Script to run after npm install, sets variables to .env file, and the variables can be later utilized.

after_install.js

const fs = require('fs');
const nl = require('os').EOL;
const LineByLineReader = require('line-by-line');
const cp = require('child_process');

afterInstall('.env', 'utf8');

function afterInstall(filename, encoding) {
  const envVars = {
    'GIT_COMMIT_HASH': {
      exists: false,
      value: undefined
    }
  };

  getHash().then(value => {
    envVars.GIT_COMMIT_HASH.value = value;
    parseFile(filename, encoding, envVars);
    // If you add more values that need to be set later on, refactor this bit
    // so that parseFile is triggered after they are all complete.
    // I'm too lazy to do it now. Just setInterval and check that none
    // of the values are undefined and then continue.
  });
}

function parseFile(filename, encoding, envVars) {
  const variables = envVars;
  const keys = Object.keys(variables);
  const lr = new LineByLineReader(filename, { encoding: encoding });

  const step1 = () => {
    // First, read the file, and check if any of the envvars exist already.
    readFile(filename, encoding).then(data => {
      keys.forEach(key => {
        if (data.indexOf(key) > -1) {
          variables[key].exists = true;
        }
      });
      step2();
    }).catch(err => console.error(err));
  };

  const step2 = () => {
    // Then, read the file line-by-line, and if the value exists, replace it.

    lr.on('error', err => console.error('Line reader failed: ', err));
    lr.on('line', line => {
      lr.pause();
      keys.forEach(key => {
        if (line.indexOf(key) > -1) {
          replaceInFile(line, `${key}=${variables[key].value}`, filename, encoding).then((response) => {
            console.log(`Replaced ${key} value with ${variables[key].value}`);
            variables[key].exists = true;
            lr.resume();
          }).catch(err => {
            console.error(err);
            lr.resume();
          });
        } else {
          /* Line didn't match our keys. Ignore it */
          lr.resume();
        }
      });
    });

    lr.on('end', step3);
  };

  const step3 = () => {
    // Loop through the object once more and check for anything that has not been set.
    keys.forEach(key => {
      if (variables[key].exists === false) {
        addToFile(key, variables[key].value, filename, encoding)
          .then(response => {
            variables[key].exists = true;
            console.log(`Added ${key}=${variables[key].value} to the file`);
          })
          .catch(err => {
            console.error(err);
          });
      }
    });
  };

  step1();
}

function getHash() {
  return new Promise((resolve, reject) => {
    cp.exec('git rev-parse HEAD', (err, stdout, stderr) => {
      if (err) {
        reject(err);
      } else if (stderr) {
        reject(err);
      }

      resolve(stdout);
    });
  });
}

function replaceInFile(line, new_line, filename, encoding) {
  return new Promise((resolve, reject) => {
    readFile(filename, encoding).then(data => {
      const newdata = data.replace(line, new_line);

      fs.writeFile(filename, newdata, encoding, err => {
        if (err) {
          reject(err);
        }
        resolve('replaced');
      });

    }).catch(err => {
      reject(err);
    });
  });
}

function addToFile(key, value, filename, encoding) {
  return new Promise((resolve, reject) => {
    readFile(filename, encoding).then(data => {
      const newdata = `${data}${key}=${value}${nl}`;
      fs.writeFile(filename, newdata, encoding, err => {
        if (err) {
          reject(err);
        }
        resolve('added');
      });
    }).catch(err => {
      if (err) {
        reject(err);
      }
    })
  });
}

function readFile(filename, encoding) {
  return new Promise((resolve, reject) => {
    fs.readFile(filename, encoding, (err, data) => {
      if (err) {
        reject(err);
      }
      resolve(data);
    });
  });
}