comp.lang.javascript FAQ - Quick Answers 2008-10-13

comp.lang.javascript FAQ - Quick Answers- 10, Updated October 11, 2008

1 Meta-FAQ meta-questions

To cope with a desire for additional explanation and detail
relating to some aspects of the FAQ, an additional resource
is available at:-

<URL: http://www.jibbering.com/faq/faq_notes/faq_notes.html >

It has been provided separately to avoid increasing the size of
the FAQ to a point where it would be unreasonable to post it to
the group.

2 Javascript Tips
2.1 _Which newsgroups deal with javascript_?
2.2 _What questions are on-topic for CLJ_?
2.3 _What should I do before posting to CLJ_?
2.4 _Why was my post not answered_?
2.5 _What is ECMAScript_?
2.6 _What is JScript_?
2.7 _What are object models_?
2.8 _What is the document object model_?
2.9 _Internationalisation and Multinationalisation in javascript.
2.10 _I have a question that is not answered in here or
in any of the resources mentioned here but I'm sure it has been
answered in CLJ. Where are the CLJ archives located_?
2.11 _What does the future hold for ECMAScript_?

3 Javascript Resources
3.1 _What books cover javascript_?
3.2 _What online resources are available_?

4 Quick Answers: Numbers
4.1 _How do I convert a Number into a String with exactly 2 decimal places_?

Most implementations fail with certain numbers, for example 0.07.
The following works successfully for M>0, N>0:

function Stretch(Q, L, c) { var S = Q
if (c.length > 0) {
while (S.length < L) {
S = c+S;
}
}
return S;
}

function StrU(X, M, N) { // X>=0.0
var T, S = String(Math.round(X * Number("1e" + N)));
if (S.search && S.search(/\D/) != -1) {
return '' + X;
}
with (String(Stretch(S, M+N, '0')))
return substring(0, T = (length-N)) + '.' + substring(T);
}

function Sign(X) {
return X > 0 ? "+" : X < 0 ? "-" : " ";
}

function StrS(X, M, N) {
return Sign(X) + StrU(Math.abs(X), M, N);
}

Number.prototype.toFixed = function(n){
return StrS(this, 1, n);
};



<URL: http://www.merlyn.demon.co.uk/js-round.htm >
<URL: http://msdn2.microsoft.com/en-us/library/sstyff0z.aspx >


4.2 _Why does simple decimal arithmetic give strange results_?

Otherwise, use Math.round on the results of expressions which
should be of integer value.

4.3 _Why does K = parseInt('09') set K to 0_?

<URL: http://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Functions/parseInt >
<URL: http://msdn2.microsoft.com/en-us/library/x53yedee.aspx >
<URL: http://docs.sun.com/source/816-6408-10/toplev.htm#1064173 >
<URL: http://www.jibbering.com/faq/faq_notes/faq_notes.html#FAQN4_12 >


4.4 _Why does 1+1 equal 11_? or How do I convert a string to a number?

Variables are not typed; their values are. The conversion between a
string and a number happens automatically. Since plus (+) is
also used as in string concatenation, '1' + 1 is equal to '11'.
The string determines what + does. To overcome this, first convert the
string to a number. For example: +varname or Number(varname) or
parseInt(varname, 10) or parseFloat(varname).
Form control values are strings, as is the result from a prompt
dialog. Convert these to numbers before performing addition by using
the unary + operator: +'1' + 1 result is 2.

Additional Notes:
<URL: http://www.jibbering.com/faq/faq_notes/type_convert.html >
<URL: http://msdn2.microsoft.com/en-us/library/67defydd.aspx >


4.5 _How do I generate a random integer from 1 to N_?
Math.random() returns a value R such that 0 <= R < 1.0; therefore:


// positive integer expected
function Random(j) {
return Math.floor(j * Math.random());
}



- gives an evenly distributed random integer in the range from
0 to x-1 inclusive; use Random(N)+1 for 1 to N.

<URL: http://msdn2.microsoft.com/en-us/library/41336409.aspx >
<URL: http://docs.sun.com/source/816-6408-10/math.htm >

How to Deal and Shuffle, see in:
<URL: http://www.merlyn.demon.co.uk/js-randm.htm >



5 Quick Answers
5.1 _How do I protect my javascript code_?

With clientside javascript you can't as your code is distributed
in source form and is easily readable. With JScript, there is the
Script Encoder (see MSDN), but this is nothing more than obfuscation.
Disabling the right mouse button also does nothing to protect
your script in a Web browser.

