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var sys = require('sys'); var exec = require('child_process').exec; var cmd = 'whoami'; var child = exec( cmd, function (error, stdout, stderr) { var username=stdout.replace('\r\n',''); } ); var username = ? How can I find username outside from exec function ?
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var myCallback = function(data) { console. log('got data: '+data); }; var usingItNow = function(callback) { callback('get it? '); }; Now open node or browser console and paste the above definitions.
Yes. The print( ) function takes another function as a parameter and calls it inside. This is valid in JavaScript and we call it a “callback”. So a function that is passed to another function as a parameter is a callback function.
A callback function is a function that is passed as an argument to another function, to be “called back” at a later time. A function that accepts other functions as arguments is called a higher-order function, which contains the logic for when the callback function gets executed.
You need to use the . call() or . apply() methods on the callback to specify the context which the method is called upon. The callback method remote_submit does not know what this will be anymore and thus when it calls the callback methods they're executed like normal functions not on an object.
You can pass the exec function a callback. When the exec function determines the username, you invoke the callback with the username.
var child = exec(cmd, function(error, stdout, stderr, callback) { var username = stdout.replace('\r\n',''); callback( username ); });
Due to the asynchronous nature of JavaScript, you can't do something like this:
var username; var child = exec(cmd, function(error, stdout, stderr, callback) { username = stdout.replace('\r\n',''); }); child(); console.log( username ); This is because the line console.log( username ); won't wait until the function above finished.
Explanation of callbacks:
var getUserName = function( callback ) { // get the username somehow var username = "Foo"; callback( username ); }; var saveUserInDatabase = function( username ) { console.log("User: " + username + " is saved successfully.") }; getUserName( saveUserInDatabase ); // User: Foo is saved successfully. If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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