外文翻译-data access数据访问

长江大学工程技术学院毕业设计（论文）外文翻译外文题目DATAACCESS数据访问译文题目数据访问系部信息系专业班级计算机科学与技术61001班学生姓名指导教师/讲师辅导教师完成日期2014年4月18日DATAACCESS一THEREAREFOURBASICPLACESFROMWHICHDATAONEOFTHECLASSICCOMPUTERSCIENCEPROBLEMSISDETERMININGWHEREDATASHOULDBESTOREDFOROPTIMALREADINGANDWRITINGWHEREDATAISSTOREDISRELATEDTOHOWQUICKLYITCANBERETRIEVEDDURINGCODEEXECUTIONTHISPROBLEMINJAVASCRIPTISSOMEWHATSIMPLIFIEDBECAUSEOFTHESMALLNUMBEROFOPTIONSFORDATASTORAGESIMILARTOOTHERLANGUAGES,THOUGH,WHEREDATAISSTOREDCANGREATLYAFFECTHOWQUICKLYITCANBEACCESSEDLATERTHEREAREFOURBASICPLACESFROMWHICHDATACANBEACCESSEDINJAVASCRIPT11LITERALVALUESANYVALUETHATREPRESENTSJUSTITSELFANDISNTSTOREDINAPARTICULARLOCATIONJAVASCRIPTCANREPRESENTSTRINGS,NUMBERS,BOOLEANS,OBJECTS,ARRAYS,FUNCTIONS,REGULAREXPRESSIONS,ANDTHESPECIALVALUESNULLANDUNDEFINEDASLITERALS12VARIABLESANYDEVELOPERDEFINEDLOCATIONFORSTORINGDATACREATEDBYUSINGTHEVARKEYWORD13ARRAYITEMSANUMERICALLYINDEXEDLOCATIONWITHINAJAVASCRIPTARRAYOBJECT14OBJECTMEMBERSASTRINGINDEXEDLOCATIONWITHINAJAVASCRIPTOBJECT二ACCESSINGINFORMATIONFROMARRAYITEMSANDOBJECTMEMBERSISMOREEXPENSIVEEACHOFTHESEDATASTORAGELOCATIONSHASAPARTICULARCOSTASSOCIATEDWITHREADINGANDWRITINGOPERATIONSINVOLVINGTHEDATAINMOSTCASES,THEPERFORMANCEDIFFERENCEBETWEENACCESSINGINFORMATIONFROMALITERALVALUEVERSUSALOCALVARIABLEISTRIVIALACCESSINGINFORMATIONFROMARRAYITEMSANDOBJECTMEMBERSISMOREEXPENSIVE,THOUGHEXACTLYWHICHISMOREEXPENSIVEDEPENDSHEAVILYONTHEBROWSEROLDERBROWSERSUSINGMORETRADITIONALJAVASCRIPTENGINES,SUCHASFIREFOX3,INTERNETEXPLORER,ANDSAFARI32,SHOWAMUCHLARGERAMOUNTOFTIMETAKENTOACCESSVALUESVERSUSBROWSERSTHATUSEOPTIMIZINGJAVASCRIPTENGINESTHEGENERALTRENDS,HOWEVER,REMAINTHESAMEACROSSALLBROWSERSLITERALVALUEANDLOCALVARIABLEACCESSTENDTOBEFASTERTHANARRAYITEMANDOBJECTMEMBERACCESSTHEONEEXCEPTION,FIREFOX3,OPTIMIZEDARRAYITEMACCESSTOBEMUCHFASTEREVENSO,THEGENERALADVICEISTOUSELITERALVALUESANDLOCALVARIABLESWHENEVERPOSSIBLEANDLIMITUSEOFARRAYITEMSANDOBJECTMEMBERSWHERESPEEDOFEXECUTIONISACONCERNTOTHATEND,THEREARESEVERALPATTERNSTOLOOKFOR,AVOID,ANDOPTIMIZEINYOURCODE三MANAGINGSCOPETHECONCEPTOFSCOPEISKEYTOUNDERSTANDINGJAVASCRIPTNOTJUSTFROMAPERFORMANCEPERSPECTIVE,BUTALSOFROMAFUNCTIONALPERSPECTIVESCOPEHASMANYEFFECTSINJAVASCRIPT,FROMDETERMININGWHATVARIABLESAFUNCTIONCANACCESSTOASSIGNINGTHEVALUEOFTHISTHEREAREALSOPERFORMANCECONSIDERATIONSWHENDEALINGWITHJAVASCRIPTSCOPES,BUTTOUNDERSTANDHOWSPEEDRELATESTOSCOPE,ITSNECESSARYTOUNDERSTANDEXACTLYHOWSCOPEWORKS31SCOPECHAINSANDIDENTIFIERRESOLUTIONEVERYFUNCTIONINJAVASCRIPTISREPRESENTEDASANOBJECTMORESPECIFICALLY,ASANINSTANCEOFFUNCTIONFUNCTIONOBJECTSHAVEPROPERTIESJUSTLIKEANYOTHEROBJECT,ANDTHESEINCLUDEBOTHTHEPROPERTIESTHATYOUCANACCESSPROGRAMMATICALLYANDASERIESOFINTERNALPROPERTIESTHATAREUSEDBYTHEJAVASCRIPTENGINEBUTARENOTACCESSIBLETHROUGHCODEONEOFTHESEPROPERTIESISSCOPE,ASDEFINEDBYECMA262,THIRDEDITIONTHEINTERNALSCOPEPROPERTYCONTAINSACOLLECTIONOFOBJECTSREPRESENTINGTHESCOPEINWHICHTHEFUNCTIONWASCREATEDTHISCOLLECTIONISCALLEDTHEFUNCTIONSSCOPECHAINANDITDETERMINESTHEDATATHATAFUNCTIONCANACCESSEACHOBJECTINTHEFUNCTIONSSCOPECHAINISCALLEDAVARIABLEOBJECT,ANDEACHOFTHESECONTAINSENTRIESFORVARIABLESINTHEFORMOFKEYVALUEPAIRSWHENAFUNCTIONISCREATED,ITSSCOPECHAINISPOPULATEDWITHOBJECTSREPRESENTINGTHEDATATHATISACCESSIBLEINTHESCOPEINWHICHTHEFUNCTIONWASCREATEDFOREXAMPLE,CONSIDERTHEFOLLOWINGGLOBALFUNCTION。WHENTHEADDFUNCTIONISCREATED,ITSSCOPECHAINISPOPULATEDWITHASINGLEVARIABLEOBJECTTHEGLOBALOBJECTREPRESENTINGALLOFTHEVARIABLESTHATAREGLOBALLYDEFINEDTHISGLOBALOBJECTCONTAINSENTRIESFORWINDOW,NAVIGATOR,ANDDOCUMENT,TONAMEAFEWTHEADDFUNCTIONSSCOPECHAINISLATERUSEDWHENTHEFUNCTIONISEXECUTEDSUPPOSETHATTHEFOLLOWINGCODEISEXECUTEDEXECUTINGTHEADDFUNCTIONTRIGGERSTHECREATIONOFANINTERNALOBJECTCALLEDANEXECUTIONCONTEXTANEXECUTIONCONTEXTDEFINESTHEENVIRONMENTINWHICHAFUNCTIONISBEINGEXECUTEDEACHEXECUTIONCONTEXTISUNIQUETOONEPARTICULAREXECUTIONOFTHEFUNCTION,ANDSOMULTIPLECALLSTOTHESAMEFUNCTIONRESULTINMULTIPLEEXECUTIONCONTEXTSBEINGCREATEDTHEEXECUTIONCONTEXTISDESTROYEDONCETHEFUNCTIONHASBEENCOMPLETELYEXECUTEDANEXECUTIONCONTEXTHASITSOWNSCOPECHAINTHATISUSEDFORIDENTIFIERRESOLUTIONWHENTHEEXECUTIONCONTEXTISCREATED,ITSSCOPECHAINISINITIALIZEDWITHTHEOBJECTSCONTAINEDINTHEEXECUTINGFUNCTIONSSCOPEPROPERTYTHESEVALUESARECOPIEDOVERINTOTHEEXECUTIONCONTEXTSCOPECHAININTHEORDERINWHICHTHEYAPPEARINTHEFUNCTIONONCETHISISCOMPLETE,ANEWOBJECTCALLEDTHEACTIVATIONOBJECTISCREATEDFORTHEEXECUTIONCONTEXTTHEACTIVATIONOBJECTACTSASTHEVARIABLEOBJECTFORTHISEXECUTIONANDCONTAINSENTRIESFORALLLOCALVARIABLES,NAMEDARGUMENTS,THEARGUMENTSCOLLECTION,ANDTHISTHISOBJECTISTHENPUSHEDTOTHEFRONTOFTHESCOPECHAINWHENTHEEXECUTIONCONTEXTISDESTROYED,SOISTHEACTIVATIONOBJECTEACHTIMEAVARIABLEISENCOUNTEREDDURINGTHEFUNCTIONSEXECUTION,THEPROCESSOFIDENTIFIERRESOLUTIONTAKESPLACETODETERMINEWHERETORETRIEVEORSTORETHEDATADURINGTHISPROCESS,THEEXECUTIONCONTEXTSSCOPECHAINISSEARCHEDFORANIDENTIFIERWITHTHESAMENAMETHESEARCHBEGINSATTHEFRONTOFTHESCOPECHAIN,INTHEEXECUTIONFUNCTIONSACTIVATIONOBJECTIFFOUND,THEVARIABLEWITHTHESPECIFIEDIDENTIFIERISUSEDIFNOT,THESEARCHCONTINUESONTOTHENEXTOBJECTINTHESCOPECHAINTHISPROCESSCONTINUESUNTILEITHERTHEIDENTIFIERISFOUNDORTHEREARENOMOREVARIABLEOBJECTSTOSEARCH,INWHICHCASETHEIDENTIFIERISDEEMEDTOBEUNDEFINEDTHESAMEAPPROACHISTAKENFOREACHIDENTIFIERFOUNDDURINGTHEFUNCTIONEXECUTION,SOINTHEPREVIOUSEXAMPLE,THISWOULDHAPPENFORSUM,NUM1,ANDNUM2ITISTHISSEARCHPROCESSTHATAFFECTSPERFORMANCE四IDENTIFIERRESOLUTIONPERFORMANCEIDENTIFIERRESOLUTIONISNTFREE,ASINFACTNOCOMPUTEROPERATIONREALLYISWITHOUTSOMESORTOFPERFORMANCEOVERHEADTHEDEEPERINTOTHEEXECUTIONCONTEXTSSCOPECHAINANIDENTIFIEREXISTS,THESLOWERITISTOACCESSFORBOTHREADSANDWRITESCONSEQUENTLY,LOCALVARIABLESAREALWAYSTHEFASTESTTOACCESSINSIDEOFAFUNCTION,WHEREASGLOBALVARIABLESWILLGENERALLYBETHESLOWESTOPTIMIZINGJAVASCRIPTENGINESARECAPABLEOFTUNINGTHISINCERTAINSITUATIONSKEEPINMINDTHATGLOBALVARIABLESALWAYSEXISTINTHELASTVARIABLEOBJECTOFTHEEXECUTIONCONTEXTSSCOPECHAIN,SOTHEYAREALWAYSTHEFURTHESTAWAYTORESOLVETHEGENERALTRENDACROSSALLBROWSERSISTHATTHEDEEPERINTOTHESCOPECHAINANIDENTIFIEREXISTS,THESLOWERITWILLBEREADFROMORWRITTENTOBROWSERSWITHOPTIMIZINGJAVASCRIPTENGINES,SUCHASCHROMEANDSAFARI4,DONTHAVETHISSORTOFPERFORMANCEPENALTYFORACCESSINGOUTOFSCOPEIDENTIFIERS,WHEREASINTERNETEXPLORER,SAFARI32,ANDOTHERSSHOWAMOREDRASTICEFFECTITSWORTHNOTINGTHATEARLIERBROWSERS,SUCHASINTERNETEXPLORER6ANDFIREFOX2,HADINCREDIBLYSTEEPSLOPESANDWOULDNOTEVENAPPEARWITHINTHEBOUNDSOFTHISGRAPHATTHEHIGHPOINTIFTHEIRDATAHADBEENINCLUDEDGIVENTHISINFORMATION,ITSADVISABLETOUSELOCALVARIABLESWHENEVERPOSSIBLETOIMPROVEPERFORMANCEINBROWSERSWITHOUTOPTIMIZINGJAVASCRIPTENGINESAGOODRULEOFTHUMBISTOALWAYSSTOREOUTOFSCOPEVALUESINLOCALVARIABLESIFTHEYAREUSEDMORETHANONCEWITHINAFUNCTIONCONSIDERTHEFOLLOWINGEXAMPLETHISFUNCTIONCONTAINSTHREEREFERENCESTODOCUMENT,WHICHISAGLOBALOBJECTTHESEARCHFORTHISVARIABLEMUSTGOALLTHEWAYTHROUGHTHESCOPECHAINBEFOREFINALLYBEINGRESOLVEDINTHEGLOBALVARIABLEOBJECTYOUCANMITIGATETHEPERFORMANCEIMPACTOFREPEATEDGLOBALVARIABLEACCESSBYFIRSTSTORINGTHEREFERENCEINALOCALVARIABLEANDTHENUSINGTHELOCALVARIABLEINSTEADOFTHEGLOBALFOREXAMPLE,THEPREVIOUSCODECANBEREWRITTENASFOLLOWSTHEUPDATEDVERSIONOFINITUIFIRSTSTORESAREFERENCETODOCUMENTINTHELOCALDOCVARIABLEINSTEADOFACCESSINGAGLOBALVARIABLESTHREETIMES,THATNUMBERISCUTDOWNTOONEACCESSINGDOCINSTEADOFDOCUMENTISFASTERBECAUSEITSALOCALVARIABLEOFCOURSE,THISSIMPLISTICFUNCTIONWONTSHOWAHUGEPERFORMANCEIMPROVEMENT,BECAUSEITSNOTDOINGTHATMUCH,BUTIMAGINELARGERFUNCTIONSWITHDOZENSOFGLOBALVARIABLESBEINGACCESSEDREPEATEDLYTHATISWHERETHEMOREIMPRESSIVEPERFORMANCEIMPROVEMENTSWILLBEFOUND五SCOPECHAINAUGMENTATIONGENERALLYSPEAKING,ANEXECUTIONCONTEXTSSCOPECHAINDOESNTCHANGETHEREARE,HOWEVER,TWOSTATEMENTSTHATTEMPORARILYAUGMENTTHEEXECUTIONCONTEXTSSCOPECHAINWHILEITISBEINGEXECUTEDTHEFIRSTOFTHESEISWITHTHEWITHSTATEMENTISUSEDTOCREATEVARIABLESFORALLOFANOBJECTSPROPERTIESTHISMIMICSOTHERLANGUAGESWITHSIMILARFEATURESANDISUSUALLYSEENASACONVENIENCETOAVOIDWRITINGTHESAMECODEREPEATEDLYTHEINITUIFUNCTIONCANBEWRITTENASTHEFOLLOWINGTHISREWRITTENVERSIONOFINITUIUSESAWITHSTATEMENTTOAVOIDWRITINGDOCUMENTELSEWHERETHOUGHTHISMAYSEEMMOREEFFICIENT,ITACTUALLYCREATESAPERFORMANCEPROBLEMWHENCODEEXECUTIONFLOWSINTOAWITHSTATEMENT,THEEXECUTIONCONTEXTSSCOPECHAINISTEMPORARILYAUGMENTEDANEWVARIABLEOBJECTISCREATEDCONTAININGALLOFTHEPROPERTIESOFTHESPECIFIEDOBJECTTHATOBJECTISTHENPUSHEDTOTHEFRONTOFTHESCOPECHAIN,MEANINGTHATALLOFTHEFUNCTIONSLOCALVARIABLESARENOWINTHESECONDSCOPECHAINOBJECTANDARETHEREFOREMOREEXPENSIVETOACCESSBYPASSINGTHEDOCUMENTOBJECTINTOTHEWITHSTATEMENT,ANEWVARIABLEOBJECTCONTAININGALLOFTHEDOCUMENTOBJECTSPROPERTIESISPUSHEDTOTHEFRONTOFTHESCOPECHAINTHISMAKESITVERYFASTTOACCESSDOCUMENTPROPERTIESBUTSLOWERTOACCESSTHELOCALVARIABLESSUCHASBDFORTHISREASON,ITSBESTTOAVOIDUSINGTHEWITHSTATEMENTASSHOWNPREVIOUSLY,ITSJUSTASEASYTOSTOREDOCUMENTINALOCALVARIABLEANDGETTHEPERFORMANCEIMPROVEMENTTHATWAYTHEWITHSTATEMENTISNTTHEONLYPARTOFJAVASCRIPTTHATARTIFICIALLYAUGMENTSTHEEXECUTIONCONTEXTSSCOPECHAINTHECATCHCLAUSEOFTHETRYCATCHSTATEMENTHASTHESAMEEFFECTWHENANERROROCCURSINTHETRYBLOCK,EXECUTIONAUTOMATICALLYFLOWSTOTHECATCHANDTHEEXCEPTIONOBJECTISPUSHEDINTOAVARIABLEOBJECTTHATISTHENPLACEDATTHEFRONTOFTHESCOPECHAININSIDEOFTHECATCHBLOCK,ALLVARIABLESLOCALTOTHEFUNCTIONARENOWINTHESECONDSCOPECHAINOBJECTFOREXAMPLENOTETHATASSOONASTHECATCHCLAUSEISFINISHEDEXECUTING,THESCOPECHAINRETURNSTOITSPREVIOUSSTATETHETRYCATCHSTATEMENTISVERYUSEFULWHENAPPLIEDAPPROPRIATELY,ANDSOITDOESNTMAKESENSETOSUGGESTCOMPLETEAVOIDANCEIFYOUDOPLANONUSINGATRYCATCH,MAKESURETHATYOUUNDERSTANDTHELIKELIHOODOFERRORATRYCATCHSHOULDNEVERBEUSEDASTHESOLUTIONTOAJAVASCRIPTERRORIFYOUKNOWANERRORWILLOCCURFREQUENTLY,THENTHATINDICATESAPROBLEMWITHTHECODEITSELFTHATSHOULDBEFIXEDYOUCANMINIMIZETHEPERFORMANCEIMPACTOFTHECATCHCLAUSEBYEXECUTINGASLITTLECODEASNECESSARYWITHINITAGOODPATTERNISTOHAVEAMETHODFORHANDLINGERRORSTHATTHECATCHCLAUSECANDELEGATETO,ASINTHISEXAMPLEHEREAHANDLEERRORMETHODISTHEONLYCODETHATISEXECUTEDINTHECATCHCLAUSETHISMETHODISFREETOHANDLETHEERRORINANAPPROPRIATEWAYANDISPASSEDTHEEXCEPTIONOBJECTGENERATEDFROMTHEERRORSINCETHEREISJUSTONESTATEMENTEXECUTEDANDNOLOCALVARIABLESACCESSED,THETEMPORARYSCOPECHAINAUGMENTATIONDOESNOTAFFECTTHEPERFORMANCEOFTHECODE六DYNAMICSCOPESBOTHTHEWITHSTATEMENTANDTHECATCHCLAUSEOFATRYCATCHSTATEMENT,ASWELLASAFUNCTIONCONTAINING,AREALLCONSIDEREDTOBEDYNAMICSCOPESADYNAMICSCOPEISONETHATEXISTSONLYTHROUGHEXECUTIONOFCODEANDTHEREFORECANNOTBEDETERMINEDSIMPLYBYSTATICANALYSISLOOKINGATTHECODESTRUCTUREFOREXAMPLETHEEXECUTEFUNCTIONREPRESENTSADYNAMICSCOPEDUETOTHEUSEOFTHEVALUEOFWCANCHANGEBASEDONTHEVALUEOFCODEINMOSTCASES,WWILLBEEQUALTOTHEGLOBALWINDOWOBJECT,BUTCONSIDERTHEFOLLOWINGINTHISCASE