​

function nInt(f, a, b, n = 10000) {

  let dx = (b - a) / n;

  let sum = 0;

  let xi;

  for (let i = 1; i <= n; i++) {

    xi = a + i * dx;

    sum += f(xi) * dx;

  }

  return sum;

}

​

console.log(nInt(f, 0, 1));