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Dynamic HTML: The Definitive Reference, 2rd Ed.Dynamic HTML: The Definitive ReferenceSearch this book

12.7. Control Statements

 
breakNN 2 IE 3 ECMA 1

Stops execution of the current loop and returns control to the next script statement following the end of the current loop. Note that without a label parameter, the scope of the break statement is its own loop. To break out of a nested loop, assign labels to each nested layer, and use the desired label as a parameter with the break statement. See the label statement (available only starting with Navigator 4 and Internet Explorer 4).

Syntax

break [label]

Example

See the label statement.

 
catchNN 2 IE 3 ECMA 1

See try.

 
continueNN 2 IE 3 ECMA 1

Stops execution of the current iteration through the loop and returns to the top of the loop for the next pass (executing the update expression if one is specified in a for loop). If you are using nested loop constructions, assign labels to each nested layer, and use the desired label as a parameter with the continue statement. See the label statement (available only starting with Navigator 4 and Internet Explorer 4).

Syntax

continue [label]

Example

outerLoop:
for (var i = 0; i <= maxValue1; i++) {
    for (var j = 0; j <= maxValue2; j++) {
        if (j*i == magic2) {
            continue outerLoop;
        }
    }
}
 
do/whileNN 4 IE 4 ECMA 3

Executes statements in a loop while a condition is true. Because the condition is tested at the end of the loop, the statements inside it are always executed at least one time. It is imperative that the expression that makes up the condition have some aspect of its value potentially altered in the statements. Otherwise, an infinite loop occurs.

Syntax

do {
    statements
} while (condition)

Example

var i = 1;
do {
    window.status = "Loop number " + i++;
} while (i <= 10)
window.status = "";
 
forNN 2 IE 3 ECMA 1

This is a construction that allows repeated execution of statements, usually for a controlled number of times.

Syntax

for ([initExpression]; [condition]; [updateExpression]) {
    statements
}

Example

var userEntry = document.forms[0].entry.value;
var oneChar;
for (var i = 0; i < userEntry.length; i++) {
    oneChar = userEntry.charAt(i);
    if (oneChar < "0" || oneChar > "9") {
        alert("The entry must be numerals only.");
    }
}
 
for/inNN 2 IE 3 ECMA 1

This is a variation of the regular for loop that can extract the property names and values of an object. Only properties (and, in Netscape 6, methods) that are set to be enumerable by the browser internals appear in the output of this construction. Opera 6 supports this construction only for custom script-generated objects.

Syntax

for (varName in objectRef) {
    statements
}

Example

function showProps( ) {
    objName = "image";
    obj = document.images[0];
    var msg = "";
    for (var i in obj) {
        msg += objName + "." + i + "=" + obj[i] + "\n";
    }
    alert(msg);
}
 
ifNN 2 IE 3 ECMA 1

This is a simple conditional statement that provides one alternate execution path.

Syntax

if (condition) {
    statement(s) if true
}

Example

if (myDateObj.getMonth( ) == 1) {
    calcMonthLength( );
}
 
if/elseNN 2 IE 3 ECMA 1

This is a conditional statement that provides two execution paths depending on the result of the condition. You can nest another if or if/else statement inside either path of the if/else statement.

Syntax

if (condition) {
    statement(s) if true
} else {
    statement(s) if false
}

Example

var theMonth = myDateObj.getMonth( );
if (theMonth == 1) {
    monLength = calcLeapMonthLength( );
} else {
    monLength = calcMonthLength(theMonth);
}
 
labelNN 4 IE 4 ECMA 3

You can assign a label identifier to any block of executing statements, including control structures. The purpose of the label is to allow break and continue statements within deeply nested control structures to exit to a nested level that may be at levels beyond the scope of the normal break and continue statements.

Syntax

labelName:

Example

outerLoop:
for (var i = 0; i <= maxValue1; i++) {
    for (var j = 0; j <= maxValue2; j++) {
        if (i == magic1 && j == magic2) {
            break outerLoop;
        }
    }
}
 
returnNN 2 IE 3 ECMA 1

Stops execution of the current function. A return statement can be located anywhere within the function, including inside control structures. You can optionally specify a value to be returned to the calling statement. This return value can be any JavaScript data type. If a return statement that returns a value is in a loop or other control structure, there must be a return statement for each branch of the execution tree, including a default return statement if execution should reach the main execution scope near or at the end of the function.

