內(nèi)容摘要：1.WhatisJavaScript?JavaScriptisaplatform-independent,event-driven,interpretedclient-sidescriptingand

內(nèi)容正文：1.WhatisJavaScript?JavaScriptisaplatform-independent,event-driven,interpretedclient-sidescriptingandprogramminglanguagedevelopedbyNetscapeCommunicationsCorp.andSunMicrosystems.2.Howtoreadandwriteafileusingjavascript?I/Ooperationslikereadingorwritingafileisnotpossiblewithclient-sidejavascript.

However,thiscanbedonebycodingaJavaappletthatreadsfilesforthescript.3.Howtodetecttheoperatingsystemontheclientmachine?Inordertodetecttheoperatingsystemontheclientmachine,thenavigator.appVersionstring(property)shouldbeused.

4.Wherearecookiesactuallystoredontheharddisk?Thisdependsontheuser'sbrowserandOS.

InthecaseofNetscapewithWindowsOS,allthecookiesarestoredinasinglefilecalledcookies.txtc:\ProgramFiles\Netscape\Users\username\cookies.txtInthecaseofIE,eachcookieisstoredinaseparatefilenamely.c:\Windows\Cookies\5.Whatcanjavascriptprogramsdo?GenerationofHTMLpageson-the-flywithoutaccessingtheWebserver.

TheusercanbegivencontroloverthebrowserlikeUserinputvalidation

Simplecomputationscanbeperformedontheclient'smachine

Theuser'sbrowser,OS,screensize,etc.canbedetected

DateandTimeHandling6.HowtosetaHTMLdocument'sbackgroundcolor?document.bgcolorpropertycanbesettoanyappropriatecolor.7.Whatdoesthe"AccessisDenied"IEerrormean?

The"AccessDenied"errorinanybrowserisduetothefollowingreason.

Ajavascriptinonewindoworframeistriestoaccessanotherwindoworframewhosedocument'sdomainisdifferentfromthedocumentcontainingthescript.8.IsajavascriptscriptfasterthananASPscript?

Yes.Sincejavascriptisaclient-sidescriptitdoesrequirethewebserver'shelpforitscomputation,soitisalwaysfasterthananyserver-sidescriptlikeASP,PHP,etc..9.AreJavaandJavaScripttheSame?

No.javaandjavascriptaretwodifferentlanguages.

Javaisapowerfulobject-orientedprogramminglanguagelikeC++,CwhereasJavascriptisaclient-sidescriptinglanguagewithsomelimitations.10.Howtoembedjavascriptinawebpage?

javascriptcodecanbeembeddedinawebpagebetween<scriptlangugage="javascript"></script>tags11.Howtoaccessanexternaljavascriptfilethatisstoredexternallyandnotembedded?

Thiscanbeachievedbyusingthefollowingtagbetweenheadtagsorbetweenbodytags.

<scriptsrc="abc.js"></script>

whereabc.jsistheexternaljavscriptfiletobeaccessed.12.Whatisthedifferencebetweenanalertboxandaconfirmationbox?

AnalertboxdisplaysonlyonebuttonwhichistheOKbuttonwhereastheConfirmboxdisplaystwobuttonsnamelyOKandcancel.13.Whatisapromptbox?

Apromptboxallowstheusertoenterinputbyprovidingatextbox.14.Howtohidejavascriptcodefromoldbrowsersthatdontrunit?

Ans.

Usethebelowspecifiedstyleofcomments

<scriptlanguage=javascript><!--

javascriptcodegoeshere//-->orUsethe<NOSCRIPT>somehtmlcode</NOSCRIPT>tagsandcodethedisplayhtmlstatementsbetweentheseandthiswillappearonthepageifthebrowserdoesnotsupportjavascript15.Howtocommentjavascriptcode?

Ans.

Use//forlinecommentsand

/**/forblockcomments16.Namethenumericconstantsrepresentingmax,minvalues

Ans.Number.MAX_VALUE

Number.MIN_VALUE17.Whatdoesjavascriptnullmean?Ans.Thenullvalueisauniquevaluerepresentingnovalueornoobject.

Itimpliesnoobject,ornullstring,novalidbooleanvalue,nonumberandnoarrayobject.18.Whatdoesundefinedvaluemeaninjavascript?Ans.Undefinedvaluemeansthevariableusedinthecodedoesntexistorisnotassignedanyvalueorthepropertydoesntexist.

19.Whatisthedifferencebetweenundefinedvalueandnullvalue?Ans.(i)Undefinedvaluecannotbeexplicitlystatedthatisthereisnokeywordcalledundefinedwhereasnullvaluehaskeywordcallednull

(ii)typeofundefinedvariableorpropertyreturnsundefinedwhereas

typeofnullvaluereturnsobject20.Whatisvariabletypinginjavascript?Ans.Itisperfectlylegaltoassignanumbertoavariableandthenassignastringtothesamevariableasfollowsexamplei=10;

i="string";Thisiscalledvariabletyping21.Doesjavascripthavetheconceptlevelscope?Ans.No.Javascriptdoesnothaveblocklevelscope,allthevariablesdeclaredinsideafunctionpossessthesamelevelofscopeunlikec,c++,java.22.Whatareundefinedandundeclaredvariables?Ans.Undeclaredvariablesarethosethatarenotdeclaredintheprogram(donotexistatall),tryingtoreadtheirvaluesgivesruntimeerror.Butifundeclaredvariablesareassignedthenimplicitdeclarationisdone.Undefinedvariablesarethosethatarenotassignedanyvaluebutaredeclaredintheprogram.Tryingtoreadsuchvariablesgivesspecialvaluecalledundefinedvalue.23.Whatis===operator?Ans.====isstrictequalityoperator,itreturnstrueonlywhenthetwooperandsarehavingthesamevaluewithoutanytypeconversion.

24.Whatdoesthedeleteoperatordo?Ans.Thedeleteoperatorisusedtodeleteallthevariablesandobjectsusedintheprogram,butitdoesnotdeletevariablesdeclaredwithvarkeyword.25.Whatdoesbreakandcontinuestatementsdo?Ans.Continuestatementcontinuesthecurrentloop(iflabelnotspecified)inanew