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内容摘要:1.WhatisJavaScript?JavaScriptisaplatform-independent,event-driven,interpretedclient-sidescriptingand
内容正文: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
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