Reentrancy in JavaScript

Question

I would like to improve my understanding of the word reentrant.

Is this function reentrant?

function* foo() {
  yield 1;
  yield 2;
}

And this one?

function foo() {
  return 1;
}

And this one?

var x = 0;
function foo() {
  return x++;
}

And this one?

function foo() {
  setTimeout(foo, 1000);
}

Answer 1

A reentrent function is a function whose execution can be resumed:

In computing, a computer program or subroutine is called reentrant if it can be interrupted in the middle of its execution and then safely called again ("re-entered") before its previous invocations complete execution.

In browser/node JavaScript, all multiprocessing is cooperative (no interrupts or context switches). A regular function always runs to completion in JavaScript. (1)

So in your case - the only reentrent function is the first one since it doesn't run its code to completion and can be resumed at a later point.

• The second function is just a regular function.
• The third one uses an outer scope, which is kind of similar because it lets a function hold some state. It's not the same thing though since the function can't be resumed.
• The fourth one just runs to completion immediately (it schedules another invokation of it - but that's up to the platform and not JavaScript).

Indeed - one can say that generators enable cooperative multitasking in JavaScript with a reentrent syntax. Before generators all code ran to completion.

_{(1) Or it never halts, but it is never interrupted. Also - in common platforms. There are platforms (like Rhino) that break the rule. They're very rare and don't use the same concurrency execution model as browser/node JS.}