Jest coverage collection from multi-project and merging them into single coverage .lcov file

collectCoverageFiles.ts

import fs from 'fs-extra';
import glob from 'glob';
import { exec } from 'child_process';

const coverageOutputDir = '.nyc_output';

fs.emptyDirSync(coverageOutputDir);

glob('**/coverage-final.json', (err, coverageFiles) => {
  if (err) {
    throw err;
  }

  if (!coverageFiles.length) {
    throw new Error('Error finding coverage files');
  }

  coverageFiles.forEach((coverageFile, index) => {
    if (!coverageFile) {
      throw new Error('Error finding coverage files');
    }

    const [fileName, fileExtention] = (coverageFile
      .split('/')
      .pop() as string).split('.');

    const destination = `${coverageOutputDir}/${fileName}-${index +
      1}.${fileExtention}`;

    fs.copyFile(coverageFile, destination, copyFileError => {
      if (copyFileError) {
        throw copyFileError;
      }

      // tslint:disable-next-line no-console
      console.log(`${coverageFile} was copied to ${destination}`);
    });
  });

  exec(
    `./node_modules/nyc/bin/nyc.js report --reporter=lcov --report-dir=${coverageOutputDir}`,
  );
});