我们如何从 node.js 中的回调函数访问变量？
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How can we access variable from callback function in node.js?
提问by Devang Bhagdev
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 ?
如何在 exec 函数之外找到用户名？
回答by Matthias Holdorf
You can pass the exec function a callback. When the exec function determines the username, you invoke the callback with the username.
您可以向 exec 函数传递一个回调。当 exec 函数确定用户名时，您使用用户名调用回调。
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:
由于 JavaScript 的异步特性，你不能做这样的事情：
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.
这是因为该行console.log( username );不会等到上面的函数完成。
Explanation of callbacks:
回调说明：
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.
回答by Sahil Chitkara
You can write the "exec" statement in a function that has a callback... Like This
您可以在具有回调的函数中编写“exec”语句......像这样
var sys = require('sys');
var exec = require('child_process').exec;
var cmd = 'whoami';
function execChild(callback){
var child = exec( cmd,
function (error, stdout, stderr)
{
username=stdout.replace('\r\n','');
callback(username);
}
)};
execChild(function(username){
console.log(username);
});
