optimizations.js

// Let's say we have a product that validates emails. 
// Person visits our websites gives us a list of emails and we validate that those emails are real.

// Our value is "Great customer experience".
// We have two engineers in our company and they both inline with our values.
// However they disagree on qualities that our code should have. 
// Person "A" believes that we should optimize for maintenability.
// Person "B" is sure that we should optimize for change. 
// They both send to implement an email validator.

// Code optimized for maintenability
// Let's say that "maintenability" means the following for person "A":
// - I know what is happening in my application
// - Application should handle error states explicitly 

const validateTotal = Metric.counter("validate_total");

function validate(emails) {
  validateTotal.observe(1);
  
  const validateEmail = (email) => {
    email.valid = (
      validateAddress(email) && checkDNS(email) && checkSMTPVRFY(email)
    );
    return email;
  };
  
  return emails.map(validateEmail);
}

// Code optimized for change
// Let's say that "change" means the following for person "A":
// - It is easy to plug-in new behaviour 
// - No need for configuration as it is easier to change code than maintain configuration.

function validate(emails, validators = [validateAddress, checkDNS, checkSMTPVRFY]) {
  const validateWith = (validators) => (emails, email) => {
    email.valid = validators.reduce((r, v) => r && v(email), true);
    return email;
  };
  
  return emails.map(validateWith(validators));
}

/// Both will be called the same:

let vaidatedEmails = validate([
  { address: "one@example.com", valid: false },
  { address: "two@example.com", valid: false },
]);