post-eleventy-build.js

let configured = false;
let finished = false;
const methodsToCall = [];

function setup() {
  const Eleventy = require("@11ty/eleventy");

  const originalWrite = Eleventy.prototype.write;
  Eleventy.prototype.write = async function(...args) {
    const out = await originalWrite.apply(this, args);

    finished = true;

    while (methodsToCall.length) {
      const method = methodsToCall.shift();
      await method();
    }

    return out;
  };
}

/**
 * @param {function(): void} method to call once Eleventy is done
 */
module.exports = function postEleventyBuild(method) {
  if (!configured) {
    configured = true;
    setImmediate(setup);
  }
  if (finished) {
    throw new Error(`Eleventy has already finished`);
  }
  methodsToCall.push(method);
};