Node js子进程事件发射器与回调

Question

Is there any difference between attaching callbacks or event listeners for child process in nodejs. like -

const execute = require('child-process').exec;
const process = execute('ping -n 1 www.google.com'); // or ping -c 1 www.google.com for mac

process.stdout.on('data', data => {
    console.log(data)
})  

In the above code, I am using an event listener for the output and I am getting stdout data in windows but can't get the output in macOS. But if I use callback like -

const execute = require('child-process').exec;

execute('ping -c 1 www.google.com', (error, stdout, stderr) => {
   console.log(stdout);
})

I am getting the output data in both windows and mac. Is there any difference using callback or event listeners(both are asynchronous)?

Answer 1

The callback is called when the execution of that async task is completed. However, the events have to be fired based on the observer. Every event has listeners and when an event is fired its related listener function starts the executions.

• You can attach multiple listeners to the same event. Callbacks are one-to-one notifications, events - one-to-many.
• You can't return value from the event. Events are one-way messages. Often, callbacks follow (error, data1, data2, data3, ...) signature because a single callback responsible for normal and error data flow (and async libraries usually expect this behaviour)
• EventEmitter-based API, on the other hand, tend to separate error and non-error messages
• "error" event is special in event emitter: if there is no listener for it, EventEmitter throws an exception. With callbacks, it's your responsibility to check the first error parameter.

You can check this link on stackoverflow for difference b/w callback and events.