Is the Promise constructor callback executed asynchronously?

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It depends on the implementation of the promise. If we check the spec. You can find the final spec here – since this answer was originally written, it has been finalized.

Here is the relevant excerpt (you can find the original source here)

  1. Let completion be Call(executor, undefined, «resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]]»).
  2. If completion is an abrupt completion, then
    • Let status be Call(resolvingFunctions.[[Reject]], undefined, «completion.[[value]]»).
    • ReturnIfAbrupt(status).

The ES6 standard indicates that the fulfillment of a promise is always asynchronous (See section 25.4.5.3, Promise.prototype.then and accompanying section 25.4.5.3.1, PerformPromiseThen). I have placed the relevant material below.

PerformPromiseThen

  1. Else if the value of promise’s [[PromiseState]] internal slot is “fulfilled”,
    • Let value be the value of promise’s [[PromiseResult]] internal slot.
    • Perform EnqueueJob(“PromiseJobs”, PromiseReactionJob, «fulfillReaction, value»).
  2. Else if the value of promise’s [[PromiseState]] internal slot is “rejected”,
    • Let reason be the value of promise’s [[PromiseResult]] internal slot.
    • Perform EnqueueJob(“PromiseJobs”, PromiseReactionJob, «rejectReaction, reason»).

TLDR: the function passed to the promise is executed synchronously, but subsequent then calls are always executed asynchronously.

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