Determine whether a method is synchronous or asynchronous

Question

In node.js, is it possible to determine (using a function) whether a method is synchronous or asynchronous?

I\'d like to write a function that does the following:

<

Answer 1

From a language standpoint this is not possible, which I believe llambda's answer proves.

• Functions can do things asynchronously but return something synchronously; say, return the number of async tasks that were fired off.
• Functions can synchronously return promises... that represent asynchronous information. I would call such a method asynchronous but the language can't really tell that.
• In some perverse sense every asynchronous function "returns" something... undefined if nothing else.

From an engineering standpoint:

• read the documentation.
• If the function accepts a callback, it is likely asynchronous. Look at the function signature.
• Read the code.
• Use "common sense." If the function does IO and returns a result from IO it must be, in some case, asynchronous. This includes file reading, reading from standard input, saving to a database, and HTTP/AJAX requests. Note streams are often used for this, which represents an asynchronous task, but the callbacks are special.

Furthermore there are functions that mix the two.

function(callback) {
    if(ready) {
        callback();
    }
    else {
        setTimeout(callback, 5000);
    }
}

Arguably this is very evil, and correct practice would be

if(ready) {
    process.nextTick(callback);
}

so the function has uniform behavior.

However there is a hacky way to tell if anything asynchronous happened, at least in Node.js. See this discussion.

// untested!! please read the documentation on these functions before using yourself
var work = process._getActiveHandles().length + process._getActiveCallbacks().length;
foo;
var newWork = (process._getActiveHandles().length + process._getActiveCallbacks().length) - work;
if(newWork > 0) {
    console.log("asynchronous work took place.");
}

This works because asynchronous work cannot resolve on the same tick, by definition, and because Node.js is single threaded.