!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> Lecture 8: JavaScript (part 2)

Lecture 8

JavaScript (part 2)

Ketan Mayer-Patel

University of North Carolina

How to Learn A Language

Basic data types
Variables
Assignment and operators
Arrays
Conditional and looping execution
Functions
Objects

Basic Data Types

Numbers
Strings
Booleans

Numbers

Literal syntax
- Normal decimal syntax
- Hex syntax: 0x
- Octal syntax: 0
Internally represented as floating point numbers.
- 3/5 = 0.6 (not 0)
- Math.floor(x), Math.ceil(x)

Numbers, cont'd

Converting to a string
- toString(radix)
- This is a method of numbers
Special numbers
- Infinity
- NaN
- Methods for testing
  - isNan()
  - isFinite()
Example 1 - Numbers

Strings

Double or single quoted
Escape sequences
- \t, \n, \", \', \\, \uXXXX
Concatenation with +
- "Hello" + " World" = "Hello World"
Length property

  var x = "Hello";
  x.length ; // Evaluates to 5

Can also do this to literals:

  "Hello".length; // Evaluates to 5

Strings, cont'd

Converting to a number

  var num = Number("123");

Indexing and substrings
- charAt(pos)
- substring(s,e)
Example 2 - Strings

Booleans

true
false
When numbers/strings are evaluated as booleans
- 0 or empty string = false
- Non-zero or non-empty string = true
When booleans are evaluated as numbers
- false = 0
- true = 1
Example 3 - Booleans

Variables

Untyped
Declared with "var":
- var x;
- var x,y,z;
Scope defined by declaration
- Global if outside of a function
- Local if inside a function
  - Locally declared variables will "mask" global variables.
  - It does not matter where inside a function a local variable is declared.
    - If declared locally, it is local everywhere in that function.
- Function parameters are local to function

Variables, cont'd

null
- Special value that indicates a variable is not anything specific at all.
undefined
- Special value that indicates a variable has never been set.
Although distinct, null and undefined test positively for equality
Example 4 - Variables

Operators

Property access: .
Numerical operators: +, -, *, /, %
String operators: +
Comparison: <, <=, >, >=
Logical: &&, ||, !
Equality test: ==, !=

Assignment

Assignment via =
Assignment with operation:
- +=, -=, *=, /=