React Diff Viewer

May 23, 2020

Kyle Matthews, author of the Gatsbyjs project, tweeted asking whether anyone had made a React component making use of the diff package from NPM. Since I had made one for work a few weeks back, and since this website is largely powered by the framework that he put out into the world for free, I thought I'd rewrite a version of the diff component and share it. This feature is inspired by various string comparison tools, commonly used to help make sense of Git versioning. GitHub, VSCode, and some other tools have some realy good examples of this.

example diff of a VSCode editor

Example provided int the VSCode docs

My use case never didn't require the "line-by-line" style comparison for git, but was more focused on an expectation of finding small differences between different people's inputs. I went with an approach that could be customized to either show a diff by letters or by words.

Here's the example I built!

Use with Prism JS

It's also possible to use this for an inline text diff for code samples using Prism js!

var str = "text before diff";
const fooprofessionalVariable = "bar";
let textAfterDiff;

This gets a bit tricky, because Prism JS isn't aware of the React lifecycle, and replaces every element inside the target code tag with custom spans. That can cause the code to break pretty easily. Fortunately, you can solve this by using multiple code tags, on either side of your Diff-ed text.

Also, you will need to modify the React Diff component to run on any text that is included in the string that you want highlighting on. That will end up looking like this:

// PrismDiff.js
import React from "react";
import * as diff from "diff";
import PropTypes from "prop-types";
import Prism from "prismjs";

const styles = {
  added: {
    color: "green",
    backgroundColor: "#b5efdb",
    textShadow: "none",
  },
  removed: {
    color: "red",
    backgroundColor: "#fec4c0",
    textShadow: "none",
  },
};

const PrismDiff = ({ string1 = "", string2 = "", mode = "characters" }) => {
  let groups = [];

  if (mode === "characters") groups = diff.diffChars(string1, string2);
  if (mode === "words") groups = diff.diffWords(string1, string2);

  const mappedNodes = groups.map(group => {
    const { value, added, removed } = group;
    let nodeStyles;
    if (added) nodeStyles = styles.added;
    if (removed) nodeStyles = styles.removed;

    // Using dangerouslySetInnerHTML with the Node rendering API
    // Note: is dangerous
    return (
      <span
        style={nodeStyles}
        dangerouslySetInnerHTML={{
          __html: Prism.highlight(
            value,
            Prism.languages.javascript,
            "javascript"
          ),
        }}
      />
    );
  });

  return <span>{mappedNodes}</span>;
};

PrismDiff.propTypes = {
  string1: PropTypes.string,
  string2: PropTypes.string,
  mode: PropTypes.oneOf(["characters", "words"]),
};

export default PrismDiff;
// PrismExample.js
import React, { useLayoutEffect } from "react";
import Prism from "prismjs";
import PrismDiff from "./PrismDiff";

const PrismExample = () => {
  useLayoutEffect(() => {
    Prism.highlightAll();
  }, []);

  return (
    <pre>
      <code className="language-javascript">var str = "text before diff";</code>
      {`
`}
      <PrismDiff
        string1={'const foo = "bar"'}
        string2={'const professionalVariable = "bar";'}
        mode="words"
      />
      {`
`}
      <code className="language-javascript">let textAfterDiff;</code>
    </pre>
  );
};

export default PrismExample;

Hope this helps as you make your own content!

There is also an official Prism js plugin that supports highlighting for line diffs, but I don't think it handles the string comparison for you.

https://prismjs.com/plugins/diff-highlight/

© Kyle Peacock 2021