When is the body of a Promise executed?

Immediately, yes, by specification.

From the MDN:

The executor function is executed immediately by the Promise implementation, passing resolve and reject functions (the executor is called before the Promise constructor even returns the created object)

Here it is in the ECMAScript specification (of course harder to read...): http://www.ecma-international.org/ecma-262/6.0/#sec-promise-executor

This guarantee may be important, for example when you're preparing several promises you then pass to all or race, or when your executors have synchronous side effects.

From the EcmaScript specification

The executor function is executed immediately by the Promise implementation, passing resolve and reject functions (the executor is called before the Promise constructor even returns the created object)

Consider the following code:

let asyncTaskCompleted = true

const executorFunction = (resolve, reject) => {
  console.log("This line will be printed as soon as we declare the promise");
  if (asyncTaskCompleted) {
    resolve("Pass resolved Value here");
  } else {
    reject("Pass reject reason here");
  }

}

const myPromise = new Promise(executorFunction)

When we execute the above code, executorFunction will be called automatically as soon as we declare the Promise, without us having to explicitly invoke it.

You can see from below the body is executed immediately just by putting synchronous code in the body rather than asynchronous:

function doSomethingAsynchronous() {
  return new Promise((resolve) => {
    console.log("a");
    resolve("promise result");
  });
}
doSomethingAsynchronous();
console.log("b");

The result shows the promise body is executed immediately (before 'b' is printed).

The result of the Promise is retained, to be released to a 'then' call for example:

function doSomethingAsynchronous() {
  return new Promise((resolve) => {
    console.log("a");
    resolve("promise result");
  });
}

doSomethingAsynchronous().then(function(pr) {
  console.log("c:" + pr);
});
console.log("b");

Result:

a
b
c:promise result

Same deal with asynchronous code in the body except the indeterminate delay before the promise is fulfilled and 'then' can be called (point c). So a and b would be printed as soon as doSomethingAsynchronous() returns but c appears only when the promise is fulfilled ('resolve' is called).

What looks odd on the surface once the call to then is added, is that b is printed before c even when everything is synchronous.

Surely a would print, then c and finally b?

The reason why a, b and c are printed in that order is because no matter whether code in the body is async or sync, the then method is always called asynchronously by the Promise.

In my mind, I imagine the then method being invoked by something like setTimeout(()=>{then(pr)},0) in the Promise once resolve is called. I.e. the current execution path must complete before the function passed to then will be executed.

Not obvious from the Promise specification why it does this?

My guess is it ensures consistent behavior regarding when then is called (always after current execution thread finishes) which is presumably to allow multiple Promises to be stacked/chained together before kicking off all the then calls in succession.

Yes, when you construct a Promise the first parameter gets executed immediately.

In general, you wouldn't really use a promise in the way you did, as with your current implementation, it would still be synchronous.

You would rather implement it with a timeout, or call the resolve function as part of an ajax callback

function doSomethingAsynchronous() {
  return new Promise((resolve) => {
    setTimeout(function() {
      const result = doSomeWork();
      resolve(result);
    }, 0);
  });
}

The setTimeout method would then call the function at the next possible moment the event queue is free