EventTarget.addEventListener()

EventTarget.addEventListener() 方法能將指定的事件監聽器註冊到 EventTarget 實作物件上。EventTarget 可能是 Document 中的 Element 物件、Document 物件本身、Window 物件，或是其它支援事件的物件（如：XMLHttpRequest）。

語法

target.addEventListener(type, listener[, options]);
target.addEventListener(type, listener[, useCapture]);
target.addEventListener(type, listener[, useCapture, wantsUntrusted  ]); // Gecko/Mozilla only

參數

type

表示所監聽的 event type 名稱之字串。

listener

當監聽的事件發生時負責接收事件物件（Event 的實作物件）的物件。這個物件必須是事件監聽器（實作了 EventListener 介面），或是一個簡單的 JavaScript 函式。

options 選擇性

An options object that specifies characteristics about the event listener. The available options are:

• capture: A Boolean that indicates that events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree.  
• passive: A Boolean indicating that the listener will never call preventDefault(). If it does, the user agent should ignore it and generate a console warning.
• mozSystemGroup: Available only in code running in XBL or in Firefox' chrome, it is a Boolean defining if the listener is added to the system group.

useCapture 選擇性

若為true, useCapture表示使用者想要初始接取機制。在初始接取機制後，所有指定型態的事件會在發送到任何上方DOM樹的EventTarget之前先發送到註冊的listener，從樹中冒泡而上的事件不會觸發用來接取機制的傾聽器，Event bubbling and capturing are two ways of propagating events that occur in an element that is nested within another element, when both elements have registered a handle for that event. The event propagation mode determines the order in which elements receive the event .參考DOM Level 3 Events和 JavaScript Event order關於更詳盡的解釋。useCapture的初始值為false.

註：For event listeners attached to the event target; the event is in the target phase, rather than capturing and bubbling phases. Events in the target phase will trigger all listeners on an element regardless of the useCapture parameter.

註：useCapture became optional only in more recent versions of the major browsers; for example, it was not optional prior to Firefox 6. You should provide this parameter for broadest compatibility.

wantsUntrusted

If true, the listener will receive synthetic events dispatched by web content (the default is false for chrome and true for regular web pages). This parameter is only available in Gecko and is mainly useful for the code in add-ons and the browser itself. See Interaction between privileged and non-privileged pages for an example.

範例

一個簡易的事件監聽

HTML 內容

<table id="outside">
    <tr><td id="t1">one</td></tr>
    <tr><td id="t2">two</td></tr>
</table>

JavaScript 內容

// Function to change the content of t2
function modifyText() {
  var t2 = document.getElementById("t2");
  if (t2.firstChild.nodeValue == "three") {
    t2.firstChild.nodeValue = "two";
  } else {
    t2.firstChild.nodeValue = "three";
  }
}

// add event listener to table
var el = document.getElementById("outside");
el.addEventListener("click", modifyText, false);

在上述的範例中， modifyText() 是一個監聽器，監聽透過 addEventListener() 註冊的 click 事件。任何在這個表格內的點擊都會上浮 （bubble up）到這個事件回調（handler）並執行 modifyText() 。

如果你要傳入任何參數到監聽器函式，你可以使用一個匿名函式。

匿名函數的事件監聽

HTML 內容

<table id="outside">
    <tr><td id="t1">one</td></tr>
    <tr><td id="t2">two</td></tr>
</table>

JavaScript 內容

// 改變 t2 內容的函數
function modifyText(new_text) {
  var t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;    
}
 
// 為表格增加事件監聽的函數
var el = document.getElementById("outside");
el.addEventListener("click", function(){modifyText("four")}, false);

備註

為甚麼使用 addEventListener?

addEventListener 是註冊一個事件監聽器的方式，正如 W3C DOM 中所定義的。他的好處有這些：

• It allows adding more than a single handler for an event. This is particularly useful for DHTML libraries or Mozilla extensions that need to work well even if other libraries/extensions are used.
• It gives you finer-grained control of the phase when the listener gets activated (capturing vs. bubbling)
• It works on any DOM element, not just HTML elements.

