No argument passed to then function?

[lxiaokun]

pool.start().then(function(data) {
console.log(data); // data is undefined, which is inconsistent with Promise API
})

[timdp]

I think it's perfectly valid to fulfil a promise with undefined. Otherwise, async noop () {} would be invalid too. Do you have a reference to the ECMAScript spec or another document that states that the result of a promise cannot be undefined?