Javascript Questions Set3

1) What is event delegation?

Event delegation is when you bind an event listener to a parent (or ancestor) element rather than the element(s) you are particularly interested in. When the event is triggered you can check the event target to make sure it was actually the triggered on the element of interest.

Event bubbling provides the foundation for event delegation in browsers. Now you can bind an event handler to a single parent element, and that handler will get executed whenever the event occurs on any of its child nodes (and any of their children in turn). This is event delegation. Here’s an example of it in practice:

Let’s say that we have a parent UL element with several child elements:

  • Item 1
  • Item 2
  • Item 3

Let’s also say that something needs to happen when each child element is clicked. You could add a separate event listener to each individual LI element, but what if LI elements are frequently added and removed from the list? Adding and removing event listeners would be a nightmare, especially if addition and removal code is in different places within your app. The better solution is to add an event listener to the parent UL element. But if you add the event listener to the parent, how will you know which element was clicked?

Simple: when the event bubbles up to the UL element, you check the event object’s target property to gain a reference to the actual clicked node. Here’s a very basic JavaScript snippet which illustrates event delegation:

// Get the element, add a click listener...
document.getElementById("parent-list").addEventListener("click", function(e) {
// e.target is the clicked element!
// If it was a list item
if(e.target && e.target.nodeName == "LI") {
    // List item found!  Output the ID!
    console.log("List item ", e.target.id.replace("post-"), " was clicked!");
       }
 });

2) What are the 3 ways to bind an event to an element ?
1) HTML Event Handlers
< input type="text" id="username" onblur="checkUserName()">
Not recommended any more. Better to separate JS and HTML

2) DOM Event Handlers
function checkUserName() { blah } ;
var el = document.getElementById(‘username’);
el.onblur = checkUserName;

Disadvantage is that you can only attach one function to each event Handler.

3) DOM Event Listeners
Advantage : They can deal with more than one function at a time.
var el = document.getElementById(‘username’);
el.addEventListener(‘blur’, checkUserName, false);

3) How do you convert arguments into an actual array. arguments are pseudo array in js.

var args = Array.prototype.slice.call(arguments);

4) How would you simulate Function.prototype.bind in older browser that don’t support it

Function.prototype.bind = Function.prototype.bind || function(context){
  var self = this;

  return function(){
    return self.apply(context, arguments);
  };
}

4) How to pass parameters to callback functions?

By default you cannot pass arguments to a callback function. For example:

function callback() {
  console.log('Hi human');
}

document.getElementById('someelem').addEventListener('click', callback);

But using closure concept you can do so :

function callback(a, b) {
  return function() {
    console.log('sum = ', (a+b));
  }
}

var x = 1, y = 2;
document.getElementById('someelem').addEventListener('click', callback(x, y));