资源描述：
《js变量作用域》由会员上传分享，免费在线阅读，更多相关内容在工程资料-天天文库。
1、js变量作用域Eachlanguagehastheconceptofvariable,whichisanelementforstoringinformation.Forexample,thefollowingfunction:lfunction,Student(name,age,from)2{3this.name=name:4this・age=age;5this・from=from;6this.ToString=function(){78return,z，my,information,is,nam
2、e:z，,〃+this.name+〃，〃age〃,〃+this.age+〃,+this・from;9}10}TheStudentclasshasthreevariables,name(name),age(age),andfrom(nativeplace)・Thesethreevariablesmakeuptheinformationdescribinganobject・Ofcourse,thereisalsoawaytoreturninformationfromStudent・Butdowedef
3、ineavariablethatcanexistallthetimeandcanstillbeaccessedandusedanywhereuntilitisdestroyed?Thinkcarefully,theaboverequirementisexcessive,becausesomeofthevariablesinafunctionisnolongerused,butifthevariablesstillexist,takeupsystemresources,theysaid:"stood
4、Maokengnotpull#$%〃・Sowehaveadiscussiononthetimelyandon-demanddestructionofvariables・Well,let'sgetdowntothepoint・AsfarasI'mconcerned,JSsupportsseveraltypesofvariables:localvariables,classvariables,privatevariables,instancevariables,staticvariables,andg
5、lobalvariables・Next,wewilldiscussandstudy.Localvariable:Localvariablesgenerallyreferstotheeffectivevariablesintherangeofvariablesis{},effectivestatementsinsidetheblock,suchas:lfunctionfoo(flag)2{3varsum二0;4if(flag二true)6varindex;7for(index=0;index<10;
6、index++)(89sum+二index;10}11}12document.write("indexis":〃+index+〃，/
，/);13returnsum;14}15//document.write("sumis":〃+sum+〃〃〈br〉〃)；16document.write("resultis〃：〃+foo〃(true)+，z
z，);Whenthecodeisexecuted,theoutputresultsare:，zindexis:undefined〃and"res
7、ultis:0〃・Wecanseethatthevalueoftheindexvariablethatwewanttooutputisundefined,thatis,undefined・Sowecanseethattheindexvariableisdestroyedaftertheendoftheifstatementblock・Whataboutthe"sunTvariables?Thisvariableisdestroyedaftertheexecutionofthefoo()functi
8、onsection・IfyouremovethestatementIarmotated,andthenexecute,youwillfindthatthesystemwillbewrong・ItisworthnotingthatifIchangetheabovefoo()functiontothefollowing:lfunctionfoo(flag)2{3varsum二0;4for(VaRindex=0;index<10;index++;){56sum+二index;7}6doc