The alternative, older way to register event listeners is described below.

Adding a listener during event dispatch

If an EventListener is added to an EventTarget while it is processing an event, it will not be triggered by the current actions but may be triggered during a later stage of event flow, such as the bubbling phase.

Multiple identical event listeners

If multiple identical EventListeners are registered on the same EventTarget with the same parameters, the duplicate instances are discarded. They do not cause the EventListener to be called twice, and since the duplicates are discarded, they do not need to be removed manually with the removeEventListener method.

The value of this within the handler

It is often desirable to reference the element on which the event handler was fired, such as when using a generic handler for a set of similar elements.

When attaching a handler function to an element using addEventListener(), the value ofthis inside the handler is a reference to the element. It is the same as the value of the currentTarget property of the event argument that is passed to the handler.

If an event attribute (e.g., onclick) is specified on an element in the HTML source, the JavaScript code in the attribute value is effectively wrapped in a handler function that follows the same rule as above for binding the value of this; an occurrence of this within the code represents a reference to the element. Note that the value of this inside a function called from within the codebehaves as per standard rules. Therefore, given the following example:

<table id="t" onclick="modifyText();">
  . . .

The value of this within modifyText() when called from the onclick event will be a reference to the global (window) object.

This is an example with and without bind:

var Something = function(element) {
  this.name = 'Something Good';
  this.onclick1 = function(event) {
    console.log(this.name); // undefined, as this is the element
  };
  this.onclick2 = function(event) {
    console.log(this.name); // 'Something Good', as this is the binded Something object
  };
  element.addEventListener('click', this.onclick1, false);
  element.addEventListener('click', this.onclick2.bind(this), false); // Trick
}

A problem in the example above is that you cannot remove the listener with bind. Another solution is using a special function called handleEvent to catch any events:

var Something = function(element) {
  this.name = 'Something Good';
  this.handleEvent = function(event) {
    console.log(this.name); // 'Something Good', as this is the Something object
    switch(event.type) {
      case 'click':
        // some code here...
        break;
      case 'dblclick':
        // some code here...
        break;
    }
  };

  // Note that the listeners in this case are this, not this.handleEvent
  element.addEventListener('click', this, false);
  element.addEventListener('dblclick', this, false);

  // You can properly remove the listners
  element.removeEventListener('click', this, false);
  element.removeEventListener('dblclick', this, false);
}

Internet Explorer 的 attachEvent 方法

在版本早於 IE 9 的 Internet Explorer, 必須使用 attachEvent 而非 addEventListener 監聽事件. 為了支援 IE, 可以參考下面的範例:

if (el.addEventListener) {
  el.addEventListener('click', modifyText, false); 
} else if (el.attachEvent)  {
  el.attachEvent('onclick', modifyText);
}

There is a drawback to attachEvent, the value of this will be a reference to the window object instead of the element on which it was fired.

相容性

You can work around the addEventListener, removeEventListener, Event.preventDefault and Event.stopPropagation not being supported by IE 8 using the following code at the beginning of your script. The code supports the use of handleEvent and also the DOMContentLoaded event.

(function() {
  if (!Event.prototype.preventDefault) {
    Event.prototype.preventDefault=function() {
      this.returnValue=false;
    };
  }
  if (!Event.prototype.stopPropagation) {
    Event.prototype.stopPropagation=function() {
      this.cancelBubble=true;
    };
  }
  if (!Element.prototype.addEventListener) {
    var eventListeners=[];
    