,CREATESALOCALWINDOWVARIABLEINEXECUTE,SOWENDSUPEQUALTOTHELOCALWINDOWINSTEADOFTHEGLOBALTHEREISNOWAYTOKNOWIFTHISISTHECASEUNTILTHECODEISEXECUTED,WHICHMEANSTHEVALUEOFTHEWINDOWIDENTIFIERCANNOTBEPREDETERMINEDOPTIMIZINGJAVASCRIPTENGINESSUCHASSAFARISNITROTRYTOSPEEDUPIDENTIFIERRESOLUTIONBYANALYZINGTHECODETODETERMINEWHICHVARIABLESSHOULDBEACCESSIBLEATANYGIVENTIMETHESEENGINESTRYTOAVOIDTHETRADITIONALSCOPECHAINLOOKUPBYINDEXINGIDENTIFIERSFORFASTERRESOLUTIONWHENADYNAMICSCOPEISINVOLVED,HOWEVER,THISOPTIMIZATIONISNOLONGERVALIDTHEENGINESNEEDTOSWITCHBACKTOASLOWERHASHBASEDAPPROACHFORIDENTIFIERRESOLUTIONTHATMORECLOSELYMIRRORSTRADITIONALSCOPECHAINLOOKUPFORTHISREASON,ITSRECOMMENDEDTOUSEDYNAMICSCOPESONLYWHENABSOLUTELYNECESSARY七CLOSURES,SCOPE,ANDMEMORYCLOSURESAREONEOFTHEMOSTPOWERFULASPECTSOFJAVASCRIPT,ALLOWINGAFUNCTIONTOACCESSDATATHATISOUTSIDEOFITSLOCALSCOPETHEUSEOFCLOSURESHASBEENPOPULARIZEDTHROUGHTHEWRITINGSOFDOUGLASCROCKFORDANDISNOWUBIQUITOUSINMOSTCOMPLEXWEBAPPLICATIONSTHEREIS,HOWEVER,APERFORMANCEIMPACTASSOCIATEDWITHUSINGCLOSURESTOUNDERSTANDTHEPERFORMANCEISSUESWITHCLOSURES,CONSIDERTHEFOLLOWINGTHEASSIGNEVENTSFUNCTIONASSIGNSANEVENTHANDLERTOASINGLEDOMELEMENTTHISEVENTHANDLERISACLOSURE,ASITISCREATEDWHENTHEASSIGNEVENTSISEXECUTEDANDCANACCESSTHEIDVARIABLEFROMTHECONTAININGSCOPEINORDERFORTHISCLOSURETOACCESSID,ASPECIFICSCOPECHAINMUSTBECREATEDWHENASSIGNEVENTSISEXECUTED,ANACTIVATIONOBJECTISCREATEDTHATCONTAINS,AMONGOTHERTHINGS,THEIDVARIABLETHISBECOMESTHEFIRSTOBJECTINTHEEXECUTIONCONTEXTSSCOPECHAIN,WITHTHEGLOBALOBJECTCOMINGSECONDWHENTHECLOSUREISCREATED,ITSSCOPEPROPERTYISINITIALIZEDWITHBOTHOFTHESEOBJECTSSINCETHECLOSURESSCOPEPROPERTYCONTAINSREFERENCESTOTHESAMEOBJECTSASTHEEXECUTIONCONTEXTSSCOPECHAIN,THEREISASIDEEFFECTTYPICALLY,AFUNCTIONSACTIVATIONOBJECTISDESTROYEDWHENTHEEXECUTIONCONTEXTISDESTROYEDWHENTHERESACLOSUREINVOLVED,THOUGH,THEACTIVATIONOBJECTISNTDESTROYED,BECAUSEAREFERENCESTILLEXISTSINTHECLOSURESSCOPEPROPERTYTHISMEANSTHATCLOSURESREQUIREMOREMEMORYOVERHEADINASCRIPTTHANANONCLOSUREFUNCTIONINLARGEWEBAPPLICATIONS,THISMIGHTBECOMEAPROBLEM,ESPECIALLYWHEREINTERNETEXPLORERISCONCERNEDIEIMPLEMENTSDOMOBJECTSASNONNATIVEJAVASCRIPTOBJECTS,ANDASSUCH,CLOSURESCANCAUSEMEMORYLEAKSWHENTHECLOSUREISEXECUTED,ANEXECUTIONCONTEXTISCREATEDWHOSESCOPECHAINISINITIALIZEDWITHTHESAMETWOSCOPECHAINOBJECTSREFERENCEDINSCOPE,ANDTHENANEWACTIVATIONOBJECTISCREATEDFORTHECLOSUREITSELFNOTETHATBOTHIDENTIFIERSUSEDINTHECLOSURE,IDANDSAVEDOCUMENT,EXISTPASTTHEFIRSTOBJECTINTHESCOPECHAINTHISISTHEPRIMARYPERFORMANCECONCERNWITHCLOSURESYOUREOFTENACCESSINGALOTOFOUTOFSCOPEIDENTIFIERSANDTHEREFOREAREINCURRINGAPERFORMANCEPENALTYWITHEACHACCESSITSBESTTOEXERCISECAUTIONWHENUSINGCLOSURESINYOURSCRIPTS,ASTHEYHAVEBOTHMEMORYANDEXECUTIONSPEEDCONCERNSHOWEVER,YOUCANMITIGATETHEEXECUTIONSPEEDIMPACTBYFOLLOWINGTHEADVICEFROMEARLIERINTHISCHAPTERREGARDINGOUTOFSCOPEVARIABLESSTOREANYFREQUENTLYUSEDOUTOFSCOPEVARIABLESINLOCALVARIABLES,ANDTHENACCESSTHELOCALVARIABLESDIRECTLY八OBJECTMEMBERSMOSTJAVASCRIPTISWRITTENINANOBJECTORIENTEDMANNER,EITHERTHROUGHTHECREATIONOFCUSTOMOBJECTSORTHEUSEOFBUILTINOBJECTSSUCHASTHOSEINTHEDOCUMENTOBJECTMODELDOMANDBROWSEROBJECTMODELBOMASSUCH,THERETENDSTOBEALOTOFOBJECTMEMBERACCESSOBJECTMEMBERSAREBOTHPROPERTIESANDMETHODS,ANDTHEREISLITTLEDIFFERENCEBETWEENTHETWOINJAVASCRIPTANAMEDMEMBEROFANOBJECTMAYCONTAINANYDATATYPESINCEFUNCTIONSAREREPRESENTEDASOBJECTS,AMEMBERMAYCONTAINAFUNCTIONINADDITIONTOTHEMORETRADITIONALDATATYPESWHENANAMEDMEMBERREFERENCESAFUNCTION,ITSCONSIDEREDAMETHOD,WHEREASAMEMBERREFERENCINGANONFUNCTIONDATATYPEISCONSIDEREDAPROPERTYASDISCUSSEDEARLIERINTHISCHAPTER,OBJECTMEMBERACCESSTENDSTOBESLOWERTHANACCESSINGDATAINLITERALSORVARIABLES,ANDINSOMEBROWSERSSLOWERTHANACCESSINGARRAYITEMSTOUNDERSTANDWHYTHISISTHECASE,ITSNECESSARYTOUNDERSTANDTHENATUREOFOBJECTSINJAVASCRIPT九PROTOTYPESOBJECTSINJAVASCRIPTAREBASEDONPROTOTYPESAPROTOTYPEISANOBJECTTHATSERVESASTHEBASEOFANOTHEROBJECT,DEFININGANDIMPLEMENTINGMEMBERSTHATANEWOBJECTMUSTHAVETHISISACOMPLETELYDIFFERENTCONCEPTTHANTHETRADITIONALOBJECTORIENTEDPROGRAMMINGCONCEPTOFCLASSES,WHICHDEFINETHEPROCESSFORCREATINGANEWOBJECTPROTOTYPEOBJECTSARESHAREDAMONGSTALLINSTANCESOFAGIVENOBJECTTYPE,ANDSOALLINSTANCESALSOSHARETHEPROTOTYPEOBJECTSMEMBERSANOBJECTISTIEDTOITSPROTOTYPEBYANINTERNALPROPERTYFIREFOX,SAFARI,ANDCHROMEEXPOSETHISPROPERTYTODEVELOPERSAS_PROTO_OTHERBROWSERSDONOTALLOWSCRIPTACCESSTOTHISPROPERTYANYTIMEYOUCREATEANEWINSTANCEOFABUILTINTYPE,SUCHASOBJECTORARRAY,THESEINSTANCESAUTOMATICALLYHAVEANINSTANCEOFOBJECTASTHEIRPROTOTYPECONSEQUENTLY,OBJECTSCANHAVETWOTYPESOFMEMBERSINSTANCEMEMBERSALSOCALLED“OWN“MEMBERSANDPROTOTYPEMEMBERSINSTANCEMEMBERSEXISTDIRECTLYONTHEOBJECTINSTANCEITSELF,WHEREASPROTOTYPEMEMBERSAREINHERITEDFROMTHEOBJECTPROTOTYPECONSIDERTHEFOLLOWINGEXAMPLEINTHISCODE,THEBOOKOBJECTHASTWOINSTANCEMEMBERSTITLEANDPUBLISHERNOTETHATTHEREISNODEFINITIONFORTHEMETHODTOSTRINGBUTTHATTHEMETHODISCALLEDANDBEHAVESAPPROPRIATELYWITHOUTTHROWINGANERRORTHETOSTRINGMETHODISAPROTOTYPEMEMBERTHATTHEBOOKOBJECTISINHERITINGTHEPROCESSOFRESOLVINGANOBJECTMEMBERISVERYSIMILARTORESOLVINGAVARIABLEWHENBOOKTOSTRINGISCALLED,THESEARCHFORAMEMBERNAMED“TOSTRING“BEGINSONTHEOBJECTINSTANCESINCEBOOKDOESNTHAVEAMEMBERNAMEDTOSTRING,THESEARCHTHENFLOWSTOTHEPROTOTYPEOBJECT,WHERETHETOSTRINGMETHODISFOUNDANDEXECUTEDINTHISWAY,BOOKHASACCESSTOEVERYPROPERTYORMETHODONITSPROTOTYPEYOUCANDETERMINEWHETHERANOBJECTHASANINSTANCEMEMBERWITHAGIVENNAMEBYUSINGTHEHASOWNPROPERTYMETHODANDPASSINGINTHENAMEOFTHEMEMBERTODETERMINEWHETHERANOBJECTHASACCESSTOAPROPERTYWITHAGIVENNAME,YOUCANUSETHEINOPERATORFOREXAMPLEINTHISCODE,HASOWNPROPERTYRETURNSTRUEWHEN“TITLE“ISPASSEDINBECAUSETITLEISANOBJECTINSTANCETHEMETHODRETURNSFALSEWHEN“TOSTRING“ISPASSEDINBECAUSEITDOESNTEXISTONTHEINSTANCEWHENEACHPROPERTYNAMEISUSEDWITHTHEINOPERATOR,THERESULTISTRUEBOTHTIMESBECAUSEITSEARCHESTHEINSTANCEANDPROTOTYPE十PROTOTYPECHAINSTHEPROTOTYPEOFANOBJECTDETERMINESTHETYPEORTYPESOFWHICHITISANINSTANCEBYDEFAULT,ALLOBJECTSAREINSTANCESOFOBJECTANDINHERITALLOFTHEBASICMETHODS,SUCHASTOSTRINGYOUCANCREATEAPROTOTYPEOFANOTHERTYPEBYDEFININGANDUSINGACONSTRUCTORFOREXAMPLETHEBOOKCONSTRUCTORISUSEDTOCREATEANEWINSTANCEOFBOOKTHEBOOK1INSTANCESPROTOTYPE_PROTO_ISBOOKPROTOTYPE,ANDBOOKPROTOTYPESPROTOTYPEISOBJECTTHISCREATESAPROTOTYPECHAINFROMWHICHBOTHBOOK1ANDBOOK2INHERITTHEIRMEMBERSFIGURE210SHOWSTHISRELATIONSHIPNOTETHATBOTHINSTANCESOFBOOKSHARETHESAMEPROTOTYPECHAINEACHINSTANCEHASITSOWNTI