Loop Statements

JavaScript supports a compact set of statements, specifically control flow statements, that you can use to incorporate a great deal of interactivity in Web pages. This chapter provides an overview of these statements.

Any expression is also a statement. See Expressions and Operators for complete information about expressions.

Use the semicolon (;) character to separate statements in JavaScript code.

See the JavaScript Reference for details about the statements in this chapter.

Block Statement

A block statement is used to group statements. The block is delimited by a pair of curly brackets:


Block statements are commonly used with control flow statements (e.g. if, for, while).

while (x < 10) {

Here, { x++; } is the block statement.

Important: JavaScript does not have block scope. Variables introduced within a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not introduce a scope. Although "standalone" blocks are valid syntax, you do not want to use standalone blocks in JavaScript, because they don't do what you think they do, if you think they do anything like such blocks in C or Java. For example:

var x = 1;
  var x = 2;
alert(x); // outputs 2

This outputs 2 because the var x statement within the block is in the same scope as the var x statement before the block. In C or Java, the equivalent code would have outputted 1.

Conditional Statements

A conditional statement is a set of commands that executes if a specified condition is true. JavaScript supports two conditional statements: if...else and switch.

if...else Statement

Use the if statement to execute a statement if a logical condition is true. Use the optional else clause to execute a statement if the condition is false. An if statement looks as follows:

if (condition) {
} else {

condition can be any expression that evaluates to true or false. See Boolean for an explanation of what evaluates to true and false. If condition evaluates to true, statement_1 is executed; otherwise, statement_2 is executed. statement_1 and statement_2 can be any statement, including further nested if statements.

You may also compound the statements using else if to have multiple conditions tested in sequence, as follows:

if (condition_1) {
} else if (condition_2) {
} else if (condition_n) {
} else {

To execute multiple statements, group them within a block statement ({ ... }) . In general, it's good practice to always use block statements, especially when nesting if statements:

if (condition) {
} else {
It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code. For example, do not use the following code:
if (x = y) {
  /* do the right thing */

If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:

if ((x = y)) {
  /* do the right thing */

The following values will evaluate to false:

  • false
  • undefined
  • null
  • 0
  • NaN
  • the empty string ("")

All other values, including all objects evaluate to true when passed to a conditional statement.

Do not confuse the primitive boolean values true and false with the true and false values of the Boolean object. For example:

var b = new Boolean(false);
if (b) // this condition evaluates to true

In the following example, the function checkData returns true if the number of characters in a Text object is three; otherwise, it displays an alert and returns false.

function checkData() {
  if (document.form1.threeChar.value.length == 3) {
    return true;
  } else {
    alert("Enter exactly three characters. " +
      document.form1.threeChar.value + " is not valid.");
    return false;

switch Statement

A switch statement allows a program to evaluate an expression and attempt to match the expression's value to a case label. If a match is found, the program executes the associated statement. A switch statement looks as follows:

switch (expression) {
   case label_1:
   case label_2:

The program first looks for a case clause with a label matching the value of expression and then transfers control to that clause, executing the associated statements. If no matching label is found, the program looks for the optional default clause, and if found, transfers control to that clause, executing the associated statements. If no default clause is found, the program continues execution at the statement following the end of switch. By convention, the default clause is the last clause, but it does not need to be so.

The optional break statement associated with each case clause ensures that the program breaks out of switch once the matched statement is executed and continues execution at the statement following switch. If break is omitted, the program continues execution at the next statement in the switch statement.

In the following example, if fruittype evaluates to "Bananas", the program matches the value with case "Bananas" and executes the associated statement. When break is encountered, the program terminates switch and executes the statement following switch. If break were omitted, the statement for case "Cherries" would also be executed.

switch (fruittype) {
   case "Oranges":
      document.write("Oranges are $0.59 a pound.<br>");
   case "Apples":
      document.write("Apples are $0.32 a pound.<br>");
   case "Bananas":
      document.write("Bananas are $0.48 a pound.<br>");
   case "Cherries":
      document.write("Cherries are $3.00 a pound.<br>");
   case "Mangoes":
      document.write("Mangoes are $0.56 a pound.<br>");
   case "Papayas":
      document.write("Mangoes and papayas are $2.79 a pound.<br>");
      document.write("Sorry, we are out of " + fruittype + ".<br>");
document.write("Is there anything else you'd like?<br>");

Loop Statements

A loop is a set of commands that executes repeatedly until a specified condition is met. JavaScript supports the for, do while, and while loop statements, as well as label (label is not itself a looping statement, but is frequently used with these statements). In addition, you can use the break and continue statements within loop statements.