Syntax

return [value]

Example

function validateNumber(form) {
    var oneChar;
    for (var i = 0; i < userEntry.length; i++) {
        oneChar = form.entry.value.charAt(i);
        if (oneChar < "0" || oneChar > "9") {
            return false;
        }
    }
    return true;
}
 
switch/caseNN 4 IE 4 ECMA 3

Provides a shortcut to execution paths for numerous conditions of an expression. The optional break statement at the end of each case block shortcuts execution of the switch statement, and also prevents the inadvertent execution of the default block, if present.

Syntax

switch (expression) {
    case label1:
        statements
        [break;]
    case label2:
        statements
        [break;]
    ...
    [default:
        statements]
}

Example

var productList = document.forms[0].prodList;
var chosenItem = productList.options[productList.selectedIndex].value;
switch(chosenItem) {
    case "Small Widget":
        document.forms[0].price.value = "44.95";
        break;
    case "Medium Widget":
        document.forms[0].price.value = "54.95";
        break;
    case "Large Widget":
        document.forms[0].price.value = "64.95";
        break;
    default:
        document.forms[0].price.value = "Nothing Selected";
}
 
throwNN 6 IE 5 ECMA 3

Triggers an exception condition, passing a value along with the exception. Although the value you pass can be a simple string, ideally you should pass an instance of the JavaScript Error object filled with sufficient information for a catch statement to act intelligently on the error. A throw statement must be enclosed in the try portion of a try-catch construction.

Syntax

throw value;

Example

function processNumber(inputField) {
    try {
        var inpVal = parseInt(inputField.value, 10);
        if (isNaN(inpVal)) {
            var msg = "Please enter a number only.";
            var err = new Error(msg);
            if (!err.message) {
                err.message = msg;
            }
            throw err;
        }
        // process number
    }
    catch (e) {
        alert(e.message);
        inputField.focus( );
        inputField.select( );
    }
}
 
try/catchNN 6 IE 5 ECMA 3

This construction provides a nondisruptive way to trap for errors (exceptions) and handle them gracefully. Both parts of this exception-handling construction are required. If an error occurs in the try portion, execution immediately branches to the catch portion, where your scripts can display alert dialogs, modify data, or any other task that keeps the JavaScript interpreter from triggering a disruptive error message. Exceptions that occur naturally (i.e., they are not thrown by a throw statement) pass an instance of the Error object as a parameter to the catch section. Statements inside the catch section can examine properties of the error object to determine how to handle exceptions that land there. Thus, one catch portion can handle errors of various types.

You can use try/catch constructions only in browsers that support them. To protect older browsers from seeing this construction, place all affected code inside a <script> tag that explicitly requires JavaScript 1.5 or later (with the language = "JavaScript1.5" attribute.

Syntax

try {
    statement(s) that could cause error
}
catch (errorInfo) {
    process error(s) gracefully
}

Example

function insertOneNode(baseNode, newNode, position) {
    try {
        baseNode.insertBefore(newNode, baseNode.childNodes[position]);
    }
    catch (e) {
        // handle W3C DOM Exception types
        switch (e.name) {
            case "HIERARCHY_REQUEST_ERR" :
                // process bad tree hierarchy reference
                break;
            case "NOT_FOUND_ERR" :
                // process bad refNode reference
                break;
            default :
                // process all other exceptions
        }
    }
    return true;
}
 
whileNN 2 IE 3 ECMA 1

Executes statements in a loop as long as a condition is true. Because the condition is tested at the beginning of the loop, it is conceivable that under the right conditions, the statements inside the loop do not execute. It is imperative that the expression that makes up the condition have some aspect of its value potentially altered in the statements. Otherwise an infinite loop occurs.

Syntax

while (condition) {
    statements
}

Example

var i = 0;
while (!document.forms[0].radioGroup[i].checked) {
    i++;
}
alert("You selected item number " + (i+1) + ".");
 
withNN 2 IE 3 ECMA 1

The with statement adds an object to the scope of every statement nested within. This can shorten the code of some statement groups that rely on a particular object reference. Note that with constructions are generally very inefficient. You can achieve better performance by assigning the object reference to a local variable, and using that variable in your function.

Syntax

with (objectRef) {
    statements
}

Example

with (document.forms[0]) {
    name1 = firstName.value;
    name2 = lastName.value;
    mail = eMail.value;
}


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