introducing variable manner js + + Data Type

A, js way of introduction

External introduced, the introduction of the first to write external, must be written in the script tags
   <script type = "text / javascript " src = "js / day01.js"> head tag
   </ script>
    Embedded introduction
   <script type = "text "> / javascript
   ; Alert ( 'Hello')
   </ Script>
</ head>
    linked -js introduced into the tag, with the prefix javascript
<body>
   intermediate double quotation marks can not be nested double quotes
   <a href =" javascript: alert ( 'hello') ">
       click me pop
   </a>
</ body>

Two, js variable

js naming convention:

js variables begin with a letter, case sensitive, do not use the key names
camel nomenclature - the first letter lowercase, uppercase funCtion behind the
variable name must be unique

js variable definitions:

var x = 1;

Left the right side of an assignment, code execution from the top down

E.g:

    <Script type = " text / JavaScript " > 
        X = " Xiong hello " ; / * "character string" * / 
        Alert (X); 
        / * display defined variables * / 
        var A; 
        A = 12 is ; 
        Alert (A) ; / * undefine no assignment error * / 
        / * the first definition of the assignment * / 
        var C; 
        Alert (C); 
        / * empty type * / 
        var D = null ; 
        Alert (D); 
        / * empty string * / 
        var = E "";
        alert(e);
    
        
    </script>

var NUM = 10 ;
     var STR = " white " ;
     var In Flag = to true ;
     var NLL = null ; 
    
    the console.log ( typeof NUM); 
    the console.log (String (NLL); // output string is not null value

This is the type of data acquisition js

Third, the data type

js several data types

Value type ( Number ): contains integer and floating point;

Type String ( String): string must be double quotes or single quotes;

Boolean type ( Boolean): Only two values true or false;

Undefined type ( Undefined ): designed to determine a no initial value has been created but variable;

Mutual conversion between:

+ Values are operators on both sides, it is again arbitrary string connector

1, converts the string values:

There are a = 3;

a=a.toString()+3;

alert(a);a=33

2, and numeric string conversion, taking plastic float

There are b = 5;

var sum=b.parseInt(8)+4;

alert(sum);sum=8+4=12

NAN:not a number

size:

String -> float; integer variable = parseFloat (string).

Get string between js:

<Script type = " text / JavaScript " >
     var STR = " the I LOVE you " ;
     / * Get the character at the specified index = .charAt string variable (index) * / 
    var A = str.charAt ( . 4 ); 
    Alert ( a); 
    / * all uppercase characters in the string * / 
    var B = str.toUpperCase (); 
    Alert (B); 
    / * all lowercase characters in a string * / 
    var C = STR. the toLowerCase (); 
    Alert (C); 
    / * returns the specified string in a position large string, the first letter of eh first occurrence of * / 
    var D = str.indexOf ( " LOVE " );
    Alert (D); 
    / * taken from a specified position to the end of the string of * / 
    var E = str.substring ( 2 ); L 
    Alert (E); 
    / * taken from the specified start location to the specified end * / 
    var F = str.substring ( 2 , 6 ); // 2 where to start, 6 intercepts 4, obtained by subtracting 4love, no header trailer Love 
    Alert (F);   
     </ Script>

introducing variable manner js + + Data Type

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