Part 2_ Common Language Constructs

SOURCE: https://www.youtube.com/watch?v=XKRZ2fOlt7Q

In part one of this lecture, we left off speaking about strict equality versus regular equality. In this lecture, we're going to go ahead and pick up speaking about what JavaScript considers boolean true and what JavaScript considers boolean false. 

So like all languages, Java Script has a specific set of things that it considers to be false or true when you evaluate boolean expressions. So let me go ahead and comment out this whole section, and let's go to the next section where it's talking about the if statement. But we're actually going to list everything that the if statement would consider true, and everything that the if statement we consider false, or actually the other way around. False first. Okay, so in this if statement I listed every single thing that the if statement will consider False. The first of all it's going to consider the keyword false is false. Now then you see something that is an or operator. Now the way the or operator works, is that if something evaluates to true on it's left side, it does not check anymore what is going to be to it's right side. And since I know that to it's left side at the moment, is going to be false. I can be sure that it's going to evaluate this, and if this false, which it is, it's going to try to evaluate this for truthfulness. And if this is true, it will stop and will not evaluate anymore. However, null also get's coerced to being false in JavaScript. So therefore, we keep continuing, saying no which is again false or undefined. Undefined is a keyword that also gets coerced to false in JavaScript. Next, it's an empty string, so it's basically double quotes with nothing in the middle. That also gets coerced into being false. Zero gets coerced to being false. And NAN, not a number, gets coerced to being false. So point is that everyone of those things is false, that means this line will actually never execute. And in fact, it's going to go to the else clause, and execute all is false. Let's go ahead and save that. And you can see it says all false. Now let me show you a very quick trick that if you ever forget, if you have something and you're not sure is it supposed to evaluate to false or true. Well, JavaScript provides these wrapper objects around the primitives. So even though boolean is a primitive type, there's a wrapper object called Boolean with a capital B. That you can actually pass things to, to see what it would coerce to. So, for example, if I pass it a null, that would coerce to false. And there you go, it shows you false. If I pass it something like an empty string, it'll coerce it to false. But if I pass it anything else that is not in the list of false things that we listed right here in that if statement. So if I pass it something like, hello world. 

That will evaluate to true, which brings us to the next thing of what is true in JavaScript? Let's go to this if statement, and here what is true in JavaScript? Well, first of all the true keyword is evaluated as true in JavaScript. After the keyword true, you see two ampersands. And two ampersands signify the logical and operator. And the way the logical and operator works, it will only return true if both things on the left and on its right are true. So if both of these things are true, this will be true. And again, since we have a non-empty string here, this will evaluate to true. Then we ending it, ending it again with a number one which is a number not a 0. Anything not a zero also evaluates a true. And just in case you think a -1 would evaluate to false, it does not. It actually also evaluates to true. Any number that is non zero will evaluate to true. And if we end it with the string false, even though the string itself has the value false, but since it's a non-empty string, that will evaluate to true. So if we save this, we'll see the statement all true. Go ahead and save that. The first one is all false from this clause. And the second one says all true. 

Okay. So we're going to stop here for part two of this lecture. And in part three of the lecture, we're going to pick up speaking about whether or not putting curly braces in the same line or next line is a matter of style with Java Script, or really an established best practice. 

Video Images