Lecture 50, Part 1: Arrays

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En provenance du cours de Johns Hopkins University

HTML, CSS, and Javascript for Web Developers

4159 notes

Johns Hopkins University

HTML, CSS, and Javascript for Web Developers

4159 notes

Cours 4 sur 6 dans Specialization Ruby on Rails Web Development

Do you realize that the only functionality of a web application that the user directly interacts with is through the web page? Implement it poorly and, to the user, the server-side becomes irrelevant! Today’s user expects a lot out of the web page: it has to load fast, expose the desired service, and be comfortable to view on all devices: from a desktop computers to tablets and mobile phones. In this course, we will learn the basic tools that every web page coder needs to know. We will start from the ground up by learning how to implement modern web pages with HTML and CSS. We will then advance to learning how to code our pages such that its components rearrange and resize themselves automatically based on the size of the user’s screen. You’ll be able to code up a web page that will be just as useful on a mobile phone as on a desktop computer. No “pinch and zoom” required! Last but certainly not least, we will get a thorough introduction to the most ubiquitous, popular, and incredibly powerful language of the web: Javascript. Using Javascript, you will be able to build a fully functional web application that utilizes Ajax to expose server-side functionality and data to the end user.

À partir de la leçon

Introduction to Javascript

What fun would a web site be if there was no functionality to it? In this module, we are going to concentrate on learning the fundamentals of the Javascript language. A lot of even seasoned developers “tinker” with Javascript without really understanding how the language works. That leads to viewing the language as more of a nuisance instead of a powerful tool. That is why we are going to concentrate not only on the “how” but also on the “why”, so you are empowered by the Javascript language features, not confused by them. We’ll cover just about everything - from common language constructs and Javascript types to objects, functions, arrays, closures, and scope isolation.

Lecture 50, Part 1: Arrays8:28

Lecture 50, Part 2: Arrays4:26

Lecture 51: Closures6:54

Lecture 52, Part 1: Fake Namespaces6:05

Lecture 52, Part 2: Immediately Invoked Function Expressions (IIFEs)8:55

Module 4 Wrap-Up0:44

Rencontrer les enseignants

Yaakov Chaikin
Adjunct Professor, Graduate Computer Science
Whiting School of Engineering

[SOUND] In

this lecture we're going to speak about arrays in JavaScript.

I'm assuming you already know what arrays are, but

if you don't arrays are just a collection of data.

In JavaScript however, since we have dynamic typing,

arrays take on pretty interesting set of properties.

So right now I'm in sublime text and I'm looking at script.js and I'm located in

the lecture50 folder, okay so let me close the browser, and let's create an array.

So let's call it var array, and we'll say new array.

Once I created the array I can now set items into that array.

And arrays are zero based, so I can say zero and let's say in the first position,

which is position zero, I'll place a string, and I'll use my name, Yaakov.

In the second position, which is position one, I'll place a number, let's say two.

In the third position, which is two, I'll place a function, and

let's say it's going to be a function that you could pass a name to.

And let's put a semicolon there, and all this function is going to

do is just going to say hello and whatever the name is, plus name, okay.

So in the third position, which is right here, or

the fourth position I should say, I am going to place an object literal.

We'll put a semicolon over there.

And object literal will say course, that's the property name,

and the name of the course is HTML, CSS & JS.

Okay, so here's my array.

And if I do a console.log on the array itself,

you'll see that we have the array, and as you can see inside the array,

we have different types of objects stored there.

Some are just primitives, and some are strings, some are functions,

which are also objects, and some are just objects.

So because of dynamic typing, in JavaScript I'm able to store different

types of objects in the same array, which is not true of most languages.

Most languages when you have an array, and you store data in that array,

the data in one particular array has to be of the same type, and if you want to have

more than one type, you have to separate it into separate arrays.

And just like I set the values using this brackets operator,

I can also retrieve the values using the brackets operator.

So for example, I could say console.log and

I can output array sub one, or sub one right here.

So array sub one meaning I want to go to the second item in the array,

which is right here and I want to output it to the screen using a console.log.

So if I save that and I see it prints out 2.

