A statement is setting a variable equal to a value. A statement is also a function call, i.e. document.write(). Statements define what the script will do and how it will be done.A Javascript code consists of Javascript statements. Each statement is a set of expressions. JavaScript statements should be terminated with a semicolon (;). Statements are executed sequentially unless altered by some statements which test a condition and branch or loop according to the result.

Javascript statements can be categorized as :
 

Conditional StatementsConditional statements are used to control your scripts so that different actions can be taken depending on the situation. Suppose you may want to display a special message during the holidays. This condition would depend on what day it was, and if it was a holiday, then a special holiday message would be displayed to your visitors. Without proper knowledge of the conditional statement your scripts will not be as lively or dynamic as they could possibly be.

(1) if...else Statement

General Format:
if (condition){
blockstatements1
}
else{
blockstatements2
}

If the condition evaluates to true then the block of blockstatements1 is executed. Otherwise the block of blockstatements2  is executed. The else clause is optional if there are no statements which need to be executed if the condition is false. Also, there could be only 1 statement under an if clause.

Example1:
if (Age >= 21){
message = "Age = " + Age;
alert("Hey ! I am an adult !" + message);
}
else{
alert("Hey ! I am a Teenager !");
}

Example2:
if (Age > 21){
message = "Age = " + Age;
alert("Hey ! I am an adult !" + message);
}

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(2) Switch Statement

The syntax is :

switch (expression){
    case label1 :
        statement;
        break;
    case label2 :
        statement;
        break;
         ...
    default :
        statement;
}

How switch works:

1. First the expression is evaluated
2. Then program looks for a label matching the value of expression and executes the associated statement. The optional break statement associated with each case label makes sure 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.
3. If no matching label is found, the program looks for the optional default statement, and if found, executes the associated statement.
4. If no default statement is found, the program continues execution at the statement following the end of switch.

eg.

switch (Entertainment){
   case "Movies" :
      alert('Movies');;
      break;
   case "Music" :
      alert('Music');;
      break;
   case "MTV" :
      alert('MTV');;
      break;
   default :
    alert('None of above');;
}

In the above example, if Entertainment evaluates to "Movies", the program matches the value with case "Entertainment" and executes the associated statement. Since there is a break statement,program terminates switch and executes the statement following switch.

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Loop Statements A loop statement checks to see if some condition is true, and if that condition is true it will execute a chunk of code.

(1) for Statement

The syntax is:

for (initial-expression;condition-expression;increment-expression)
{  statements;}

A for loop repeats until the condition-expression evaluates to false. Initial-expression is executed first.Then the condition-expression is evaluated. If the value of condition-expresion is true, the loop statements are executed. If the value of condition is false, the for loop terminates.   Then increment-expression is evaluated and then the loop starts again.

Eg.

for (var Age=0; Age < 100; Age++)
{document.write("Age= " + Age) }

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(2)do...while Statement

The do...while statement repeats until a specified condition evaluates to false.

The syntax is:
do
{  statements}
while (condition)

eg.

Age=0
do{
    Age = Age +1 ;
    document.write(Age);
}
while (Age<100);

the do loop iterates at least once, even if initial condition is false and repeats until Age is no longer less than 100.

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The syntax is:

while (condition)
{   statements}

A while statement executes its statements as long as a specified condition evaluates to true. The  while loop will not execute the statements even once if the condition is initially false.

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The labeled statement looks as follows:

label :
statements

Eg:
 

label1:
if (4==i) {
document.write("You've entered " + i + ".<BR>");
label2 :
if (2==j) {
document.write("You've entered " + j + ".<BR>");
break label2;
document.write("The sum is " + (i+j) + ".<BR>");
        }
document.write(i + "-" + j + "=" + (i-j) + ".<BR>");
   }

A statement labeled label1 contains a statement labeled label1. If break is encountered, the program breaks out of the label2 statement and continues with the remainder of the label1 statement. If break had a label of label1, the program would break out of the label1 statement and continue at the statement following label1.

