JavaScript | For Loop

Looping in programming languages is a feature that facilitates the execution of a set of instructions repeatedly until some condition evaluates and becomes false. We come across for loop which provides a brief and systematic way of writing the loop structure.

Syntax:

for (statement 1 ; statement 2 ; statement 3)
  • Statement 1 is the initialization of the counter. It is executed once before the execution of the code block.
  • Statement 2 is the testing statement that defines the condition for executing the code block It must return a boolean value. It is also an entry controlled loop as the condition is checked before the execution of the loop statements.
  • Statement 3 is increment or decrement of counter & executed (every time) after the code block has been executed.
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<script type = "text/javaScript"> 
// JavaScript program to illustrate for loop
    var x; 
    
    // for loop begins when x=2 
    // and runs till x <=4 
    for (x = 2; x <= 4; x++)  
    { 
        document.write("Value of x:" + x + "<br />"); 
    } 
    
< /script> 

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Output : 


Value of x:2
Value of x:3
Value of x:4



Flow chart :

Image


Statement 1

We can initialize the counter variable externally rather than in statement 1. This shows us clearly that statement 1 is optional. We can leave the portion empty with a semicolon. For example:













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<script> 


    var x = 2;  


    


    for ( ; x <= 4; x++)   


    {  


        document.write("Value of x:" + x + "<br />");  


    }   


</script> 



















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Statement 2

This statement checks the boolean value of the testing condition. If the testing condition is true, the for loop will execute further. It is executed every time the for loop runs before the loop enters its body. If this statement returns true, the loop will start over again, otherwise it will end and move over to the code following the loop body. This is also an optional statement and Javascript treats it as true if left blank. If this statement is omitted, the loop runs indefinitely if the loop control isn’t broken manually. It is explained below: 











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<script> 


 var x = 2;  


 bool  


    for ( ; ; x++)   


    {  


        document.write("Value of x:" + x + "<br />");  


        break; 


    }   


</script> 



















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Statement 3

It is a controlled statement that controls the increment/decrement of the counter variable. It is also optional by nature and can be done inside the loop body. For example:-











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<script> 


var i = 0; 


var len = 2; 


var gfg = ""; 


for (; i < len; ) {  


  gfg += cars[i] + "<br>"; 


//can be increased inside loop 


  i++; 


} 


</script> 



















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for/in


There is another advanced loop called for/in loop which run through all the properties of an object. The loop will be executed once for each property of the object.


Syntax : for (var in object) { statements to be executed } 


Example:











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<!DOCTYPE html> 


<html> 


<body style = "text-align:center;">  


  


<h1 style = "color:green;" >  


GeeksForGeeks  


</h1>  


  


<button onclick="GFG()">Try it</button> 


  


<p id="demo"></p> 


  


<script> 


function GFG() { 


  var Platform= {fname:"geeks", Mname:"for", lname:"geeks", };  


    


  var text = ""; 


  var x; 


  for (x in Platform) { 


    text += Platform[x] + " "; 


  } 


  document.getElementById("demo").innerHTML = text; 


} 


</script> 


  


</body> 


</html> 



















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Output :

Image


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