Eleventy URL Linter to check for changed URLs when swapping from `slug` to `slugify` filters (via @pdehaan)

.eleventy.js

const assert = require("assert");
const inspect = require("util").inspect;

module.exports= (eleventyConfig) => {
  // ever so slightly modified from @pdehaan’s original:
  // https://github.com/11ty/eleventy/issues/278#issuecomment-873367464

  const slugFn = eleventyConfig.getFilter("slug");
  const slugifyFn = eleventyConfig.getFilter("slugify");
  const slugErrors = new Set();

  eleventyConfig.addFilter("slug", function (str="") {
    const slugValue = slugFn(str);

    try {
      assert.strictEqual(slugValue, slugifyFn(str));
    } catch (err) {
      // Only display a unique error once.
      if (!slugErrors.has(err.message)) {
        console.error(`\nslug-vs-slugify filter mismatch for ${inspect(str)}\n${err.message}`);
        slugErrors.add(err.message);
        process.exitCode = 2;
      }
    } finally {
      return slugValue;
    }
  });
};

This is now bundled into the Eleventy Upgrade Helper plugin: https://github.com/11ty/eleventy-upgrade-help