Furthermore, since at position three, or really at index two here, we have

a function, I can invoke this function very easily using therein notation.

So I'll say array, and it's at position two, so I'll say position two.

And now since at position two is the value, and

what's sitting in here in this position two is the value of the function.

So that's the function value, not the return of the function, but

just the function value.

Which means in order to invoke it all I have to do is put parentheses after it and

once I do, I can save it, and undefined and

the reason it says hello undefined is because I never passed name.

So, let's go ahead and say Yaakov, and save that.

Now it says Hello Yaakov, but I can even do something more fun than that.

I can actually pass in the array because I already have Yaakov stored right here.

So I can say I'm passing in an array, and it's in the position zero so now what

I'm doing is I'm retrieving position zero of this array, which is Yaakov string,

and I'm passing it to position two value which is the value of the function.

So the function is getting executed against the value Yaakov.

So if I save that, I'll get again the same thing, hello Yaakov.

Likewise, I could certainly retrieve a property

of a literal object that is stored inside that array.

So I could say here console.log, and I could say array sub three, or index three,

and I could say .course, and that should print out HTML, CSS, and JavaScript.

Let's go ahead and save that, and it says HTML CSS & JS.

Okay, I can also create arrays using a short hand array imitation.

So let me go ahead and comment this out so it doesn't crowd our console.

So we'll save it and we'll clear the console.

Let's go ahead and say, short hand array creation.

Okay, so what we're going to do is say, var names, and

we're going to create an array simply by providing an empty brackets.

So empty brackets now creates an array in the variable names, and

what I could do is, I could start listing my names, Yaakov, John, Joe, let's say.

Okay, so if I do console.log now, on the names,

I should see that array right there, Yaakov, John, and Joe.

So that works, great.

So now you see that there's two ways to create an array.

One is kind of a long hand notation where you say new array, and

the other one simply by kind of like an array literal something like that,

but we just use the brackets and you separate the array values by a comma.

And again I could put whatever I want in here, so if I wanted to let's say, I can

actually use spacing here like this and if I wanted to I could create an object here.

So let's see, I can create an object and say, name: "Yaakov", and comma, there's

another object here if I wanted to have an object, we'll do name: "John" and I

can actually leave it here because arrays are not restricted to have the same type.

So if I save that and I'll see object, object Joe, right?

So if I actually opened that up, it will let me open this up and

we'll see Yaakov, name Yaakov, and the same one is second one is named John.

Okay, but let's undo that for

now and go back to the regular array we had before, and let's go ahead and

loop over this array printing out each name, let's say, say hello to each name.

So for that we're going to use a regular for loop.

We'll say, var i = zero, and

we need to keep looping until it's the end of the array.

Well, there's three items in the array and

each array object has a special property called length.

And it's not a function,

it's just a property that the JavaScript engine sets for you.

So array names that length, we'll give me the length of the array,

in this case it will be three.

So what I'm saying is that I want my loop variable to be zero, one and two and

when it's three it's no longer going to be less than the length of the array and

it will quit.

Okay, so let's go ahead and print hello to the console.

So we'll say console.log and we'll say, "Hello" space,

and then we'll say names[i].

So now every time through the loop the i is going to change and

it's going to say hello to a different name.

Let's go ahead and maybe comment that out so we don't see that on the console, and

we save it and we say hello Yaakov, hello John, hello Joe.

Now what's interesting about arrays in JavaScript is that they can even

be sparse.

So I'm going to go ahead and make some room here and I can say something like

names and I can say name[100] and I could set that value to be let's say Jim.

So now we have an array that has zero, one and

two index and an 100 index and in between there's nothing there.

So if I actually go ahead and take that here, and paste that loop right here and

execute it, what we're going to see is it said, hello Yaakov, hello John, hello Joe.

And it starts that again, hello Yaakov, hello Joe and then 97 times it just says

hello undefined because what we're doing here is we're saying I want you to

retrieve names with an index of four, and five, and six, and so on.

Those indices just don't exist in this particular array, but 100 does exist,

so therefore you see when it gets to 100, it prints out hello Jim.

There is one more issue with arrays that you should be aware of, and

we'll discuss that issue in part two of this lecture.

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