Promise.all() does not seem to work

Oh, and another weird one with arrow functions: they don't seem to be passed values if defined with parentheses:

function go() {
  var p1 = new Promise(function (res, rej) {
    setTimeout(() => res(1), 250);
  });

  p1.then(x => {
    console.log('Bare arrow:', x);
  });

  p1.then((x) => {
    console.log('Parentheses arrow:', x);
  });

  p1.then(function (x) {
    console.log('Regular:', x);
  });
}

Output:

Bare arrow: 1
Parentheses arrow: undefined
Regular: 1