Your code is likely protected under copyright laws. See:

<URL: http://www.wipo.int/about-ip/en/copyright.html >
<URL: http://webdesign.about.com/od/copyright/Copyright_Issues_on_the_Web_Intellectual_Property.htm >


5.2 _How can I disable the back button in a web browser_?

You can't. The browser's history cannot be modified. However, you
can use location.replace(url); in some browsers to replace
the current page in the history.

<URL: http://msdn2.microsoft.com/en-us/library/ms536712.aspx >
<URL: http://docs.sun.com/source/816-6408-10/location.htm#1194240 >


5.3 _How can I access the client-side filesystem_?

Security means that by default you can't. In a more restricted
environment, there are options. For example, using LiveConnect
to connect to Java with Netscape, and using the FileSystemObject
in IE. Check Google Groups archives <URL: http://groups.google.com/group/comp.lang.javascript/topics >
for previous posts on the subject.

<URL: http://msdn2.microsoft.com/en-us/library/z9ty6h50.aspx >
<URL: http://www.javaworld.com/javaworld/jw-10-1998/jw-10-apptowin32.html >


5.4 _How can I see in javascript if a web browser accepts cookies_?

Write a cookie and read it back and check if it's the same.

<URL: http://www.w3schools.com/js/js_cookies.asp >

Additional Notes:

<URL: http://www.jibbering.com/faq/faq_notes/cookies.html >
<URL: http://www.ietf.org/rfc/rfc2965.txt >
<URL: http://www.galasoft-lb.ch/myjavascript/consulting/2001012701/ >
<URL: http://www.cookiecentral.com/ >


5.5 _How can I prevent access to a web page by using javascript_?

In practice you can't. While you could create a suitable
encryption system with a password in the page, the level of
support you need to do this means it's always simpler to do it
server-side. Anything that "protects" a page
other than the current one is definitely flawed.

5.6 _How do I communicate between frames in a web browser_?

Note that it is not possible to communicate between frames on
different domains this way.

5.7 _How do I find the size of the window_?

Note: The dimensions cannot be determined accurately until after
the document has finished loading.

<URL: http://msdn2.microsoft.com/en-us/library/ms533566.aspx >
<URL: http://developer.mozilla.org/en/DOM/window.innerWidth >
<URL: http://dev.opera.com/articles/view/using-capability-detection/ >


5.8 _How do I check to see if a child window is open, before opening another_?


var myWin;
function openWin(aURL) {
if (!myWin || myWin.closed ) {
myWin = window.open(aURL,'myWin');
} else {
myWin.location = aURL;
myWin.focus();
}
}


<URL: http://msdn2.microsoft.com/en-us/library/ms533574.aspx >
<URL: http://docs.sun.com/source/816-6408-10/window.htm#1201877 >


5.9 _Why does framename.print() not print the correct frame in IE_?

IE prints the frame that has focus when you call the print
method frameref.focus();frameref.print();
<URL: http://msdn2.microsoft.com/en-us/library/ms976105.aspx >


5.10 _How do I get the value of a form control_?

Third Exception: File inputs. Most current browsers do not allow
reading of type="file" input elements in a way that is useful.

5.11 _How do I close a window and why does it not work on the first one_?

Use windowRef.close(), where windowRef is a window object
reference, such as window, top, parent, self, or a reference
obtained from the window.open() method. You can only close
windows opened by scripts, no others.

<URL: http://msdn2.microsoft.com/en-us/library/ms536367.aspx >
<URL: http://docs.sun.com/source/816-6408-10/window.htm#1201822 >
<URL: http://developer.mozilla.org/en/docs/DOM:window.open#FAQ >


5.12 _How do I modify the current page in a browser_?

Note: Make sure the element exists on the page (and has been parsed) before trying to
reference it.