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(5) break Statement
The break statement can be used in a while, for, and labeled statement. This stops the execution of the loop, drops out of loop to the next statement following the loop.

The syntax is:
break               (In a while or for statement)
break label       (In a labeled statement)

Eg:

function ExampleFunction() {
var Age = 0
while (Age < 1000) {
    if (Age == 100) break;
    Age++
}
alert('out of loop');
}

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(6) Continue Statement

The continue statement can be used in a while, for, and labeled statement. Continue statement terminates the current loop and continues execution of the loop with the next iteration (if condition is still true). ie., In a while loop, it jumps back to the condition. In a for loop, it jumps to the increment-expression. In a labeled statement it continues the defined labeled statement.

Eg1: Continue in a While Loop

Age = 0
x=0
while (Age <= 100) {
 Age++
// when Age=60 , continue  is executed and control goes to statement Age++
 if (Age == 60) continue; 

// This statement will be skipped if Age=60
document.write(Age + "<BR>");
}

The above while loop prints all ages except for 60.

Eg2:
 

Age=0
label1 :
while (Age < 100) {
document.write(Age + "<BR>");
Age += 1;
label :
while (x>4) {
document.write(x + "<BR>");
x-=1;
if ((x%2)==0);
continue label;
document.write(x + " is odd.<BR>");
}
document.write("Age = " + Age + "<br>");
document.write("j = " + j + "<Br>");
}

The statement labeled label1 contains a statement labeled label. When continue is encountered, the program terminates the current iteration of label2 and begins the next iteration. Each time continue is encountered, label reiterates until its condition returns false. When false is returned, the remainder of the label1 statement is completed, and label1 reiterates until its condition returns false. When false is returned, the program continues at the statement following label1. If continue had a label of label1, the program would continue at the top of the label1 statement.

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(1)for...in
The for...in statement iterates over all the properties of an object. For each distinct property, JavaScript executes the specified statements.

The syntax is:
for (variable in object)
{   statements }

Eg: The  following function  iterates through all the object's properties and returns a string that lists the property names and their values.

Function DisplayProperties(Object, ObjectName) {
     for (var eachproperty in Object) {
         document.write(ObjectName + "." + eachproperty + " = " + Object[eachproperty] + '<BR>');
       }
}

A complete example is given below:

<head>
<script>
function DisplayProperties(Object, ObjectName) {
     for (var eachproperty in Object) {
         document.write(ObjectName + "." + eachproperty + " = " + Object[eachproperty] + '<BR>');
       }
}

employee=new Object();
employee.name="Bob";
employee.sex="male";
employee.country="US";

DisplayProperties(employee, 'employee')

</script>
</head>

Result of above program:
employee.name = Bob
employee.sex = male
employee.country = US
 

(2)with Statement

The syntax is:
with (object)
{   statements}

The with statement establishes the default object for a set of statements. Within the set of statements, any property references that do not specify an object are assumed to be for the default object.

eg:

<head>
<script>
employee=new Object();
employee.name="Bob";
employee.sex="male";
employee.country="US";

// This is the with statement which acts as the default object
with (employee) {
document.write(name + "/"  + sex + "/" + country);
}

</script>
</head>

Result of above program:
Bob/male/US

The  with statement specifies that the employee object is the default object. The statements following the with statement refer to properties name,sex and country without specifying the  object, employee. It is assumed that employee object  be used for these references.

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Comments are used in  a javascript code to explain what a script or function does. Using comments in a Javascript code is recommended very much as this could be useful for others to understand the logic behind each script.

JavaScript  ignores comments. There are two different ways to give comments.
 

A Complete example:

<head>
<script>
var Age=0; // initialize variable Age
var Country="USA"; // Initialize variable Country

/* Display Country */
alert("country="+ Country)
// Display Age
alert("Age="+ Age)

</script>
</head>