The evolution of a JavaScript programmer

Every JavaScript programmer grows up eventually. Here's the same function, factorial(6), as written at each stage of the journey.

// Newbie programmer
function factorial(x) {
  if (x == 0) {
    return 1;
  } else {
    return x * factorial(x - 1);
  }
}
document.write(factorial(6));

// First year programmer, studied Java
class FactorialCalculator {
  constructor(number) {
    this.number = number;
  }

  getResult() {
    var result = 1;
    for (var i = 1; i <= this.number; i++) {
      result = result * i;
    }
    return result;
  }
}
console.log(new FactorialCalculator(6).getResult());

// Callback era
function factorial(n, callback) {
  if (n <= 1) return setTimeout(function () { callback(1); }, 0);
  factorial(n - 1, function (result) {
    setTimeout(function () {
      callback(n * result);
    }, 0);
  });
}
factorial(6, function (result) { console.log(result); });

// Discovered Promises
const factorial = (n) =>
  n <= 1
    ? Promise.resolve(1)
    : factorial(n - 1).then((result) => n * result);
factorial(6).then(console.log);

// async/await convert
async function factorial(n) {
  let result = 1;
  for (let i = 2; i <= n; i++) {
    result *= await i; // just to be safe
  }
  return result;
}
factorial(6).then(console.log);

// ES6 one-liner
const f = (n) => (n ? n * f(n - 1) : 1);
console.log(f(6));

// Functional programmer
console.log(
  Array.from({ length: 6 }, (_, i) => i + 1).reduce((a, b) => a * b, 1)
);

// Precision pedant
const f = (n) => (n ? n * f(n - 1n) : 1n);
console.log(String(f(6n)));

// npm developer
// $ npm install factorial-of
// added 1,284 packages, and audited 1,285 packages in 41s
// 87 vulnerabilities (12 low, 43 moderate, 32 high)
const factorialOf = require("factorial-of");
console.log(factorialOf(6));

// EXPERT PROGRAMMER
console.log(720);

See also: the evolution of a Python, Ruby, PHP, C or Perl programmer, and where this joke started, Fritz Ruehr's The Evolution of a Haskell Programmer (itself a riff on the old "Hello World" Evolution of a Programmer).