<URL: http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-FF21A306 >
<URL: http://msdn.microsoft.com/en-us/library/cc304097(VS.85).aspx >
<URL: http://msdn2.microsoft.com/en-us/library/ms533897.aspx >
<URL: http://developer.mozilla.org/en/Whitespace_in_the_DOM >
<URL: http://developer.mozilla.org/en/docs/DOM:element.innerHTML >
<URL: http://www.jibbering.com/faq/faq_notes/alt_dynwrite.html >


5.13 _How do I trim whitespace_?

Using Regular Expressions (JavaScript 1.2/JScript 3+) :


function trimString(s) {
return s.replace(/^\s+|\s+$/g,'');
}



<URL: http://docs.sun.com/source/816-6408-10/regexp.htm >
<URL: http://msdn2.microsoft.com/en-us/library/6wzad2b2.aspx >
<URL: http://en.wikipedia.org/wiki/Regular_expression >
<URL: http://www.merlyn.demon.co.uk/js-valid.htm >


5.14 _How do I force a reload from the server/prevent caching_?

To reload a page, use location.reload(). However, this depends
upon the cache headers that your server sends. To change this,
you need to alter the server configuration. A quick fix on the
client is to change the page URI so that it contains a unique
element, such as the current time. For example:
location.replace(location.href+'?d='+new Date().valueOf())
If the location.href already contains a query String, use:
location.replace(location.href+'&d='+new Date().valueOf())
<URL: http://www.mnot.net/cache_docs/ >
<URL: http://docs.sun.com/source/816-6408-10/date.htm >


5.15 _How do I get a perl/asp/php variable into client-side javascript_?

Use the server-side language to generate the javascript:


var jsvar = "<%= aspvar %>";
var jsvar = "<?php echo $phpvar ?>";


5.16 _Why do I get permission denied when accessing a frame/window_?

In the normal browser security model, it is impossible for a
script from one domain to access the properties of pages served
from another domain, or a different protocol. Any attempt to
access a property in such cases will result in a "Permission
Denied" error. Signed scripts or trusted ActiveX objects can
overcome this in limited situations.

<URL: http://msdn2.microsoft.com/en-us/library/ms533028.aspx >


5.17 _How do I make a 10 second delay_?

Other (less event driven) hosts have different wait functions,
such as WScript.Sleep() in the Windows Script Host.

<URL: http://msdn2.microsoft.com/en-us/library/ms536753.aspx >
<URL: http://docs.sun.com/source/816-6408-10/window.htm#1203758 >
<URL: http://en.wikipedia.org/wiki/Event-driven_programming >


5.18 _How do I change print settings with window.print()_?

In a normal security environment, you can't change anything. The
page style*** rules provide some options, but are not supported
in browsers yet. If you can, use an ActiveX or Plugin ScriptX and
Neptune from Meadroid to give you more control for Windows
versions of Internet Explorer, Netscape, and Opera.

<URL: http://www.meadroid.com/scriptx/ >
<URL: http://msdn2.microsoft.com/en-us/library/ms976105.aspx >


5.19 _I have <a href="javascript:somefunction()"> what ... _?

Instead, use
<a href="something.html" onclick="somefunction();return false">
where something.html is a meaningful alternative. Alternatively,
attach the click callback using an event registry.

<URL: http://www.useit.com/alertbox/20021223.html >


5.20 _My element is named myselect[], how do I access it_?

Form controls with any "illegal" characters can be accessed with
formref.elements["myselect[]"] - The bracket characters,
amongst others, are illegal in ID attributes and javascript
identifiers, so you should try to avoid them as browsers may
handle them incorrectly.

<URL: http://msdn2.microsoft.com/en-us/library/ms537449.aspx >
<URL: http://docs.sun.com/source/816-6408-10/form.htm >
<URL: http://www.jibbering.com/faq/faq_notes/faq_notes.html#FAQN4_25 >


5.21 _How do I detect Opera/Netscape/IE_?

Object/feature detection means checking that the object you wish
to use is supported by the browser before using it. This means
that you don't need to know what browsers support what methods,
and your code will automatically be usable on any browser that
can execute it.


if (document.getElementById &&
document.getElementById('el') &&
document.getElementById('el').style ) {
// We know that this browser supports getElementByID and has
// a style object, so we can set a style property.
document.getElementById('el').style.color = "red";
}


Browser bugs can often be detected and overcome in similar ways.

