Chapter 2 JavaScript Basic

Has 5 primitive types: Undefined, Null, Boolean, String, Number.

typeof  operator

Undefined return undefined

Null return object

Boolean return boolean

String return string

Number return number

Null returns object, this is an error in the original JS. Today, null is considered a placeholder for an object,  even though, it is a primitive types.

The Undefined Type

Use typeof with Object undefined, it also give "undefined", like the variables defined but not initialized.

The Null Type

alert(null == undefined);  //outputs "true"

Even though this is equal, they have different meanings.

The Boolean Type

true false

The Number Type

Number can present 32-bit integer and 64-bit floating point value.

Define floating-point value, you should use like: var fNum = 5.0; This will stored as string, and converted to number until calculation.

e-notion:  var fNum = 3.125e7, fNum2=3.125e-7;

Number.MAX_VALUE  Number.MIN_VALUE

>Number.MAX_VALUE  ->   Number.POSITIVE_INFINITY

<Number.MIN_VALUE   ->   Number.NEGATIVE_INFINITY

Function isFinite(value) returns Boolean

NaN: When convert from other type and failed, the number value is NaN, can‘t calculate.

alert(NaN == NaN);  //outputs "false"

Function isNaN(value) returns Boolean

The String Type

‘‘

""

Conversions

Converting to a string

Booleans, numbers, strings are pseudo-objects.

All objects, include   pseudo-objects, all have toString().

Boolean:   true  false

Number: 

var num = 10;

alert(num.toString());   //10

alert(num.toString(2)); // 1010

alert(num.toString(16)); // A

Converting to a number

parseInt():  validate the character in position 0 and determines if this is a valid number, if it isn‘t , return NaN, else continue untile a character isn‘t a valid number, and parse the part of valid.

var iNum1 = parseInt("123blue"); //returns 123

var iNum1 = parseInt("22.5"); //returns 22, decimal point is not valid

var iNum2 = parseInt("10", 2) ;//returns 2

var iNum3 = parseInt("0XA");//returns 10

var iNum4 = parseInt("010");//returns 8

var iNum5 = parseInt("010", 10);//returns 10

parseFloat(): if have more than one decimal point, all are invalid except the first one. No radix mode.

var iNum1 = parseFloat("12.34.56"); //returns 12.34

var iNum2 = parseInt("123blue"); //returns 123.0

var iNum3 = parseInt("010");//returns 10

var iNum4 = parseInt("0XA");//returns NaN

Type Casting

Boolean(value)

String with at least one character, a number other than 0, or an object: return true;

Empty string, the number 0, undefined or null, return false.

Number(value)

Works similar to parseInt() & parseFloat(), except that it converts the entire value.

String(value)

Like toString(), but can produce a string for a null or undefined value without error.

var s1 = String(null);   //"null"

var oNull = null;

var s2 = oNull.toString();  //won‘t work

Reference Types

ECMAScript doesn‘t have classes in the traditional sense. ECMAScript defines "object definitions" that are logically equivalent to classes in other programming languages.

The Object class

Like the java.lang.Object in java.

Properties

constructor

prototype

Methods

hasOwnProperty(property)

isPrototypeof(object)

toString()

valueOf()

The Boolean Class

var oFalseObject = new Boolean(false);

Var bResult = oFalseObject  && true;    //outputs true

Because all objects converted to true.

The Number class

toFixed(), toFixed(n): returns a string representation of a number with a specified number of decimal points.      0 <= n <= 20

toExponential(n): returns a string with the number formatted in e-notation.

toPrecision():

var oNumberObject = new Number(99);

alert(oNumberObject .toPrecision(1));  //outputs "1e+2"

alert(oNumberObject .toPrecision(2));  //outputs "99"

alert(oNumberObject .toPrecision(3));  //outputs "99.0"

The String class

The String class has length property.

charAt(): returns a string containing the character in that position.

charCodeAt():

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