外文翻译-javascript的学习

外文原文来源MASTERINGAJAX14AUG2007FROMOREILLYMEDIAINC2JAVASCRIPTREFRESHERJAVASCRIPTISANESSENTIALPIECEOFTHEAJAXPACKAGEJAVASCRIPTSERVESASTHEINTERMEDIARYBETWEENTHEBROWSERTHECLIENTANDTHESERVERSOTHATAWEBPAGECANBEDYNAMICALLYUPDATEDWITHOUTREFRESHINGTHEENTIREPAGEJAVASCRIPTWASDEVELOPEDBYBRENDANEICHANDINTRODUCEDINTHE1995RELEASEOFNETSCAPE20BECAUSEJAVASCRIPTOFFEREDAMEANSTOADDINTERACTIVITYTOHTMLPAGES,ITQUICKLYBECAMEPOPULARANDWIDELYUSEDDIFFERENTVERSIONSWEREDEVELOPEDBOTHBYNETSCAPEANDOTHERBROWSERMANUFACTURERS,BUTEVENTUALLYTHEJAVASCRIPTLANGUAGEWASSTANDARDIZEDBYTHEEUROPEANCOMPUTERMANUFACTURERSASSOCIATIONECMAASECMASCRIPTTHEECMASCRIPTSTANDARDDEFINESTHECOREOFTHEJAVASCRIPTLANGUAGEMOSTBROWSERSTODAYSUPPORTTHETHIRDEDITIONOFTHEECMA262STANDARDTHISCHAPTERPROVIDESAREVIEWOFCOREJAVASCRIPTECMASCRIPT,JAVASCRIPTOBJECTS,THEDOCUMENTOBJECTMODELDOM,ANDEVENTHANDLINGFORREADERSWHOHAVESOMEFAMILIARITYANDEXPERIENCEWITHJAVASCRIPTANDWITHBASICPROGRAMMINGCONSTRUCTSVARIABLES,OPERATORS,FUNCTIONSIFYOURENEWTOPROGRAMMINGAND/ORJAVASCRIPT,REFERTOBOOKSSUCHASBEGINNINGJAVASCRIPT,SECONDEDITIONINDIANAPOLISWILEYPUBLISHING,2004ANDPROFESSIONALJAVASCRIPTFORWEBDEVELOPERSINDIANAPOLISWILEYPUBLISHING,2005FORMORECOMPREHENSIVEINFORMATIONABOUTJAVASCRIPTINTHISCHAPTER,YOULEARNABOUTTHESETOPICSCOREJAVASCRIPTVARIABLES,OPERATORS,STATEMENTS,FUNCTIONSOBJECTORIENTEDJAVASCRIPTBUILTINOBJECTS,THEBROWSEROBJECTMODEL,USERDEFINEDOBJECTSTHEDOCUMENTOBJECTMODELDOMJAVASCRIPTANDEVENTSCOREJAVASCRIPTCOREJAVASCRIPTECMASCRIPTPROVIDESEVERYTHINGYOUNEEDTOACCOMPLISHBASICPROGRAMMINGTASKSVARIABLES,OPERATORSTHATACTONTHEVARIABLES,STATEMENTS,ANDFUNCTIONSTHATPROVIDEREUSEABLEBLOCKSOFCODETOACCOMPLISHATASKOBJECTSAREALSOPARTOFECMASCRIPT,ANDTHEYAREDISCUSSEDLATERINTHISCHAPTERINTHESECTION,“OBJECTORIENTEDJAVASCRIPT”SYNTAXTHEECMASCRIPTSTANDARDSPECIFIESJAVASCRIPTSBASICSYNTAXRULESUSINGCORRECTSYNTAXISCRUCIALIFYOURCODEHASSYNTAXERRORS,THEBROWSERSJAVASCRIPTINTERPRETERSTOPSPROCESSINGYOURCODEUNTILTHEERRORISCORRECTEDAFEWGUIDELINESCANHELPYOUFOLLOWGOODPROGRAMMINGPRACTICESANDAVOIDSYNTAXERRORSJAVASCRIPTISCASESENSITIVEPAYEXTRAATTENTIONWHENUSINGCAPITALLETTERSINTHENAMESOFVARIABLES,FUNCTIONS,ANDOBJECTSAVARIABLENAMEDMYVARISNOTTHESAMEASONENAMEDMYVARSEMICOLONSAREOPTIONALEVENTHOUGHSEMICOLONSARENOTREQUIREDATTHEENDOFEVERYCODESTATEMENT,ITSGOODPRACTICETOINCLUDETHEMINYOURCODETODELINEATEWHERETHELINEOFCODEENDSTHEYMAKETHECODEEASIERTOREAD,EASIERTODEBUG,ANDTHEYHELPDECREASETHENUMBEROFERRORSGENERATEDWHENTHEBROWSERREADSYOURCODELETTHECODEWRAPDONTINSERTLINERETURNSINTHEMIDDLEOFALINEOFCODEALINERETURNCHARACTERINTHEMIDDLEOFACODESTATEMENTISONEOFTHEMOSTCOMMONERRORSINJAVASCRIPT,ANDLINERETURNSWILLMAKEYOURCODENONFUNCTIONALADDCOMMENTSTHESYNTAXFORJAVASCRIPTCOMMENTSIS/FORASINGLELINECOMMENTAND/AND/FOROPENINGANDCLOSINGMULTILINECOMMENTSCOMMENTINGYOURCODEMAKESITMUCHEASIERFORYOUORANYONEELSETOUNDERSTANDYOURCODEWHENITSBEINGUPDATEDORDEBUGGEDREMEMBER,THEPERSONUPDATINGORDEBUGGINGJUSTMIGHTBEYOUVARIABLESAVARIABLEISATEMPORARYSTORAGECONTAINERFORDATAJAVASCRIPTVARIABLESCANBEDECLAREDUSINGTHEVARKEYWORDFOREXAMPLE,THEFOLLOWINGDECLARESANEWVARIABLENAMEDMYCOLORVARMYCOLORTHISVARIABLEDOESNTCONTAINANYINFORMATIONYETIFYOUINCLUDEDTHISVARIABLEINANALERTMESSAGE,SUCHASTHEFOLLOWING,ITSVALUEWOULDBEUNDEFINED,ASSHOWNINFIGURE21ALERT“MYCOLOR“MYCOLORFIGURE21UNDEFINEDVALUEYOUDONTHAVETODECLAREAVARIABLEASASEPARATESTEPINSTEAD,YOUCANINITIALIZEAVARIABLEDECLAREITANDASSIGNAVALUETOITATTHESAMETIME,ASSHOWNHEREVARMYCOLOR“GREEN”VARIABLENAMESMUSTSTARTWITHALETTER,ANUNDERSCORE_,ORADOLLARSIGNCAMELNOTATIONISCOMMONLYUSEDFORVARIABLENAMES,ALTHOUGHNOSPECIFICNOTATIONISREQUIREDINCAMELNOTATION,THEFIRSTWORDISLOWERCASE,ANDADDITIONALWORDSSTARTWITHACAPITALLETTER,ASSHOWNHEREVARMYFAVORITENUMBERPRIMITIVEDATATYPESEVERYJAVASCRIPTVARIABLEHASADATATYPETHATINDICATESTHEKINDOFDATATHEVARIABLECONTAINSPRIMITIVEDATATYPESSTOREDATADIRECTLYINAVARIABLETHEREAREFIVETYPESOFPRIMITIVEDATATYPESNUMBERSTRINGBOOLEANUNDEFINEDNULLTHENUMBERTYPEINCLUDESBOTHINTEGERSANDFLOATINGPOINTNUMBERSTHESTRINGTYPEINCLUDESANYGROUPOFONEORMORECHARACTERSASTRINGISDISTINGUISHEDFROMANUMBERBYENCLOSINGITINQUOTATIONMARKSFOREXAMPLE,“5”ISASTRING,WHILE5ISANUMBERTHEBOOLEANTYPEHASTWOVALUESTRUEANDFALSETHEUNDEFINEDTYPEHASONLYONEVALUEUNDEFINEDANUNDEFINEDVALUEOCCURSWHENAVARIABLEISDECLAREDBUTNOTASSIGNEDAVALUE,ASSHOWNEARLIERFORTHEMYCOLORVARIABLETHENULLTYPEALSOHASONLYONEVALUENULLANULLVALUEMEANSTHATTHEOBJECTINQUESTIONDOESNOTEXISTYOUDONTHAVETOINCLUDEADATATYPEWHENYOUDECLAREORINITIALIZEAJAVASCRIPTVARIABLEJAVASCRIPTUSESDYNAMICTYPINGTHEDATATYPEISINFERREDFROMTHECONTEXTOFTHEJAVASCRIPTSTATEMENTTHISALSOMEANSTHATTHESAMEVARIABLECANBEUSEDFORDIFFERENTTYPESOFDATAATDIFFERENTTIMES,ASSHOWNHEREVARMYNUMBER“LUCKY”/MYNUMBERCONTAINSASTRINGMYNUMBER3/MYNUMBERCONTAINSANUMBERREFERENCEDATATYPESREFERENCEDATATYPESINJAVASCRIPTCONTAINAREFERENCETOAPLACEINMEMORYTHATHOLDSTHEDATA,RATHERTHANTHEDATAITSELFTHEOBJECTTYPEASWELLASSPECIALCATEGORIESOFOBJECTSUCHASARRAYANDFUNCTIONISAREFERENCEDATATYPEITSIMPORTANTTOBECLEARABOUTWHETHERYOUREUSINGAPRIMITIVETYPEORAREFERENCETYPEIFYOUUSEAREFERENCETYPEFORAFUNCTIONPARAMETERANDTHENMAKECHANGESTOTHEDATAWITHINTHEFUNCTION,YOUCANMODIFYTHEORIGINALDATAINTHEREFERENCETYPEEVENTHOUGHTHECHANGEISMADELOCALLYWITHINAFUNCTION,THEDATAISCHANGEDGLOBALLY,ASSHOWNHEREFUNCTIONMYARRAYAVARA05VARNEXTARRAY1,2,3MYARRAYNEXTARRAYALERT“NEXTARRAY“NEXTARRAYAFTERTHEMYARRAYFUNCTIONISCALLED,THEVALUEOFNEXTARRAYIS5,2,3OPERATORSOPERATORSENABLEYOUTOPERFORMANACTIONONAVARIABLETHEBASICJAVASCRIPTOPERATORSINCLUDEASSIGNMENT,ARITHMETIC,COMPARISON,LOGICAL,ANDINCREMENT/DECREMENTOPERATORSASSIGNMENTOPERATORTHEASSIGNMENTOPERATORISUSEDFORASSIGNINGAVALUETOAVARIABLE,ASSHOWNHEREVARMYTREE“OAK”INTHISCASE,THEVALUE“OAK”ISASSIGNEDTOTHEVARIABLEMYTREEARITHMETICOPERATORSJAVASCRIPTINCLUDESOPERATORSFORALLTHEBASICARITHMETICFUNCTIONS,INCLUDINGADDITION,SUBTRACTION,MULTIPLICATION,ANDDIVISION/VARA5VARB10VARCAB/C15A“MY”B“NAME”CAB/C”MYNAME”JAVASCRIPTINCLUDESAMODULUSOPERATOR,ALSOKNOWNASTHEREMAINDEROPERATORTHEMODULUSOPERATORISUSEDTOCALCULATETHEVALUEOFTHEREMAINDERAFTERANUMBERHASBEENDIVIDEDBYANOTHERNUMBER,ASSHOWNHEREVARMYMOD165/MYMOD1USINGAMODULUSISANEASYWAYTODETERMINEIFANINTEGERHASANEVENORODDVALUECALCULATEINTEGER2IFTHERESULTIS0,THENITSANEVENNUMBERCOMPARISONOPERATOR