Loops

while

pre-tested loop
the condition expression is evaluated before executing the body of the loop

while (i < total) { 
   i++ 
} 

do while

post-tested loop (executed at least once)
the condition expression is evaluated after executing the body of the loop

do { 
   i++ 
} while (i < total) 

for

for ( i = 0; i < total; i++) { 
} 

for in

Added in ES 2009.
This traverses all enumerable properties of an object
Iterates over the property keys of an object
This cannot iterate through a collection

var myobject = { 
   name: "Steve" 
   age: 20 
}; 
for (var propertyVal in myobject) { 
   write(propertyVal); 
   write(myobject[propertyVal]); 
} 

var people = ["steve","john","people"]; 
for (i in people) { 
   alert(people[i]); 
}