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Introduction to variables and data types

From: Foundations of Programming: Fundamentals

Video: Introduction to variables and data types

As soon as we write even the simplest program in any programming language, we have to keep track of pieces of information. If we are building a loan calculator, we need to keep track of the amount of the loan, the number of months, the interest rate. If we are writing a game program, we might need to know the current score, the position of the player on the screen, how many lives do we have left, what image do we use for our player. This is all data and we create variables to hold that data.

Introduction to variables and data types

As soon as we write even the simplest program in any programming language, we have to keep track of pieces of information. If we are building a loan calculator, we need to keep track of the amount of the loan, the number of months, the interest rate. If we are writing a game program, we might need to know the current score, the position of the player on the screen, how many lives do we have left, what image do we use for our player. This is all data and we create variables to hold that data.

Variables are simply containers. What we're doing is going out to the computer memory and grabbing a little piece of it, giving it a name to use while our program is running. We grab this space and we name it and then we put a value in it, like an email address or a date or a position or a number, and then we can change that value as we need to. Variables can vary, hence the name. So in JavaScript you create a variable like this.

The word var written all lowercase, which is part of the JavaScript language, and then we name the variable. The name of the variable is up to us and it should represent the piece of data that we want to hold. So a variable called year, or one called customerEmail, or todaysDate, or while we are experimenting, just nonsense words like foo or even a single letter like x. Now the name that you use for your variable must be written as one word, there are no spaces allowed, and it can be made of letters, numbers, the dollar sign and the underscore.

No other characters are allowed but you can't actually start with a number. So this variable name for example would not work, but reversing them so the number's at the end, that would work, and we will talk more about naming our variables later on when we talk about style guidelines, but I am just going to use simple names for now. All we are doing when we create a variable is carving out a little area of memory to hold a value. Right now after this line of code runs, this variable exists.

It has a name, year, but it doesn't have a value. This is regarded as undefined and undefined has a special meaning in JavaScript. But there is no point to having a variable that stays undefined. So we can define or set the initial value of the variable when we create it. We could do that as two statements. I say var year, then year = 2011, and that puts the value in the variable. Now very important. The = sign here is setting the variable to the value 2011.

It is not a polite description. It is a command. Put the value 2011 in the variable called year. You can also combine these into one statement, to both define and create the variable and to set its value. Now here is a place where JavaScript will let you be sloppy. I won't. Now technically the word var is not even required. In JavaScript if you just write a line of code like this without var, JavaScript will go looking for an existing variable called year to put this value in, but if it doesn't find it, it will just make it.

However, we are always going to use the word var when defining our variables. There are couple of situations where leaving var off can lead to unexpected behavior, so make a habit of always using it in your JavaScript. Now once again JavaScript is case sensitive. That means if I create a variable called x with a lowercase x, that's one. If I use the word uppercase X, that's two. These are two different variables. This second line here might have been an accident. Maybe I meant to say lowercase x, but because of the automatic creation of variables without the word var, you would now have two different variables and nothing in JavaScript would actually give you an error on this.

So be careful when you're naming your variables. Now if you are creating multiple variables at the same time, you can of course do them on multiple lines like this but you can actually combine them on to one line, separating the variable names with commas. It's just a shorthand way of doing this. And similar to that if you're actually creating multiple variables and giving them all initial values you can still combine them on to one line. This makes it a little easier to read, a little shorter to write.

Now in JavaScript, once you've actually created a variable you can put anything in it. Numbers, text, dates, it could start with a number, then put some text, then put a date in it, and that might not sound unusual, but a lot of other languages don't let you do that. Creating a variable in many languages does not just mean defining a container but also saying what type of container it is and what it can hold, and while that's not essential in JavaScript, it's really useful to know. So we are going to take a quick look at that idea.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 82198 viewers

Simon Allardice
Author

 
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  1. 4m 14s
    1. Welcome
      1m 16s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 49s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 10s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 56s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 57s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 28s
  7. 13m 32s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 40s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 59s
    1. Working with arrays
      5m 47s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 26s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 3s
  13. 14m 17s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 29s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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