前端-階段一所謂閉包

所謂閉包張立理

functioninfiniteSequence(){

vari=0;

return

function(){

return

i++;}}varincrement=infiniteSequence();console.log(increment());

console.log(increment());console.log(increment());//…WARNING!!WARNING非常非常的學(xué)術(shù)性大量術(shù)語詞匯出沒也許永遠(yuǎn)都用不上內(nèi)容并非標(biāo)準(zhǔn)，有所刪減邏輯不是那么清晰只談函數(shù)，不提eval，不提newFunctionSummary引言什么是變量閉包之表象閉包之內(nèi)在關(guān)于垃圾回收作用域的歷史ECMAScriptv3ScopeChainVariableObjectIdentifierResolutionECMAScriptv5LexicalEnvironmentVariableEnvironmentGetIdentifierReference什么是變量變量是一種關(guān)聯(lián)關(guān)系（association）關(guān)聯(lián)的目標(biāo)：符號名（symbolicname)值（value）關(guān)聯(lián)是單向的–永遠(yuǎn)不可能根據(jù)值找到變量Asymbolicnameassociatedwithavalueandwhoseassociatedvaluemaybechanged.--WikipediaIdentifiernameValue‘GrayZhang’什么是變量VariableStatementvarIdentifier=AssignmentExpressionFunctionDeclarationfunctionIdentifier(FormalParameterList){

FunctionBody

}FormalParameterListIdentifier,Identifier[,…]什么是變量varname=‘GrayZhang’;functionadd(x,y){

returnx+y;

}KeywordIdentifierValue(StringLiteral)閉包之表象內(nèi)層函數(shù)可以使用外層函數(shù)作用域內(nèi)的變量functionouter(){varname=‘GrayZhang’;functioninner(){console.log(‘Hello‘+name);}inner();}外層內(nèi)層閉包之內(nèi)在Q：為什么javascript會有閉包？A：因?yàn)镋CMAScript中變量解析是一個(gè)查找過程，而非綁定過程。Q：變量存放在哪里？A：ExecutionContext中的VariableEnvironment。Q：從哪里查找變量？A：ExecutionContext中的LexicalEnvironment。Q：如何查找變量？A：自內(nèi)向外。？可執(zhí)行代碼（ExecutableCode）GlobalCodeFunctionCodeEvalCode<script>varname=‘GrayZhang’;varprefix=‘Hello‘;varphrases=[prefix,name];console.log(phrases.join(‘‘));</script>varsource=

‘varx=3;’+

‘console.log(x);’eval(source);functionsayHello(name){varprefix=‘Hello’;varphrases=[prefix,name];console.log(phrases.join(‘‘));}functiongetName(){varinput=$(‘#name’);returninput.val();}getName();執(zhí)行環(huán)境（ExecutionContext）當(dāng)進(jìn)入（開始執(zhí)行）一段可執(zhí)行代碼時(shí)，生成一個(gè)執(zhí)行環(huán)境對象。執(zhí)行環(huán)境對象通過棧（Stack）維護(hù)。新建的執(zhí)行環(huán)境對象稱為“當(dāng)前運(yùn)行的執(zhí)行環(huán)境對象”。functionenterCode(code){

varec=newExecutionContext();

control.ecStack.push(ec);

control.runningEC=ec;

control.execute(code);

