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Understanding variable scope

From: Foundations of Programming: Fundamentals

Video: Understanding variable scope

So one more thing you should be aware of when working with functions and this goes not just for JavaScript, but for most modern programming languages. If you create a function and define a variable inside that function using the word var, that variable is only available inside that function. This is because of something called variable scope. And every variable has a scope. That simply means where is this variable visible? Who can see it? Who can use it? So let's say we define this simple function and then we call it.

Understanding variable scope

So one more thing you should be aware of when working with functions and this goes not just for JavaScript, but for most modern programming languages. If you create a function and define a variable inside that function using the word var, that variable is only available inside that function. This is because of something called variable scope. And every variable has a scope. That simply means where is this variable visible? Who can see it? Who can use it? So let's say we define this simple function and then we call it.

What's going to happen is we will jump into the function, we will create the variable, and we will pop up the alert box with the contents of that variable which is 500. Control returns to right after we called it and if try and pop up the content of x again, this will be regarded as undefined. It will say I don't know what x means, because the x variable is not available outside simple function. This is because variables declared inside the functions are referred to as local variables.

They have local scope. They are only available within the opening and closing curly braces of that function. If I need to have the contents of this visible outside, I could do a couple of things. One is I could return a value using the Return keyword and we saw that little earlier. The other way that I could do it do is to declare, to create the variable outside the function. So up at the top I create var x. That's outside all functions and that's referred to as a global variable. That means we could just use the word x to refer to it anywhere, whether I'm inside a function or outside a function.

Now I want to be careful of is not using the word var to declare that variable again. So inside simple function I don't want to say var x. I just want to say x = 500. So now we could call it inside the function and it would be 500. Then we could call it outside the function and it would be 500 as well. Now in JavaScript we only worry about scope when we talking about functions. It doesn't apply to other code blocks like creating variables inside loops or if statements and so on.

But with other languages variable scope can apply on those levels too. So you have to be quite careful about where your variables are defined and who can get to them.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 92347 viewers

Simon Allardice
Author

 
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  1. 4m 15s
    1. Welcome
      1m 17s
    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 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  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 58s
    1. Working with arrays
      5m 46s
    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 25s
    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 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    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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