<URL: http://developer.mozilla.org/en/docs/Using_Web_Standards_in_your_Web_Pages:Developing_Cross_Browser/Cross_Platform_Pages >
<URL: http://www.jibbering.com/faq/faq_notes/not_browser_detect.html >
<URL: http://dev.opera.com/articles/view/using-capability-detection/ >
<URL: http://developer.apple.com/internet/webcontent/objectdetection.html >


5.22 _How do I disable the right mouse button_?

The oncontextmenu is a proprietary method and is not supported on
all browsers.
<body oncontextmenu="return false">
5.23 _How do I change the confirm box to say yes/no or default to cancel_?

The buttons on a confirm box cannot be changed, nor can you
specify a default button. However, you should be able to change
the question so that "OK" is suitable as the default.
e.g. change "We will now buy ourselves a Porsche with your credit
card, Do you want to continue with this transaction _OK_ Cancel"
to "We will now buy ourselves a Porsche with your credit card,
Would you like to abandon this transaction? _OK_ Cancel"

5.24 _How do I log-out a user when they leave my site_?

This cannot be done reliably. Here's why:


The user may disable javascript so the log-out script will
never execute.

The user may not be on-line when they close your web page.

Javascript errors elsewhere in the page may prevent the script
executing.

The browser may not support the onunload event, or may not fire
it under certain circumstances, so the log-out function will
not execute.


The URL below has more information.

<URL: http://groups.google.com/groups?selm=BlmZ7.55691%244x4.7344316%40news2-win.server.ntlworld.com >


5.25 _How do I format the lastModified date with javascript_?

Apparently, new Date() reads document.lastModified
correctly, though problems
can occur if the browser returns only two digits for the year.
In particular, time offset, field order and separators may vary.
It is also reliant on the server's clock having been correctly
set at the time of upload. See the URL
below.

<URL: http://www.merlyn.demon.co.uk/js-date2.htm#lM >


5.26 _Why are my rollovers so slow_?

Images are cached by the browser depending on the headers sent by
the server. If the server does not send sufficient information
for the browser to decide the image is cacheable, the browser
will check if the image has been updated every time you change the
src of an image (in some user settings). To overcome this you
must send suitable headers.

<URL: http://www.mnot.net/cache_docs/ >


5.27 _How do I change the text in the url/location bar_?

This text can only be altered by changing the URL of the page.

5.28 _How do I prompt a "Save As" dialog for an accepted mime type_?

Some browsers accept the Content-Disposition header, but this
must be added by the server. Taking the form:-
Content-Disposition: attachment; filename=filename.ext
<URL: http://classicasp.aspfaq.com/general/how-do-i-prompt-a-save-as-dialog-for-an-accepted-mime-type.html >
<URL: http://support.microsoft.com/kb/q260519/ >


5.29 _How do I run a server side script_?

Mozilla, Opera 7.6+, Safari 1.2+, and Windows IE 7
provide the XMLHttpRequest object
(Windows IE versions 5+, provides ActiveX to acheive an analagous
effect). XMLHttpRequest can send HTTP requests to
the server, and provides access the responseText or responseXML
(when the response is XML), and HTTP header information.

<URL: http://jibbering.com/2002/4/httprequest.html >
<URL: http://www.w3.org/TR/XMLHttpRequest/ >
<URL: http://developer.mozilla.org/en/XMLHttpRequest >
<URL: http://msdn.microsoft.com/en-us/library/ms537505(VS.85).aspx >


5.30 _I have window.status="Moomin"; why doesn't the statusbar change_?

When changing the status in an event (e.g. onmouseover) you
should return true from the event. Also a number of browsers
require a short delay before setting the status to overcome their
default behaviour with the statusbar.
onevent="setTimeout('window.status=\'Moomin\'',15);"
Most browsers are configured, by default, to disallow scripts from setting
the status bar text.

5.31 _How do I modify the current browser window_?

In a default security environment you are very limited in how much
you can modify the current browser window. You can use
window.resizeTo or window.moveTo to resize or move a
window respectively, but that is it. Normally you can only
suggest chrome changes in a window.open
<URL: http://msdn2.microsoft.com/en-us/library/ms536651.aspx >
<URL: http://developer.mozilla.org/en/docs/DOM:window.open >


5.32 _How do I POST a form to a new window_?