Another statement, for...in, executes statements repeatedly but is used for object manipulation. See Object Manipulation Statements.

The loop statements are:

for Statement

A for loop repeats until a specified condition evaluates to false. The JavaScript for loop is similar to the Java and C for loop. A for statement looks as follows:

for ([initialExpression]; [condition]; [incrementExpression])

When a for loop executes, the following occurs:

  1. The initializing expression initialExpression, if any, is executed. This expression usually initializes one or more loop counters, but the syntax allows an expression of any degree of complexity. This expression can also declare variables.
  2. The condition expression is evaluated. If the value of condition is true, the loop statements execute. If the value of condition is false, the for loop terminates. If the condition expression is omitted entirely, the condition is assumed to be true.
  3. The statement executes. To execute multiple statements, use a block statement ({ ... }) to group those statements.
  4. The update expression incrementExpression, if there is one, executes, and control returns to step 2.

The following function contains a for statement that counts the number of selected options in a scrolling list (a Select object that allows multiple selections). The for statement declares the variable i and initializes it to zero. It checks that i is less than the number of options in the Select object, performs the succeeding if statement, and increments i by one after each pass through the loop.


function howMany(selectObject) {
   var numberSelected = 0;
   for (var i = 0; i < selectObject.options.length; i++) {
      if (selectObject.options[i].selected)
   return numberSelected;


<form name="selectForm">
      <strong>Choose some music types, then click the button below:</strong>
      <select name="musicTypes" multiple="multiple">
         <option selected="selected">R&B</option>
         <option>New Age</option>
      <input type="button" value="How many are selected?"
         onclick="alert ('Number of options selected: ' + howMany(document.selectForm.musicTypes))"/>

do...while Statement

The do...while statement repeats until a specified condition evaluates to false. A do...while statement looks as follows:

while (condition);

statement executes once before the condition is checked. To execute multiple statements, use a block statement ({ ... }) to group those statements. If condition is true, the statement executes again. At the end of every execution, the condition is checked. When the condition is false, execution stops and control passes to the statement following do...while.

In the following example, the do loop iterates at least once and reiterates until i is no longer less than 5.

do {
   i += 1;
} while (i < 5);

while Statement

A while statement executes its statements as long as a specified condition evaluates to true. A while statement looks as follows:

while (condition)

If the condition becomes false, statement within the loop stops executing and control passes to the statement following the loop.

The condition test occurs before statement in the loop are executed. If the condition returns true, statement is executed and the condition is tested again. If the condition returns false, execution stops and control is passed to the statement following while.


Example 1
The following while loop iterates as long as n is less than three:

n = 0;
x = 0;
while (n < 3) {
   x += n;

With each iteration, the loop increments n and adds that value to x. Therefore, x and n take on the following values:

  • After the first pass: n = 1 and x = 1
  • After the second pass: n = 2 and x = 3
  • After the third pass: n = 3 and x = 6

After completing the third pass, the condition n < 3 is no longer true, so the loop terminates.

Example 2
Avoid infinite loops. Make sure the condition in a loop eventually becomes false; otherwise, the loop will never terminate. The statements in the following while loop execute forever because the condition never becomes false:

while (true) {
   alert("Hello, world");

label Statement

A label provides a statement with an identifier that lets you refer to it elsewhere in your program. For example, you can use a label to identify a loop, and then use the break or continue statements to indicate whether a program should interrupt the loop or continue its execution.

The syntax of the label statement looks like the following:

label :

The value of label may be any JavaScript identifier that is not a reserved word. The statement that you identify with a label may be any statement.

In this example, the label markLoop identifies a while loop.

while (theMark == true) {

break Statement

Use the break statement to terminate a loop, switch, or in conjunction with a label statement.

  • When you use break without a label, it terminates the innermost enclosing while, do-while, for, or switch immediately and transfers control to the following statement.
  • When you use break with a label, it terminates the specified labeled statement.

The syntax of the break statement looks like this:

  1. break;
  2. break label;

The first form of the syntax terminates the innermost enclosing loop or switch; the second form of the syntax terminates the specified enclosing label statement.

