1. Friday, Week 2 Variables! What’s a variable??? How do I use them in JavaScript??? What are they good for??? Some examples… Mini-Lab 2!!!

2. What’s a Variable? A place to store a piece of information. Think of it as a box:

3. What’s a Variable? We need to give the box a name! boxName

4. What’s a Variable? Now we can put something (only one thing!) in the box. boxName 2

5. What’s a Variable? To get that thing back, we can just use the box’s name… boxName 2

6. Variables in JavaScript We use the var keyword to create or declare a variable: var num, num2; numnum2

7. Variables in JavaScript We use the = sign to assign a value to a variable (put it in the box) num = 3; numnum2 3

8. Variables in JavaScript When we use a variable name, we get the value it contains: num2 = num + 13; numnum2 3 + 13 = 16

9. Variables in JavaScript So, after our two assignments, we have the following: numnum2 316

10. Variables in JavaScript If we re-assign a variable, its value changes: num = 4; numnum2 3 16 4

11. What are Variables Good For? Storing temporary information: Compare: // swap a and b a = b; b = a; // swap a and b var temp = a; a = b; b = temp;

12. What are Variables Good For? Storing shared information Often, we want to have some information that several functions can use We can create a global variable that is outside any function Now, all the functions in that document can “see” the global variable!

13. What are Variables Good For? Storing information from a user of a web page: var userName = document.guestbook.nameTextfield.value;

14. What are Variables Good For? Lots of other things: Loops Arrays Counters Etc…

15. Important Notes Variable names in JavaScript are case sensitive – the case matters! So, num and Num are different variable names! Variable names must begin with a letter, and can have only letters, digits, and/or the underscore (_) – no spaces!. OK – num, num2, cs105, my_num Not OK – 1num, money$, cs 105

16. Important Notes Choose descriptive variable names: Helps you remember what is in the variable. Helps the grader read your code (hint, hint!). Variables are declared with the var keyword Variable declarations can be at the very beginning of a function (local variables), or outside all functions (global variables). You do not need to type var when you use a variable – just use the variable’s name!

17. Some Examples function doSomething() { var sum, numberValues, average; …//compute sum and numberValues average = sum / numberValues; }

18. Some Examples function wholeName() { var first, last, whole; first = “Jason”; last = “Atkins”; whole = first + last; }

19. Some Examples function wholeName() { var first, last, whole; first = “Jason”; last = “Atkins”; whole = first + last; } Problem: whole now has “JasonAtkins”

20. Some Examples function wholeName() { var first, last, whole; first = “Jason”; last = “Atkins”; whole = first + “ “ + last; } Ok, better: whole now has “Jason Atkins”

21. Some Examples function wholeName() { var first, last, whole; first = document.guestbook.firstName.value; last = document.guestbook.lastName.value; whole = “Hello, “ + first + “ “ + last + “, how are you today?”; alert (whole); }

22. Mini-Lab Time!