Use the target attribute on the form, opening a window with
that name and your feature string in the onsubmit handler of the
FORM.


<form action="..." target="wndname" onsubmit="window.open('',this.target,'features');return true;">


<URL: http://www.htmlhelp.com/reference/html40/forms/form.html >


5.33 _How do I download a page to a variable_?

Within a web-page use the XMLHttpRequest object, see:

<URL: http://jibbering.com/2002/4/httprequest.html >
<URL: http://www.ajaxtoolbox.com/ >


5.34 _How do I access a property of an object using a string_?

There are two ways to access properties: the dot notation and
the square bracket notation. What you are looking for is the square
bracket notation in which the dot, and the identifier to its right,
are replaced with a set of square brackets containing a string. The
value of the string matches the identifier. For example:-


//dot notation
var bodyElement = document.body;

//square bracket notation, using an expression
var bodyElement = document["bo"+"dy"];


<URL: http://www.jibbering.com/faq/faq_notes/square_brackets.html >


5.35 _When should I use eval_?

The eval() function should _only_ be used when it is necessary to
evaluate a string supplied or composed at run-time; the string
can be anything from a simple (but unpredictable) expression such
as "12 * 2.54" to a substantial piece of javascript code.

5.36 _Why doesn't the global variable "divId" always refer to the element with id="divId"_?

Microsoft introduced a shortcut that can be used to reference
elements which include an id attribute where the
id becomes a globally-accessible property. Some browsers reproduce
this behavior. Some, most notably Gecko-based browsers (Netscape and Mozilla),
do so only in "quirks" mode. The best approach is the document.getElementById
method, which is part of the W3C DOM standard and implemented
in modern browsers (including IE from version 5.0). So an
element with id="foo" can be referenced
with:-


var el = document.getElementById("foo");



Note: make sure not to use the same id twice in the same document
and do not give an element a name that matches an id
of another in the same document or it will trigger bugs in MSIE <= 7 with
document.getElementsByName and document.getElementById.

<URL: http://developer.mozilla.org/en/docs/Using_Web_Standards_in_your_Web_Pages:Using_the_W3C_DOM#Accessing_Elements_with_the_W3C_DOM >
<URL: http://www.jibbering.com/faq/faq_notes/faq_notes.html#FAQN4_41 >


5.37 _How do I open a new window with javascript_?

New windows can be opened on browsers that support the
window.open function and are not subject to the action of any
pop-up blocking mechanism with code such as:-


var wRef;
if(window.open){
wRef = window.open("http://example.com/page.html","windowName";);
}


<URL: http://developer.mozilla.org/en/docs/DOM:window.open >
<URL: http://www.infimum.dk/HTML/JSwindows.html >


5.38 _How do I get my browser to report javascript errors_?

* Windows IE

Wait until a little yellow
triangle appears at the left end of the status bar, double click
on it and, when the error dialog box appears, check the "Always
show errors" checkbox it contains.

Or, Internet Options, Advanced, deselect "Disable Script Debugging",
select "Display a notification ...".


* Firefox
<URL: http://www.getfirebug.com/ >.


* Opera

Tools > Advanced > Error console


<URL: http://dev.opera.com/articles/view/introduction-to-opera-dragonfly/ >

* Safari
<URL: http://developer.apple.com/internet/safari/faq.html#anchor14 >

* Chrome

JavaScript Console: click the Page menu icon and select
Developer > JavaScript Console. From here, you'll be
able to view errors in the JavaScript execution, and enter
additional javascript commands to execute.



JavaScript Debugger: available as Page menu icon > Developer
Debug JavaScript, the debugger provides a command prompt from which you
can set breakpoints, backtrace, and more. Type help at the debugger
command line to get started.


<URL: http://www.google.com/chrome/intl/en/webmasters-faq.html#jsexec >

* Mac IE

Use the Preferences dialog.



5.39 _What is Ajax_?
Ajax
is shorthand for Asynchronous JavaScript And XML. The technology is
based on the XMLHttpRequest Object.
At its simplest, it is the sending/retrieving of new data from the server
without changing URL's or reloading the current page.

