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POHow do I run an object's method onEvent in javascript?
 

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  1. POHow do I run an object's method onEvent in javascript?
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    <p>I just started using javascript and I'm missing something important in my knowledge. I was hoping you could help me fill in the gap.</p> <p>So the script I'm trying to run is suppose to count the characters in a text field, and update a paragraph to tell the user how many characters they have typed. I have an object called charCounter. sourceId is the id of the text area to count characters in. statusId is the id of the paragraph to update everytime a key is pressed.</p> <pre><code>function charCounter(sourceId, statusId) { this.sourceId = sourceId; this.statusId = statusId; this.count = 0; } </code></pre> <p>There is one method called updateAll. It updates the count of characters and updates the paragraph.</p> <pre><code>charCounter.prototype.updateAll = function() { //get the character count; //change the paragraph; } </code></pre> <p>I have a start function that is called when the window loads.</p> <pre><code>function start() { //This is the problem document.getElementbyId('mytextfield').onkeydown = myCounter.updateAll; document.getElementbyId('mytextfield').onkeyup = myCounter.updateAll; } myCounter = new charCounter("mytextfield","charcount"); window.onload = start; </code></pre> <p>The above code is the problem. Why in the world can't I call the myCounter.updateAll method when the event is fired? This is really confusing to me. I understand that if you call a method <b>likeThis()</b> you'll get a value from the function. If you call it <b>likeThis</b> you are getting a pointer to a function. I'm pointing my event to a function. I've also tried calling the function straight up and it works just fine, but it will not work when the event is fired.</p> <p>What am I missing?</p> <hr> <p>Thanks for all the answers. Here's three different implementations.</p> <h1>Implementation 1</h1> <pre><code>function CharCounter(sourceId, statusId) { this.sourceId = sourceId; this.statusId = statusId; this.count = 0; }; CharCounter.prototype.updateAll = function() { this.count = document.getElementById(this.sourceId).value.length; document.getElementById(this.statusId).innerHTML = "There are "+this.count+" charactors"; }; function start() { myCharCounter.updateAll(); document.getElementById('mytextfield').onkeyup = function() { myCharCounter.updateAll(); }; document.getElementById('mytextfield').onkeydown = function() { myCharCounter.updateAll(); }; }; myCharCounter = new CharCounter('mytextfield','charcount'); window.onload = start; </code></pre> <h1>Implementation 2</h1> <pre><code>function CharCounter(sourceId, statusId) { this.sourceId = sourceId; this.statusId = statusId; this.count = 0; }; CharCounter.prototype.updateAll = function() { this.count = document.getElementById(this.sourceId).value.length; document.getElementById(this.statusId).innerHTML = "There are "+ this.count+" charactors"; }; CharCounter.prototype.start = function() { var instance = this; instance.updateAll(); document.getElementById(this.sourceId).onkeyup = function() { instance.updateAll(); }; document.getElementById(this.sourceId).onkeydown = function() { instance.updateAll(); }; }; window.onload = function() { var myCounter = new CharCounter("mytextfield","charcount"); myCounter.start(); }; </code></pre> <h1>Implementation 3</h1> <pre><code>function CharCounter(sourceId, statusId) { this.sourceId = sourceId; this.statusId = statusId; this.count = 0; }; CharCounter.prototype.updateAll = function() { this.count = document.getElementById(this.sourceId).value.length; document.getElementById(this.statusId).innerHTML = "There are "+this.count+" charactors"; }; function bind(funcToCall, desiredThisValue) { return function() { funcToCall.apply(desiredThisValue); }; }; function start() { myCharCounter.updateAll(); document.getElementById('mytextfield').onkeyup = bind(myCharCounter.updateAll, myCharCounter); document.getElementById('mytextfield').onkeydown = bind(myCharCounter.updateAll, myCharCounter); }; myCharCounter = new CharCounter('mytextfield','charcount'); window.onload = start; </code></pre>
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