}執(zhí)行環(huán)境（ExecutionContext）一個(gè)執(zhí)行環(huán)境對象包括：詞法環(huán)境–LexicalEnvironment變量環(huán)境–VariableEnvironmentThis綁定-ThisBindingExecutionContext:{LexicalEnvironment,VariableEnvironment,ThisBinding}詞法環(huán)境（LexicalEnvironment）既是一個(gè)屬性，又是一個(gè)類型。每個(gè)執(zhí)行環(huán)境對象都有且僅有一個(gè)關(guān)聯(lián)的詞法環(huán)境對象。在代碼執(zhí)行過程中，需要解析變量時(shí)，通過詞法環(huán)境對象進(jìn)行解析，從其環(huán)境數(shù)據(jù)中得到值。一個(gè)詞法環(huán)境對象包括：環(huán)境數(shù)據(jù)–environementrecords外層環(huán)境–outerenvironment詞法環(huán)境（LexicalEnvironment）存在2種詞法環(huán)境的實(shí)現(xiàn)類型DeclarativeEnvironmentObjectEnvironment區(qū)別是ObjectEnvironment可接受指定的對象作為環(huán)境數(shù)據(jù)屬性的值？什么情況會出現(xiàn)ObjectEnvironment變量環(huán)境（VariableEnvironment）每個(gè)執(zhí)行環(huán)境對象都有且僅有一個(gè)關(guān)聯(lián)的變量環(huán)境對象。變量環(huán)境僅僅是一個(gè)名字，變量環(huán)境對象的類型是詞法環(huán)境（LexicalEnvironment）。在進(jìn)入（開始執(zhí)行）代碼時(shí)，所有的變量標(biāo)識符（Identifier）會存放在當(dāng)前的變量環(huán)境對象中。變量環(huán)境中有環(huán)境數(shù)據(jù)屬性，但不使用外層環(huán)境屬性。環(huán)境數(shù)據(jù)（environmentrecords）存在于詞法環(huán)境或變量環(huán)境中。包含一個(gè)bindingobject，簡單地認(rèn)為bindingobject是一個(gè)Map對象，保存變量標(biāo)簽符（Identifier）和變量值（Value）的關(guān)系。常用方法：hadBinding(name)–查看是否有變量綁定createBinding(name)–創(chuàng)建一個(gè)變量綁定setBinding(name,value)–修改變量綁定的值getValue(name)–獲取變量綁定的值deleteBinding(name)–刪除一個(gè)變量綁定環(huán)境數(shù)據(jù)（environmentrecords）EnvironmentRecords:{bindingObject:{},hasBinding:function(name){return(nameinthis.bindingObject);},createBinding:function(name){this.bindingObject[name]=undefined;},setBinding:function(name,value){this.bindingObject[name]=value;},//…}環(huán)境數(shù)據(jù)（environmentrecords）存在2種環(huán)境數(shù)據(jù)的實(shí)現(xiàn)類型DeclarativeEnvironmentRecordsObjectEnvironmentRecordsLexicalEnvironmentEnvironmentRecordsObjectEnvironmentDeclaractiveEnvironmentObjectEnvironmentRecordsDeclaractiveEnvironmentRecords創(chuàng)建詞法環(huán)境NewDeclarativeEnvironment(e)用于創(chuàng)建一個(gè)DeclarativeEnvironment進(jìn)入函數(shù)時(shí)執(zhí)行catch表達(dá)式時(shí)NewObjectEnvironment(o,e)用于創(chuàng)建一個(gè)ObjectEnvironment執(zhí)行with表達(dá)式時(shí)創(chuàng)建詞法環(huán)境functionNewDeclarativeEnvironment(e){varenv=newLexicalEnvironment();varenvRec=newEnvironmentRecords();envRec.bindingObject={};env.environmentRecords=envRec;env.outerEnvironment=e;returnenv;}functionNewObjectEnvironment(o,e){varenv=newLexicalEnvironment();varenvRec=newEnvironmentRecords();envRec.bindingObject=o;env.environmentRecords=envRec;env.outerEnvironment=e;returnenv;}消化一下名詞解釋完了嗎？NO總結(jié)ExecutionContext:{LexicalEnvironment,VariableEnvironment,ThisBinding}EnvironmentRecords:{hasBinding(name),createBinding(name),setBinding(name,value),getValue(name),deleteBinding(name)}LexicalEnvironment:{environmentRecords,outerEnvironment}函數(shù)（Function）是一個(gè)對象（Object）。