Mozilla Documentation:

<URL: http://developer.mozilla.org/en/docs/XMLHttpRequest >

MSDN Documention:

<URL: http://msdn2.microsoft.com/en-us/library/ms535874.aspx >
<URL: http://msdn2.microsoft.com/en-us/library/ms759148.aspx >

Libraries and Tutorial Sites:

<URL: http://jibbering.com/2002/4/httprequest.html >
<URL: http://www.ajaxtoolbox.com/ >


An alternative to the XMLHttpRequest Object is Remote Scripting:

<URL: http://www.ashleyit.com/rs/main.htm >


5.40 _Why is my Ajax page not updated properly when using an HTTP GET request in Internet Explorer_?

Microsoft Internet Explorer caches the results of HTTP GET requests. To ensure that the
document is retrieved from the server, you will need to use the POST Method.

<URL: http://msdn2.microsoft.com/en-us/library/ms536648.aspx >



6 Comments and Suggestions
6.1 _Why do some posts have <FAQENTRY> in them_?
6.2 _How do I make a suggestion_?




1_Meta-FAQ meta-questions_

see <URL: http://jibbering.com/faq/#FAQ1 >
Or Wednesdays FAQ posting.

2_Javascript Tips_

see <URL: http://jibbering.com/faq/#FAQ2 >
Or Wednesdays FAQ posting.

3_Javascript Resources_

see <URL: http://jibbering.com/faq/#FAQ3 >
Or Wednesdays FAQ posting.

4_Quick Answers: Numbers_
4.1 *How do I convert a Number into a String with exactly 2 decimal places?*
4.2 *Why does simple decimal arithmetic give strange results?*
4.3 *Why does K = parseInt('09') set K to 0?*
4.4 *Why does 1+1 equal 11? or How do I convert a string to a number?*
4.5 *How do I generate a random integer from 1 to N?*


5_Quick Answers_
5.1 *How do I protect my javascript code?*
5.2 *How can I disable the back button in a web browser?*
5.3 *How can I access the client-side filesystem?*
5.4 *How can I see in javascript if a web browser accepts cookies?*
5.5 *How can I prevent access to a web page by using javascript?*
5.6 *How do I communicate between frames in a web browser?*
5.7 *How do I find the size of the window?*
5.8 *How do I check to see if a child window is open, before opening another?*
5.9 *Why does framename.print() not print the correct frame in IE?*
5.10 *How do I get the value of a form control?*
5.11 *How do I close a window and why does it not work on the first one?*
5.12 *How do I modify the current page in a browser?*
5.13 *How do I trim whitespace?*
5.14 *How do I force a reload from the server/prevent caching?*
5.15 *How do I get a perl/asp/php variable into client-side javascript?*
5.16 *Why do I get permission denied when accessing a frame/window?*
5.17 *How do I make a 10 second delay?*
5.18 *How do I change print settings with window.print()?*
5.19 *I have <a href="javascript:somefunction()"> what ... ?*
5.20 *My element is named myselect[], how do I access it?*
5.21 *How do I detect Opera/Netscape/IE?*
5.22 *How do I disable the right mouse button?*
5.23 *How do I change the confirm box to say yes/no or default to cancel?*
5.24 *How do I log-out a user when they leave my site?*
5.25 *How do I format the lastModified date with javascript?*
5.26 *Why are my rollovers so slow?*
5.27 *How do I change the text in the url/location bar?*
5.28 *How do I prompt a "Save As" dialog for an accepted mime type?*
5.29 *How do I run a server side script?*
5.30 *I have window.status="Moomin"; why doesn't the statusbar change?*
5.31 *How do I modify the current browser window?*
5.32 *How do I POST a form to a new window?*
5.33 *How do I download a page to a variable?*
5.34 *How do I access a property of an object using a string?*
5.35 *When should I use eval?*
5.36 *Why doesn't the global variable "divId" always refer to the element with id="divId"?*
5.37 *How do I open a new window with javascript?*
5.38 *How do I get my browser to report javascript errors?*
5.39 *What is Ajax?*
5.40 *Why is my Ajax page not updated properly when using an HTTP GET request in Internet Explorer?*


6_Comments and Suggestions_

see <URL: http://jibbering.com/faq/#FAQ6 >
Or Wednesdays FAQ posting.

.

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