Example 1:
The following example iterates through the elements in an array until it finds the index of an element whose value is theValue:

   if (a[i] == theValue)

Example 2: Breaking to a Label

var x = 0;
var z = 0
labelCancelLoops: while (true) {
    console.log("Outer loops: " + x);
    x += 1;
    z = 1;
    while (true) {
        console.log("Inner loops: " + z);
        z += 1;
        if (z === 10 && x === 10) {
            break labelCancelLoops;
        } else if (z === 10) {

continue Statement

The continue statement can be used to restart a while, do-while, for, or label statement.

  • When you use continue without a label, it terminates the current iteration of the innermost enclosing while, do-while or for statement and continues execution of the loop with the next iteration. In contrast to the break statement, continue does not terminate the execution of the loop entirely. In a while loop, it jumps back to the condition. In a for loop, it jumps to the increment-expression.
  • When you use continue with a label, it applies to the looping statement identified with that label.

The syntax of the continue statement looks like the following:

  1. continue;
  2. continue label;

Example 1
The following example shows a while loop with a continue statement that executes when the value of i is three. Thus, n takes on the values one, three, seven, and twelve.

i = 0;
n = 0;
while (i < 5) {
   if (i == 3)
   n += i;

Example 2
A statement labeled checkiandj contains a statement labeled checkj. If continue is encountered, the program terminates the current iteration of checkj and begins the next iteration. Each time continue is encountered, checkj reiterates until its condition returns false. When false is returned, the remainder of the checkiandj statement is completed, and checkiandj reiterates until its condition returns false. When false is returned, the program continues at the statement following checkiandj.


checkiandj :
   while (i < 4) {
      document.write(i + "<br/>");
      i += 1;
      checkj :
         while (j > 4) {
            document.write(j + "<br/>");
            j -= 1;
            if ((j % 2) == 0)
               continue checkj;
            document.write(j + " is odd.<br/>");
      document.write("i = " + i + "<br/>");
      document.write("j = " + j + "<br/>");

Object Manipulation Statements

JavaScript uses the for...in, for each...in, and with statements to manipulate objects.

for...in Statement

The for...in statement iterates a specified variable over all the properties of an object. For each distinct property, JavaScript executes the specified statements. A for...in statement looks as follows:

for (variable in object) {

The following function takes as its argument an object and the object's name. It then iterates over all the object's properties and returns a string that lists the property names and their values.

function dump_props(obj, obj_name) {
   var result = "";
   for (var i in obj) {
      result += obj_name + "." + i + " = " + obj[i] + "<br>";
   result += "<hr>";
   return result;

For an object car with properties make and model, result would be:

car.make = Ford
car.model = Mustang

Although it may be tempting to use this as a way to iterate over Array elements, because the for...in statement iterates over user-defined properties in addition to the array elements, if you modify the Array object, such as adding custom properties or methods, the for...in statement will return the name of your user-defined properties in addition to the numeric indexes. Thus it is better to use a traditional for loop with a numeric index when iterating over arrays.

for each...in Statement

for each...in is a loop statement introduced in JavaScript 1.6. It is similar to for...in, but iterates over the values of object's properties, not their names.

var sum = 0;
for each (var item in obj) {
  sum += item;
print(sum); // prints "26", which is 5+13+8


Comments are author notations that explain what a script does. Comments are ignored by the interpreter. JavaScript supports Java and C++-style comments:

  • Comments that span multiple lines are preceded by /* and followed by */:

The following example shows two comments:

// This is a single-line comment.

you can put whatever you want here. */

Exception Handling Statements

You can throw exceptions using the throw statement and handle them using the try...catch statements.

You can also use the try...catch statement to handle Java exceptions (though there is a bug 391642 with this). See Handling Java Exceptions in JavaScript and JavaScript to Java Communication for information.

