• Conditional Statements: if...else and switch
• Loop Statements: for, while, do while, label, break, and continue (label is not itself a looping statement, but is frequently used with these statements)
• Object Manipulation Statements: for...in and with
• Comments
• Exception Handling Statements: try...catch and throw

Conditional Statements

if...else Statement

if (condition) {
   statements1 
}
[else {
   statements2 
} ]

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

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

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

switch (expr) {
   case "Oranges" : 
      document.write("Oranges are $0.59 a pound.<BR>"); 
      break; 
   case "Apples" :
      document.write("Apples are $0.32 a pound.<BR>");
      break;
   case "Bananas" : 
      document.write("Bananas are $0.48 a pound.<BR>"); 
      break; 
   case "Cherries" :
      document.write("Cherries are $3.00 a pound.<BR>");
      break; 
   default :
      document.write("Sorry, we are out of " + i + ".<BR>"); 
}

document.write("Is there anything else you'd like?<BR>");

Loop Statements

for Statement

for ([initialExpression]; [condition]; [incrementExpression]) {
   statements
}

1. The initializing expression initial-expression, if any, is executed. This expression usually initializes one or more loop counters, but the syntax allows an expression of any degree of complexity.
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.
3. The statements execute.
4. The update expression incrementExpression executes, and control returns to Step 2.

<SCRIPT>
function howMany(selectObject) {
   var numberSelected=0
   for (var i=0; i < selectObject.options.length; i++) {
      if (selectObject.options[i].selected==true)
         numberSelected++
   }
   return numberSelected
}
</SCRIPT>
<FORM NAME="selectForm">
<P><B>Choose some music types, then click the button below:</B>
<BR><SELECT NAME="musicTypes" MULTIPLE>
<OPTION SELECTED> R&B
<OPTION> Jazz
<OPTION> Blues
<OPTION> New Age
<OPTION> Classical
<OPTION> Opera
</SELECT>
<P><INPUT TYPE="button" VALUE="How many are selected?"
onClick="alert ('Number of options selected: ' + howMany(document.selectForm.musicTypes))">
</FORM>

do...while Statement

do {
   statement
} while (condition)

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

while Statement

while (condition) {
   statements
}

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

• 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

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

label Statement

label : 
   statement

markLoop:
while (theMark == true)
   doSomething();
}

break Statement

• When you use break with a while, do-while, for, or switch statement, break terminates the innermost enclosing loop or switch immediately and transfers control to the following statement.
• When you use break within an enclosing label statement, it terminates the statement and transfers control to the following statement. If you specify a label when you issue the break, the break statement terminates the specified statement.

1. break
2. break [label]

for (i = 0; i < a.length; i++) {
   if (a[i] = theValue);
      break;
}

continue Statement

• In a while or for statement, continue terminates the current loop 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.
• In a label statement, continue is followed by a label that identifies a label statement. This type of continue restarts a label statement or continues execution of a labelled loop with the next iteration. continue must be in a looping statement identified by the label used by continue.

1. continue
2. continue [label]

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

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

for...in Statement

for (variable in object) {
   statements }

function dump_props(obj, obj_name) {
   var result = ""
   for (var i in obj) {
      result += obj_name + "." + i + " = " + obj[i] + "<BR>"
   }
   result += "<HR>"
   return result
}

with Statement

with (object){
   statements
}

var a, x, y
var r=10
with (Math) {
   a = PI * r * r
   x = r * cos(PI)
   y = r * sin(PI/2)
}

Comments

• Comments on a single line are preceded by a double-slash (//).
• Comments that span multiple lines are preceded by /* and followed by */:

// This is a single-line comment.

/* This is a multiple-line comment. It can be of any length, and
you can put whatever you want here. */

Exception Handling Statements

The throw Statement

throw expression

throw "Error2"    // generates an exception with a string value
throw 42          // generates an exception with the value 42
throw true        // generates an exception with the value true

// Create an object type UserException
function UserException (message) {
   this.message=message
   this.name="UserException"
}
// Create an instance of the object type and throw it
myUserException=new UserException("Value too high")
throw myUserException

The try...catch Statement

function getMonthName (mo) {
   mo=mo-1 // Adjust month number for array index (1=Jan, 12=Dec)
   var months=new Array("Jan","Feb","Mar","Apr","May","Jun","Jul",
      "Aug","Sep","Oct","Nov","Dec")
   if (months[mo] != null) {
      return months[mo]
   } else {
      throw "InvalidMonthNo"
   }
}

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

The catch Block

catch (catchID) {
   statements
}

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

The finally Block

try {
   openMyFile()  // tie up a resource
   writeMyFile(theData)
}
finally {
   closeMyFile() // always close the resource
}

Nesting try...catch Statements

Last Updated: 10/29/98 15:51:07