Process never ends when using 'process.stdin.once'

Question

In my node script, I am waiting for the user to press enter at some point:

console.log("Press enter to continue...");
await new Promise(function(resolve, reject) {
    process.stdin.once("data", function(data) {
        resolve();
    });
});

The script runs fine, and continues only after the user has pressed enter.

But at the end of the execution, the process does not terminate.

Instead, it seems to be just pending for user input (i.e., a newline is printed every time I press enter).

I'm pretty sure that the problem is related to process.stdin.once, and not to how I use the Promise in order to force synchronous execution.

I have several places in my script where I have to wait for the user to press enter, so adding process.stdin.end() before resolving the promise is out of the question here (it means that waiting for the user to press enter will work only once).

How can I resolve this problem?

Lowestoft

That cannot be all of your code, as await can only be use inside an async function. – Snowonthemountain

@Pointy: Of course it's not all of my code. I've pretty much stated that explicitly ("my script does this and that..."). And yes - this snippet is indeed invoked from an async function. – Lowestoft

OK well one never knows around here what might be lurking in unposted code. That said, the given answer is correct, though idiomatically in UNIX/Linux C code you'd probably just explicitly exit() or fall out of the main function (which I realize doesn't exist in Node programs). – Snowonthemountain

@Pointy: Technically, since I do know where my script starts and ends looking past all the asynchronous calls), I just ended up adding process.stdin.resume() at the beginning and process.stdin.pause() at the end. All the rest remained the same. Thank you. – Lowestoft

Answer 1

This is happening because as long as your stdin stream is open, node assumes you might be waiting for more data. The stream is automatically opened after you read from it in any way. An easy way to inform node you no longer want more input is to call pause(). Here's a simple example:

async function example() {
    console.log("Step 1");
    await new Promise(function(resolve, reject) {
        process.stdin.once("data", function(data) {
            resolve();
        });
    });

    console.log("Step 2");
    await new Promise(function(resolve, reject) {
        process.stdin.once("data", function(data) {
            resolve();
        });
    });

    // more steps...
}

example().then(() => {
    console.log("Done");
    process.stdin.pause();
});

Goren

So can I just call process.stdin.pause() before resolve()? – Lowestoft

Sort of - if you do that, you have to make sure you also resume() before you attempt to gather user input again. It's safe to call resume() on already-resumed stream, and same for pause(), so if you want to wrap up the "wait for user input" logic into a little helper function, you should first call resume(), then set up your once handler, and then pause() before returning. – Goren