Javascript - Unanswered questions - Part 1

What is Event delegation?

Event delegation is Javascript as it relates to the DOM. It basically means that if you attach an event listener to a DOM element that listener is not only firing on that DOM element. It's actually firing on every children in that. So, for instance if you have a navigation and so you've got an unordered list you've got list items and then you've got anchor tags inside that navigation what you have. If you add an event listener to the ul element in essence you're actually adding event listener to all of the children as well.

In short, JS event listeners fir not only on a single DOM element but on all its descendants.

What is Event Bubbling?

It's actually inverse of Event delegation. Also known as propagation, events on an DOM element will bubble up and also fire on all parents. 

What's the difference between "target" and "currentTarget"?

target is the actual element that triggered the event for example clicked, whereas currentTarget is the element with the listener attached.

Is the following block of code valid? Explain the reasons.

function foo(){

//I pity this code

}();

It's not a valid block of code, it'll through a syntax error in browser something like below.

"syntaxError: expected expression, got ')'

Basically the idea of the IIFE is I write this function and I run it. I just want to write it and then run it immediately. To make it correct add parenthesis like below.

(function foo(){

//it works

})();

What is the difference between a variable that is: null, undefined or undeclared?

Start with undeclared. Undeclared is basically when you try and use a variable that has never sort of used before. That means, you've never written out var foo='foo' or const foo='foo'. But you try to use it in some kind of manner. It's simply forgot something somewhere or you ripped out some code.

Undefined is that you've declared it but it doesn't have a value, it has not been assigned a value.

var c;

If(c==='test'){

}

//c is undefined in if statement

Null is little bit different. Null is a value. It's  just not a value, null is actually interpreted as having a value that value is null. It's a nothing value, it's a way of creating placeholder assignment for something that would may have may be other values.

You see this especially when you dealing with an API, you might have something on an object that is returned from your API, you might see the key there and the key's value is null. That's completely right. If you ever have a situation where you need to sort of zero out of a value, null is a great tool in order to do that. Because the idea of zero as a number, that's actually a value  or zero as a string that's certainly a value. So, you can use null instead.

What's the difference between == and ===?
Lets take an example:

let foo;//undefined
const bar=null;//null

//compare the two
console.log(foo == bar); //true

Get confused, cool.
Essentially what you have when you use double equals is a check for equality.  Javascript sees undefined and null as both are sort of having no value and says that those are equal even though they are not the same type of thing. So, triple equals is always going to check for equality and type.