Syntax:

for (initialValue; test; increment)
statement

Example:

for (x=0; x<3; x++)
document.write("This text is repeated three times<br>")

Syntax:

while (expression)
statement

Example:

var number=0
while (number<5){
document.write(number+"<br>")
number++
}

Syntax:

do
statement
while (expression)

Example:

var number=0
do{
document.write(number+"<br>")
number++
}
while (number<5)

Syntax:

for (variable in object)
statement

"variable" is any arbitrary variable that will hold the current property of the object in question as it is being looped. Here are two examples.

Example 1: This example writes out all the properties of the "navigator" object:

for (myprop in navigator)
document.write(myprop+"<br>")
 

Example 2: This example writes out the properties, plus their corresponding value, of the "navigator" object:

for (myprop in navigator)
document.write(myprop+": "+navigator[myprop]+"<br>")

Example:

for (i=0; i<6; i++){
if (puppies[i].name=="George")
alert("I found George!")
break
}

Example:

for (i=1; i<6; i++){
if (i==4)
continue
document.write(i+"<br>")
}
//1, 2, 3, 5 is written out.