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JavaScript | addEventListener() with Examples
  • Difficulty Level : Medium
  • Last Updated : 19 Feb, 2019

Prerequisite: JavaScript events
An event is an important part of JavaScript.A web page respond according to an event occurred. Some events are user generated and some are generated by API’s. An event listener is a procedure in JavaScript that waits for an event to occur. The simple example of an event is a user clicking the mouse or pressing a key on the keyboard.

The addEventListener() is an inbuilt function in JavaScript which takes the event to listen for, and a second argument to be called whenever the described event gets fired. Any number of event handlers can be added to a single element without overwriting existing event handlers.
Syntax:

element.addEventListener(event, listener);

Parameters:

  • event : event can be any valid JavaScript event.Events are used without “on” prefix like use “click” instead of “onclick” or “mousedown” instead of “onmousedown”.
  • listener(handler function) : It can be a JavaScript function which respond to the event occur.

JavaScript Code to show the working of addEventListener() method :
code #1:




<!DOCTYPE html>
<html>
  
<body>
    <button id="try">Click here</button>
    <h1 id="text"></h1>
    <script>
    document.getElementById("try").addEventListener("click", function(){
    document.getElementById("text").innerText = "GeeksforGeeks";
});
    </script>
</body>
  
</html>  

Output:
Before clicking the “Click here” button:

After clicking the “Click here” button:

code #2:
In this example two events “mouseover” and “mouseout” are added to the same element. If the text is hovered over then “mouseover” event occur and RespondMouseOver function invoked, similarly for “mouseout” event RespondMouseOut function invoked.




<!DOCTYPE html>
<html>
  
<body>
    <button id="clickIt">Click here</button>
  
    <p id="hoverPara">Hover over this Text !</p>
  
    <b id="effect"></b>
  
    <script>
        const x = document.getElementById("clickIt");
        const y = document.getElementById("hoverPara");
  
        x.addEventListener("click", RespondClick);
        y.addEventListener("mouseover", RespondMouseOver);
        y.addEventListener("mouseout", RespondMouseOut);
  
        function RespondMouseOver() {
            document.getElementById("effect").innerHTML +=
                       "MouseOver Event" + "<br>";
        }
  
        function RespondMouseOut() {
            document.getElementById("effect").innerHTML +=
                      "MouseOut Event" + "<br>";
        }
  
        function RespondClick() {
            document.getElementById("effect").innerHTML +=
                      "Click Event" + "<br>";
        }
    </script>
</body>
  
</html>

Output:
Before clicking the “Click here” button and hovering over text:

After clicking the “Click here” button and hovering over text:




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