How to ensure we listen to a child process' events before they occur?

Question

Here is some node.js code that spawns a linux ls command and prompts its result

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-l']);

let content = "";
ls.stdout.on('data', function(chunk){
    content += chunk.toString();
});

ls.stdout.on('end', function(){
    console.log(content);
});

This works well. However, the ls command is launched asynchronously, completely separated from the main nodeJs thread. My concern is that the data and end events on the process' stdout may have occurred before I attached event listeners.

Is there a way to attach event listeners before starting that sub-process ?

Note : I don't think I can wrap a Promise around the spawn function to make this work, as it would rely on events to be properly catched to trigger success/failure (leading back to the problem)

Answer 1

There is no problem here.

Readable streams (since node v0.10) have a (limited) internal buffer that stores data until you read from the stream. If the internal buffer fills up, the backpressure mechanism will kick in, causing the stream to stop reading data from its source.

Once you call .read() or add a data event handler, the internal buffer will start to drain and will then start reading from its source again.