How to Use emitter. on in node. js _ node. js-js tutorial

This article mainly introduces node. emitter in js. the usage of the on method is described in this article. on Method description, syntax, receive parameters, use instances and implementation source code. For more information, see Method description:

Registers a listener for a specified event.

Syntax:

The Code is as follows:

Emitter. on (event, listener)
Emitter. addListener (event, listener)

Receiving parameters:

Event (string) event Type
The callback function when the listener (function) triggers an event.

Example:

The Code is as follows:

Server. on ('connection', function (stream ){
Console. log ('someone connected! ');
});

Source code:

The Code is as follows:

EventEmitter. prototype. addListener = function (type, listener ){
Var m;
If (! Util. isFunction (listener ))
Throw TypeError ('listener must be a function ');
If (! This. _ events)
This. _ events = {};
// To avoid recursion in the case that type = "newListener "! Before
// Adding it to the listeners, first emit "newListener ".
If (this. _ events. newListener)
This. emit ('newlistener ', type,
Util. isFunction (listener. listener )?
Listener. listener: listener );
If (! This. _ events [type])
// Optimize the case of one listener. Don't need the extra array object.
This. _ events [type] = listener;
Else if (util. isObject (this. _ events [type])
// If we 've already got an array, just append.
This. _ events [type]. push (listener );
Else
// Adding the second element, need to change to array.
This. _ events [type] = [this. _ events [type], listener];
// Check for listener leak
If (util. isObject (this. _ events [type]) &! This. _ events [type]. warned ){
Var m;
If (! Util. isUndefined (this. _ maxListeners )){
M = this. _ maxListeners;
} Else {
M = EventEmitter. defaultMaxListeners;
}
If (m & m> 0 & this. _ events [type]. length> m ){
This. _ events [type]. warned = true;
Console. error ('(node) warning: possible EventEmitter memory' +
'Leak detected. % d listeners added. '+
'Use emitter. setMaxListeners () to increase limit .',
This. _ events [type]. length );
Console. trace ();
}
}
Return this;
};