Programming Tutorials

do-while, while, for and for-in loops in Javascript

By: Syed Fazal in Javascript Tutorials on 2008-08-16  

Iterative statements, also called loop statements, specify certain commands to be executed repeatedly until some condition is met. The loops are often used to iterate the values of an array (hence the name) or to work though repetitious mathematical tasks. JavaScript provides four types of iterative statements to aid in the process.

do-while

The do-while statement is a post-test loop, meaning that the evaluation of the escape condition is only done after the code inside the loop has been executed. This means that the body of the loop is always executed at least once before the expression is evaluated. Syntax:

do {

   statement

} while ( expression );

For example:

var i = 0;

do {

 i += 2;

} while (i < 10);

It’s considered best coding practice to always use block statements, even if only one line of code is to be executed. Doing so can avoid confusion about what should be executed for each condition.

while

The while statement is a pretest loop. This means the evaluation of the escape condition is done before the code inside the loop has been executed. Because of this, it is possible that the body of the loop is never executed. Syntax:

while( expression ) statement

For example:

var i = 0;

while (i < 10) {

 i += 2;

}

for

The for statement is also a pretest loop with the added capabilities of variable initialization before entering the loop and defining postloop code to be entered. Syntax:

for ( initialization ; expression ; post-loop-expression ) statement

For example:

for (var i=0; i < iCount; i++){

 alert(i);

}

This code defines a variable i that begins with the value 0 . The for loop is entered only if the conditional expression ( i < iCount ) evaluates to true , making it possible that the body of the code might not be executed. If the body is executed, the postloop expression is also executed, iterating the variable i .

for-in

The for-in statement is a strict iterative statement. It is used to enumerate the properties of an object.

Syntax:

for ( property in expression ) statement

For example:

for (sProp in window) {

 alert(sProp);

}

Here, the for-in statement is used to display all the properties of the BOM window object. The method propertyIsEnumerable() , discussed earlier, is included in JavaScript specifically to indicate whether or not a property can be accessed using the for-in statement.