JavaScript base (ii) Data Types

javascript variable number of issues

　　Definition and is not defined and undefined

<Script type = "text / JavaScript"> 
    A = 40 ; 
    the console.log (A); // may output a = 40, but not recommended, preferably with A = 40 var; 
    the console.log (B); // Not defiend IS B 
    var C; 
    the console.log (C); // A undefiend 
</ Script>

　　Two variable values ​​exchanged

<Script type = "text / JavaScript">
     // do not use a third variable, the value of two variables exchange 
    var A = 50 ;
     var B = 40 ; 
    A = A + B; // A = 90; B = 40 ; 
    B = A - B; // A = 90; B = 50; 
    A = A - B; // A = 40; B = 50; 
</ Script>

js is weakly typed language

• We will go to infer what type of value.
  • 　　js typeof name provides a method to determine what type

• For boolean data in the true and false, 0 and 1 with involvement operation. 
  • 　　Digital Plus string: convert a number to a string
  • 　　Digital Plus boolean values: true is converted to 1, false converted to 0
  • 　　+ Boolean string: String Boolean value to true or false
  • 　　Boolean Boolean +: Boolean value 0 or the value 1
• Any value can be converted boolean
  • 　　Empty string, 0 and NaN, null, undefined is false, others are to true .
• Numerical problems
  • 　　Infintiy and -Infintiy: infinite and infinitesimal
  • 　　NaN: non-numeric (Not a Number)
  • 　　NaN is not equal to any value, including NaN
  • 　　isNaN () can not be converted into any value will return true 
    • 　　isNaN(“abc”);//true
    • 　　isNaN("1");//false

js data type conversion

• toString (); null and undefined not toString () method, can be used String ();
• String();//String(null);
• Number (); // for a character string with a return NaN, the following code
• parseInt () and parseFloat (); // For the first character string return NaN, otherwise it returns the number of characters before

　　The Code

// Note 
// the parseInt ( "value", "band"); 
the parseInt ( "12.3abc"); // return 12 is 
the parseInt ( "abc12.3"); // Returns NaN3 
the parseInt ( ""); // return NaN 
Number The ( ""); // returns 0 
Number The ( "1223a"); // returns NaN

For operators

1. In addition to 0, return to Infinity; 0 modulo time to return NaN
2. ++ a; // a = a + 1; expression returns the value of a
3. a ++; // return value of a first expression in the calculation.

For js array

• Ordered list
• Any type
• Variable array size