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Javascript Basics

What is Javascript?

History

ECMAScript

Values and Primitives

Numbers

Represents both integers and reals. 64 bit representation. Integral values range: +/- 9007199254740992

example: var a = 352661; var b = 334.22;

Special values: NaN +Infinity -Infinity

test("Strange arithmetic", function() {
    var a = NaN;
    equals( a <  10, false, "bigger than 10");
    equals( a >  10, false, "smaller than 10");
    equals( a === 10, false, "not equal to 10");
    equals( a === NaN, false, "not NaN??? " );
    equals( isNaN( a ), true, "Ahh!!" );
});

Caveat: some operators only work on 32 bit numbers. A silent conversion will be performed by the operator.

Strings

example: var txt = 'Hello World!'; var txt2 = "I'm happy" + " happy";

Sequence of 16 bit elements interpreted as UTF-16.

Useful operators:

split
txt.split( ' ' );

substring
txt.substring( 5 );
txt.substring( 2, 5 );

String to number
parseInt(“1234blue”); //returns 1234
parseInt(“0xA”); //returns 10
parseInt(“22.5”); //returns 22
parseInt(“blue”); //returns NaN
parseFloat(“1234blue”); //returns 1234.0
parseFloat(“0xA”); //returns NaN
parseFloat(“22.5”); //returns 22.5

Booleans

Two possible values: true or false

var a = true;
var b = a && ( a == false );

undefined and null

Javascript declares both null and undefined which has subtle differences.

Anything unassigned or not declared has the value 'undefined'.

null is a special value meaning "no value".

equals( "object", typeof null, "type of null is 'object'");

On the other hand, undefined means that the variable has not been declared, or has not been given a value.

equals( 'undefined', typeof i, "no 'i' anywhere yeilds 'undefined' "); var i; equals( 'undefined', typeof i, "no assignment to 'i' still gives us 'undefined');

caveat: equals( true, null == undefined, "== consider them equal" ); equals( false, null === undefined, "BUT === does not" );

Objects

An object is a container of keys and values, i.e. a map. { name: 'John', age: function() { return differenceInYears( new Date(1973,3,5), new Date()) } }

Functions

Javascript is a functional language so a function is a first class citizen (unlike Java).

As in many languages a function is invoked by using parenthesis. Example: var tst = function() { return 'hello' }

tst (a pointer to the function) tst() (invocation of the function) equals( 'hello', tst(), 'invoking the function' )

First class citizen means functions can be arguments to functions and functions can be returned from functions.

When not used to this "first class citizen" it can make code look a little scary at first: _f( 'whack' )() <= a function called _f invoked with argument 'whack'. Result of that is expected to be a function that subsequently is invoked.

Statements

  • Declarations
  • Conditionals
  • Loops
  • Exceptions
  • Flow statments

Equality and Comparison

In Javascript there are two kinds of comparison operators '==' (equality) and '===' (identity). The difference is that '==' will perform type conversion whereas '===' will not. Performance wise '===' is faster if the types differ, but maybe more importantly '==' can produce confusing results:

equals( false, '' == '0' );
equals( true, 0 == '' );
equals( true, 0 == '0' );

equals( false, false == 'false' );
equals( true, false == '0' );

equals( false, false == undefined );
equals( false, false == null );
equals( true, null == undefined );

equals( true, ' \t\r\n ' == 0 );

However, because of lack of type conversion, there can be WTF with '===':

equals( true, "abc" == new String( "abc" ) );
equals( false, "abc" === new String( "abc" ) );

The negated operators are called: '!=' and '!==' respectively.

Objects

  • Object literals
  • Properties
  • Methods
  • Prototypes
  • Inheritance

Arrays

  • Array literals
  • Properties
  • Methods

JSON

  • Javascript Object Notation

Functions

  • Lambda
  • Anonymous functions
  • First Class Objects
  • Methods
  • Prototypes
  • Scope
  • Invocation forms (this)
  • Arguments

Dynamic Compilation

  • Eval
  • Function

Modules and Namespaces

  • Namespace
  • Module
  • Mixins
  • Dynamic Module

Learning Javascript

  • Online resources
  • Standard library
  • Common pitfalls

Future of Javascript