javaScript for loops

javascript for loops are a way of repeating the same block of code over and over.

the javascript for loop can executes a statement or series of statements until a specified condition is false.

syntax:

         for ( initialization  ;  condition  ;  increment)

                  {
                             the for loop code block
                  }

initialization

optional.an expression.this expression is executed only once, before the loop is executed.

condition

optional.a boolean expression.if condition is return true, loop body is executed .if condition is false, the loop is terminated.

increment/decrement

optional.an expression.the increment expression is executed at the end of every pass through the loop.

example:

 <script>
   
    var   a = [10,20,30,40,50,60,70,80,90,100] ;
          
                
       for ( var i=0 ; i < a.length ; i++){
                
            document.write("<br/>"+"the array index value a["+i+"]"+"     "+ a[i] );
                                   
        }
  
  </script>
  • first, the initial value of the variable i is evaluated.
  • then, as long as the value of i is less than to array length, the document.write statements are executed and i is reevaluated.
  • when i is greater than array length, the condition becomes false and control is transferred outside the loop.

the initialization,  condition,  increment statements are optional.so we can write for loop in number of ways.

first form

 <script>
   
    var   a = [100,200,300,400,500,600,700,800,900,1000] ;
          
       var  j = 0 ;     // initialization statement           
                 
       for (   ; j < a.length ; j++){
                
  document.write("<br/>"+"the array-----index-----value----a["+j+"]"+"  "+ a[j] );
                                   
        }
  
  </script>

second form

 <script>
   
    var   aa = [1,2,3,4,5,6,7,8,9,10] ;
          
       var  a = 0 ;     //we can put initialization statement output for loop parentheses          
                 
       for (   ; a < aa.length ;  ){
                
            document.write("<p>"+"the array________index_____ value aa["+a+"]"+"     "+ aa[a] );
           
            a++;  //you can put increment statement inside loop body                      
        }
  
  </script>

javaScript nested for loop

Nesting of loops, that is, one for statement within another for statement.

synax:

       for( initialization  ;  condition  ;  increment ){  outer for loop block        
       
                              for( initialization  ;  condition  ;  increment ){
                                 
                                                           inner for loop block
                                                        } }
   <head>  <script>
     
 for(var i=1; i<10; i++){  document.write("<br/>outer for loop  i value--"+i);
     
    for(var j=1; j<10; j++){  if(i==j){
    
                    document.write("<br/>inner for loop  i  == j value"+j); }
             
                 }}</script></head>

javaScript infinite For loop

an infinite loop in javascript occurs if a condition always evaluates to true.

in the below examples some pseudocode for an infinite loop for a web server.

third form

 <script>
   
    var   i = [1000,2000,3000,4000,5000,6000,7000,8000,9000,10000] ;
          
       var  j = 0 ;     //we can put initialization statement output for loop parentheses          
                 
       for (   ;       ; //omit loop condition  ){
                
            document.write("<br/>"+"java script infinite loop  i["+j+"]"+"     "+ i[j] );
           
            j++;  //you can put increment statement inside loop body                      
        }
  
  </script>

note:if you omit the loop condition, condition always true, in this situation loop never end.so its called infinite loop and this will crash your browser.

to solve this situation, you must provide a break statement inside the loop body.

         for ( ; ; ) {    document.write("this is infinite loop") ; break; }