A node script that inserts a portion of a snippet file into a section of another file

replace_file_snippet.js

// Example usage:
// node replace_file_snippet.js \
//   inputFile.html '<!--InsertionAreaStart-->' '<!--InsertionAreaEnd-->' \
//   snippetFile.html '<!--OptionalSnippetStart-->' '<!--OptionalSnippetEnd-->' \
//   outputFile.html

const fs = require('fs');
const path = require('path');

function main() {
  const args = process.argv.slice(2);
  const numArgs = args.length;
  const hasEnoughArgs = (numArgs === 5 || numArgs === 7);
  if (!hasEnoughArgs) {
    console.error('Usage: ',
      path.basename(process.argv[0]),
      path.basename(process.argv[1]),
      '<read_filename>',
      '<read_start_text>',
      '<read_end_text>',
      '<snippet_filename>',
      '[<snippet_start_text>',
      '<snippet_end_text>]',
      '<write_filename>');
    return;
  }

  const readFilename = args[0];
  const readStartText = args[1];
  const readEndText = args[2];
  const snippetFilename = args[3];
  let snippetStartText;
  let snippetEndText;
  let writeFilename;
  if (numArgs === 5) {
    writeFilename = args[4];
  } else {
    snippetStartText = args[4];
    snippetEndText = args[5];
    writeFilename = args[6];
  }

  changeConfig(readFilename, readStartText, readEndText,
    snippetFilename, snippetStartText, snippetEndText,
    writeFilename);
}

function changeConfig(readFilename, readStartText, readEndText,
                      snippetFilename, snippetStartText, snippetEndText,
                      writeFilename) {
  fs.readFile(readFilename, 'utf8', (err1, readData) => {
    if (err1) {
      console.error('Error reading', readFilename, ': ', err1);
      process.exit();
    }

    const readStartREP = escapeREP(readStartText);
    const readEndREP = escapeREP(readEndText);

    const readREP = `${readStartREP}[\\s\\S]*${readEndREP}`;
    const readRegExp = new RegExp(readREP, 'g');
    const hasReadPattern = readRegExp.test(readData);
    if (!hasReadPattern) {
      console.log('The pattern', readREP, 'does not exist in', readFilename);
      process.exit();
    }

    fs.readFile(snippetFilename, 'utf8', (err2, snippetData) => {
      if (err2) {
        console.error('Error reading', snippetFilename, ': ', err2);
        process.exit();
      }

      let innerSnippetData = snippetData;
      if (!!snippetStartText && !!snippetEndText) {
        const snippetStartREP = escapeREP(snippetStartText);
        const snippetEndREP = escapeREP(snippetEndText);
        const innerSnippetREP = `${snippetStartREP}([\\s\\S]*)${snippetEndREP}`;
        const innerSnippetRegExp = new RegExp(innerSnippetREP);
        const matches = snippetData.match(innerSnippetRegExp);
        if (matches.length > 1) {
          innerSnippetData = matches[1];
        }
      }

      const replacement = `${readStartText}\r\n${innerSnippetData}${readEndText}`;
      const result = readData.replace(readRegExp, replacement);

      fs.writeFile(writeFilename, result, 'utf8', (err3) => {
        if (err3) {
          console.error('Error reading', snippetFilename, ': ', err2);
          process.exit();
        }
        console.log('Updated', writeFilename);
      });
    });
  });
}

function escapeREP(rep) {
  return rep.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); // $& means the whole matched string
}

main();