    var addEventListener=function(type,listener /*, useCapture (will be ignored) */) {
      var self=this;
      var wrapper=function(e) {
        e.target=e.srcElement;
        e.currentTarget=self;
        if (listener.handleEvent) {
          listener.handleEvent(e);
        } else {
          listener.call(self,e);
        }
      };
      if (type=="DOMContentLoaded") {
        var wrapper2=function(e) {
          if (document.readyState=="complete") {
            wrapper(e);
          }
        };
        document.attachEvent("onreadystatechange",wrapper2);
        eventListeners.push({object:this,type:type,listener:listener,wrapper:wrapper2});
        
        if (document.readyState=="complete") {
          var e=new Event();
          e.srcElement=window;
          wrapper2(e);
        }
      } else {
        this.attachEvent("on"+type,wrapper);
        eventListeners.push({object:this,type:type,listener:listener,wrapper:wrapper});
      }
    };
    var removeEventListener=function(type,listener /*, useCapture (will be ignored) */) {
      var counter=0;
      while (counter<eventListeners.length) {
        var eventListener=eventListeners[counter];
        if (eventListener.object==this && eventListener.type==type && eventListener.listener==listener) {
          if (type=="DOMContentLoaded") {
            this.detachEvent("onreadystatechange",eventListener.wrapper);
          } else {
            this.detachEvent("on"+type,eventListener.wrapper);
          }
          eventListeners.splice(counter, 1);
          break;
        }
        ++counter;
      }
    };
    Element.prototype.addEventListener=addEventListener;
    Element.prototype.removeEventListener=removeEventListener;
    if (HTMLDocument) {
      HTMLDocument.prototype.addEventListener=addEventListener;
      HTMLDocument.prototype.removeEventListener=removeEventListener;
    }
    if (Window) {
      Window.prototype.addEventListener=addEventListener;
      Window.prototype.removeEventListener=removeEventListener;
    }
  }
})();

註冊 event listeners 的舊方法

addEventListener() was introduced with the DOM 2 Events 規範。在這之前，註冊 event listeners 的方法如下：

// Pass a function reference — do not add '()' after it, which would call the function!
el.onclick = modifyText;

// Using a function expression
element.onclick = function() {
  // ... function logic ...
};

這個方法取代在元素上現有的 click event listener(s)。Similarly for other events and associated event handlers such as blur (onblur), keypress (onkeypress), and so on.

因為這是 DOM 0 的一部份, 這個方法被廣泛的支援且不需跨瀏覽器的程式碼; hence it is normally used to register event listeners dynamically unless the extra features of addEventListener() are needed.

記憶體問題

var i;
var els = document.getElementsByTagName('*');

// 案例 1
for(i=0 ; i<els.length ; i++){
  els[i].addEventListener("click", function(e){/*do something*/}, false);
}

// 案例 2
function processEvent(e){
  /*do something*/
}

for(i=0 ; i<els.length ; i++){
  els[i].addEventListener("click", processEvent, false);
}

In the first case, a new (anonymous) function is created at each loop turn. In the second case, the same previously declared function is used as an event handler. This results in smaller memory consumption. Moreover, in the first case, since no reference to the anonymous functions is kept, it is not possible to call element.removeEventListener because we do not have a reference to the handler, while in the second case, it's possible to do myElement.removeEventListener("click", processEvent, false).

規範

瀏覽器相容性

[1] Although WebKit has explicitly added [optional] to the useCapture parameter as recently as June 2011, it had been working before the change. The new change landed in Safari 5.1 and Chrome 13.

[2] Before Chrome 49, the type and listener parameters were optional.

[3] Prior to Firefox 6, the browser would throw an error if the useCapture parameter was not explicitly false. Prior to Gecko 9.0 (Firefox 9.0 / Thunderbird 9.0 / SeaMonkey 2.6), addEventListener() would throw an exception if the listener parameter was null; now the method returns without error, but without doing anything.

[4] Older versions of Internet Explorer support the proprietary EventTarget.attachEvent method instead.

[5] For backwards compatibility, browsers that support options allow the third parameter to be either options or Boolean.

參見

• EventTarget.removeEventListener()
• Creating and triggering custom events
• More details on the use of this in event handlers