It's difference between native and shim

[david-ni]

when I run the following code in browser（chrome v66） that support promise native

let promiseA = new Promise(function(resolve){ resolve('A') }),
    promiseB = new Promise(function(resolve){ resolve(promiseA) }),
    promiseC = new Promise(function(resolve){ resolve('C') });

promiseB.then((arg)=>{ console.log(arg); });
promiseC.then((arg)=>{ console.log(arg); });

the result is C A
but when I run the same code with es6-promise，the result is reverse: A C
which one is right?

[Aqours]

The Promise.resolve(value) method returns a Promise object that is resolved with the given value. If the value is a thenable (i.e. has a "then" method), the returned promise will "follow" that thenable, adopting its eventual state; if the value was a promise, that object becomes the result of the call to Promise.resolve; otherwise the returned promise will be fulfilled with the value.

Unlike old-style passed-in callbacks, a promise comes with some guarantees:

• Callbacks will never be called before the completion of the current run of the JavaScript event loop.
• Callbacks added with .then even after the success or failure of the asynchronous operation, will be called, as above.
• Multiple callbacks may be added by calling .then several times, to be executed independently in insertion order.

let promiseA = new Promise(function(resolve){ resolve('A') }),
    promiseC = new Promise(function(resolve){ resolve('C') });

promiseA.then(arg => arg).then((arg)=>{ console.log(arg); });
promiseC.then((arg)=>{ console.log(arg); });

According to MDN docs, your code equal to above code. So the result is C A which is correct, and it's a bug of es6-promise.