parallel columns, making it easier to see exactly what changed at each line. It is the preferred view for code reviews because you can scan corresponding lines horizontally. Inline view is better for small changes or narrow screens, while side-by-side excels for refactoring and multi-line changes.

// JavaScript — render side-by-side diff view
function sideBySide(oldLines, newLines) {
  const pairs = [];
  let oldIdx = 0, newIdx = 0;
  while (oldIdx < oldLines.length || newIdx < newLines.length) {
    const oldLine = oldLines[oldIdx] || "";
    const newLine = newLines[newIdx] || "";
    if (oldLine === newLine) {
      pairs.push({ left: oldLine, right: newLine, type: "same" });
      oldIdx++; newIdx++;
    } else {
      if (oldIdx < oldLines.length)
        pairs.push({ left: oldLine, right: "", type: "removed" });
      if (newIdx < newLines.length)
        pairs.push({ left: "", right: newLine, type: "added" });
      oldIdx++; newIdx++;
    }
  }
  return pairs;
}

How to read a side-by-side diff

In side-by-side view, the left column shows the original file and the right column shows the modified version. Deleted lines appear highlighted in red on the left with an empty slot on the right. Added lines appear highlighted in green on the right with an empty slot on the left. Modified lines show the old version in red (left) and new version in green (right) on the same row.