Exception Types

Just about any object can be thrown in JavaScript. Nevertheless, not all thrown objects are created equal. While it is fairly common to throw numbers or strings as errors it is frequently more effective to use one of the exception types specifically created for this purpose:

throw Statement

Use the throw statement to throw an exception. When you throw an exception, you specify the expression containing the value to be thrown:

throw expression;

You may throw any expression, not just expressions of a specific type. The following code throws several exceptions of varying types:

throw "Error2";   //String type
throw 42;         //Number type
throw true;       //Boolean type
throw {toString: function() { return "I'm an object!"; } };
Note: You can specify an object when you throw an exception. You can then reference the object's properties in the catch block. The following example creates an object myUserException of type UserException and uses it in a throw statement.
// Create an object type UserException
function UserException (message){

// Make the exception convert to a pretty string when used as a string (e.g. by the error console)
UserException.prototype.toString = function (){
  return this.name + ': "' + this.message + '"';

// Create an instance of the object type and throw it
throw new UserException("Value too high");

try...catch Statement

The try...catch statement marks a block of statements to try, and specifies one or more responses should an exception be thrown. If an exception is thrown, the try...catch statement catches it.

The try...catch statement consists of a try block, which contains one or more statements, and zero or more catch blocks, containing statements that specify what to do if an exception is thrown in the try block. That is, you want the try block to succeed, and if it does not succeed, you want control to pass to the catch block. If any statement within the try block (or in a function called from within the try block) throws an exception, control immediately shifts to the catch block. If no exception is thrown in the try block, the catch block is skipped. The finally block executes after the try and catch blocks execute but before the statements following the try...catch statement.

The following example uses a try...catch statement. The example calls a function that retrieves a month name from an array based on the value passed to the function. If the value does not correspond to a month number (1-12), an exception is thrown with the value "InvalidMonthNo" and the statements in the catch block set the monthName variable to unknown.

function getMonthName (mo) {
    mo=mo-1; // Adjust month number for array index (1=Jan, 12=Dec)
    if (months[mo] != null) {
       return months[mo]
    } else {
       throw "InvalidMonthNo"          //throw keyword is used here

try {// statements to try
    monthName=getMonthName(myMonth) // function could throw exception
catch (e) {
    logMyErrors(e) // pass exception object to error handler

The catch Block

You can use a catch block to handle all exceptions that may be generated in the try block.

catch (catchID) {

The catch block specifies an identifier (catchID in the preceding syntax) that holds the value specified by the throw statement; you can use this identifier to get information about the exception that was thrown. JavaScript creates this identifier when the catch block is entered; the identifier lasts only for the duration of the catch block; after the catch block finishes executing, the identifier is no longer available.

For example, the following code throws an exception. When the exception occurs, control transfers to the catch block.

try {
   throw "myException" // generates an exception
catch (e) {
// statements to handle any exceptions
   logMyErrors(e) // pass exception object to error handler

The finally Block

The finally block contains statements to execute after the try and catch blocks execute but before the statements following the try...catch statement. The finally block executes whether or not an exception is thrown. If an exception is thrown, the statements in the finally block execute even if no catch block handles the exception.

You can use the finally block to make your script fail gracefully when an exception occurs; for example, you may need to release a resource that your script has tied up. The following example opens a file and then executes statements that use the file (server-side JavaScript allows you to access files). If an exception is thrown while the file is open, the finally block closes the file before the script fails.

try {
    writeMyFile(theData); //This may throw a error
    handleError(e); // If we got a error we handle it
}finally {
    closeMyFile(); // always close the resource

If the finally block returns a value, this value becomes the return value of the entire try-catch-finally production, regardless of any return statements in the try and catch blocks:

function f() {
    try {
        throw "bogus";
    } catch(e) {
        return true; // this return statement is suspended until finally block has completed
        alert(2); // not reachable
    } finally {
        return false; // overwrites the previous "return"
        alert(4); // not reachable
    // "return false" is executed now
    alert(5); // not reachable
f(); // alerts 0, 1, 3; returns false

Nesting try...catch Statements

You can nest one or more try...catch statements. If an inner try...catch statement does not have a catch block, the enclosing try...catch statement's catch block is checked for a match.

Utilizing Error objects

Depending on the type of error, you may be able to use the 'name' and 'message' properties to get a more refined message. 'name' provides the general class of Error (e.g., 'DOMException' or 'Error'), while 'message' generally provides a more succinct message than one would get by converting the error object to a string.

If you are throwing your own exceptions, in order to take advantage of these properties (such as if your catch block doesn't discriminate between your own exceptions and system ones), you can use the Error constructor. For example:

function doSomethingErrorProne () {
   if (ourCodeMakesAMistake()) {
      throw (new Error('The message'));
   else {
try {
catch (e) {
   alert(e.name);// alerts 'Error'
   alert(e.message); // alerts 'The message' or a JavaScript error message)

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