JavaScript Learning Summary (i)

1. Character conversion

 varS1 = "01"; varS2 = "1.1"; varS3 = "Z";//the letter ' Z ' cannot be converted to a number, so or return nanvarb =false; varF = 1.1; varo ={valueOf:function() {                return-1;                }        }; S1=-S1;//value becomes numeric-1s2 =-s2;//value becomes numeric-1.1s3 =-S3;//value becomes NaNb =-C;//value becomes numeric 0f =-F;//Change to-1.1o =-O;//1 The ValueOf () method of the first execution object returns-1,--1 is 1
o = valueof () value of +o;//-1 O is -1,+-1 or-1

2. Special character arithmetic

  varRESULT1 = 5-true;//4 because true is converted to 1        varRESULT2 = NaN-1;//nan Nan is not a number, and any number doing any operation is Nan        varRESULT3 = 5-3;//2        varRESULT4 = 5-"";//5 because "" is converted to 0        varRESULT5 = 5-"2";//3 because "2" is converted to 2        varRESULT6 = 5-NULL;//5 because null is converted to 0

3. Variable-to-string operation

 var  value1 = 10;  var  value2 = true  ;  var  value3 = null  ;  var   Value4;//value4 no assignment is underfined     Converting to a string is ' underfined ' alert (String (value1));      //  " alert (String (value2)); //      alert (String (VALUE3)); //      alert (String (VALUE4)); //  "undefined"  

4. Conversion of numbers into the system

var num = ten;        Alert (num.tostring ());        // "10" Default decimal        Alert (num.tostring (2));      // "1010"        Binary Alert (num.tostring (8));      // "12" octal        Alert (Num.tostring (ten));     // "10" decimal        Alert (num.tostring ());     // "a" hex

5. String comparison operation

varRESULT1 = 5 > 3;//true        varRESULT2 = 5 < 3;//false        varRESULT3 = "Brick" < "Alphabet";//true string comparisons are compared in alphabetical order with lowercase letters behind uppercase letters        varRESULT4 = "Brick". toLowerCase () < "Alphabet". toLowerCase ();//false alphabetical order comparison        varRESULT5 = "All" < "3";//true character 2 is less than character 3        varRESULT6 = "3" <;//false when ' 23 ' is converted to 23.        varRESULT7 = "a" < 3;//false because "a" becomes NaN character ' a ' cannot be converted to a number        varRESULT8 = NaN < 3;//false NaN and any number comparison cannot be converted to numbers, so it is always false        varResult9 = NaN >= 3;//false

6. Conversion of character into the system

  var num1 = parseint ("AF", +);        // 175 Output Decimal data according to the 16 binary 10*16+15*1        var num2 = parseint ("AF");            // Nan does not have a specified binary, by default the 10 binary conversion, because AF is not in the decimal range, returns NaN         alert (NUM1);        alert (num2);

7, the use of parseint

varNUM1 = parseint ("1234blue");//1234        varnum2 = parseint ("");//The NaN character ' cannot be converted to a number        varnum3 = parseint ("0xA");//10-hexadecimal 16 binary a        varNUM4 = parseint (22.5);// A        varNUM5 = parseint ("70");//70-decimal        varNUM6 = parseint ("0xf");//15 16 binary for

8. Use of Number objects

var num1 = number ("Hello world!");  // NaN        var num2 = number ("");              // 0 empty strings can be converted to 0 this parseint () is not the same        var num3 = number ("000011");        //  One        var num4 = number (true);            // 1

9. Nan Usage

Alert (nan = = nan);       // false        Alert (IsNaN (NaN));       // true        Alert (IsNaN (ten));        // false? is a        number Alert (IsNaN ("ten"));      // false? Can is converted        to number Alert (IsNaN ("Blue"));    // true? Cannot is converted to a        number Alert (IsNaN (true));      // false? can converted to number 1

10, the system maximum number

var result = Number.MAX_VALUE + 1;        Alert (isfinite (result));     // false

11, Infinity Infinity

 Alert (5 * 6); //         30  alert (5 * NaN); //    nan  alert (Infinity * 0); //    nan  alert (Infinity * 2); //         infinity  alert ("5" * 5); //       25  alert (true  * 10); //      10  alert (false  * 10); // 0 
  

Alert (26 5); 1
Alert (Infinity% 3); NaN
Alert (3 0); NaN
Alert (5 Infinity); 5
Alert (0 10); 0
Alert (true% 25); 1
Alert (3 false); NaN

12. For In loop

 for (var in window) {             document.write (propname);             document.write ("<br/>");        }

13. Special character comparison

AlertNULL= = undefined);//trueAlertNULL= = = undefined);//falseAlert ("Nan" = = Nan);//falseAlert ("nan" = = = Nan);//falseAlert (nan = = Nan);//falseAlert (nan = = = Nan);//falseAlert (nan! = nan);//trueAlert (Nan!== nan);//trueAlert (false= = 0);//trueAlertfalse= = = 0);//falseAlerttrue= = 1);//trueAlerttrue= = = 1);//falseAlert (NULL= = 0);//falsealert (undefined = = 0);//falseAlert (5 = = "5");//trueAlert (5 = = = "5");//false

14. A Boolean object

  var message = "Hello world!" ;         var messageasboolean = Boolean (message);                 // true

15. For loop use with label

 Breakstatements, andContinuestatements can be used in conjunction with a tagged statement to return a specific location in the code. Typically, this is done when there is a loop inside the loop, for example:varnum = 0; //return to a specific locationOutermost: for(vari=0; I < 10; i++) {              for(varj=0; J < 10; J + +) {                if(i = = 5 && J = = 5) {                     Breakoutermost; } num++;    }} alert (num); //55
In the example above, the label outermost represents the first for statement. Normally, each for statement executes 10 blocks of code, which means that num++ will normally be executed 100 times, and Num should be equal to 100 when execution is complete. The break statement here has a parameter, which is the label of the statement to jump to after stopping the loop. This way, the break statement can not only jump out of the internal for statement (that is, the statement using the variable J), but also jump out of the external for statement (that is, the statement using the variable i). Therefore, the last value of NUM is 55 because the loop terminates when both I and J values are equal to 5 o'clock.

You can use the Continue statement in the same way:

var iNum = 0; outermost: for (var i=0; i<10; i++) {  for ( var j=0; j<10; J + +) {    if (i = = 5 && j = 5) {    continue  outermost;  }  iNum+ +;  }} alert (iNum);     // Output ""

In the above example, the Continue statement forces the loop to continue, not just the inner loop, but also the outer loop. This occurs when J equals 5 o'clock, which means that the internal loop will be reduced by 5 iterations, resulting in a value of INum of 95.

Tip: It can be seen that tagged statements used in conjunction with break and continue are very powerful, but overuse of them will cause trouble with debugging code. Be sure to use a label that is descriptive and do not nest too many layers of loops.

JavaScript Learning Summary (i)