包含幾個(gè)特殊的屬性[[Construct]]–newSomeFunction()[[Call]]–someFunction()[[HasInstance]]–oinstanceofSomeFunction[[Scope]]–閉包[[FormalParameters]]–參數(shù)列表[[Code]]–可執(zhí)行代碼包含可執(zhí)行代碼（ExecutableCode）和執(zhí)行狀態(tài)（State）。創(chuàng)建函數(shù)（CreateFunctionObject）新建一個(gè)普通對象（newObject()）將[[Class]]設(shè)為”function”將[[Prototype]]指向Ftotype根據(jù)默認(rèn)的規(guī)則，設(shè)置[[Call]]、[[Contruct]]及[[HasInstance]]屬性將[[Scope]]設(shè)置為當(dāng)前的LexicalEnvironment對象設(shè)置[[Code]]、[[FormalParameterList]]及name、length、prototype屬性創(chuàng)建函數(shù)（CreateFunctionObject）varfn=newObject();//[[DefaultValue]],[[HasProperty]],etc...initializeAsObject(fn);fn.[[Class]]='function';fn.[[Prototype]]=Ftotype;fn.[[Call]]=function(){/*...*/};fn.[[Construct]]=function(){/*...*/};fn.[[HasInstance]]=function(){/*...*/};fn.[[Scope]]=control.runningEC.lexicalEnvironment;fn.[[Code]]=functionBody;fn.[[FormalParameterList]]=parameterList;=functionName;fn.length=parameterList.length;totype={constructor:fn};創(chuàng)建函數(shù)（CreateFunctionObject）作用域（[[Scope]]）是函數(shù)對象的一個(gè)屬性functionhello(){varo={};=‘GrayZhang’;returno;}varperson=hello();console.log();functionouter(){varname=‘GrayZhang’;functionsay(){alert(name);}returnsay;}varinner=outer();//inner.[[Scope]]inner();進(jìn)入函數(shù)（EnteringFunctionCode）新建一個(gè)執(zhí)行環(huán)境。根據(jù)規(guī)則設(shè)置this綁定。如果thisArg是null或undefined，設(shè)置為global。如果thisArg不是Object，設(shè)置為ToObject(thisArg)。以函數(shù)的[[Scope]]屬性為參數(shù)，NewDeclarativeEnvironment創(chuàng)建一個(gè)LexicalEnvironment對象。將當(dāng)前LexicalEnvironment設(shè)置為該值。將當(dāng)前VariableEnvironment設(shè)置為該值。開始初始化參數(shù)及函數(shù)內(nèi)聲明的變量。同一對象進(jìn)入函數(shù)（EnteringFunctionCode）varec=newExecutionContext();if(thisArg==null){thisArg=global;}if(typeofthisArg!=='object'){thisArg=ToObject(thisArg);}ec.thisBinding=thisArg;varlocalEnv=NewDeclarativeEnvironment(fn.[[Scope]]);ec.lexicalEnvironment=localEnv;ec.variableEnvironment=localEnv;initializeBinding(fn.[[Code]],fn.[[FormalParameterList]]);進(jìn)入函數(shù)（EnteringFunctionCode）執(zhí)行函數(shù)時(shí)，在作用域（[[Scope]]）的基礎(chǔ)上添加詞法環(huán)境（LexicalEnvironment）GlobalEnvironmentOuterEnvironmentCurrentEnvironment//GlobalEnvironmentfunctionouter(){//OuterEnvironmentfunctioninner(){//CurrentEnvironment}}varinner=outer();//[[Scope]]===OuterEnvironmentinner();定義綁定初始化

（DeclarationBindingInstantiation）從HostingBehavior說起……functionsayHello(name){if(!name){thrownewError();}else{varprefix='Hello';alert(prefix+name);}}functionsayHello(name){

varprefix;if(!name){thrownewError();}else{prefix='Hello';alert(prefix+name);}}定義綁定初始化

（DeclarationBindingInstantiation）遍歷FormalParameterList（參數(shù)列表），對每一項(xiàng)（參數(shù)），如果VariableEnvironment中不存在，則添加并賦值。依次遍歷源碼中每個(gè)FunctionDeclaration（函數(shù)聲明），對每一項(xiàng)（函數(shù)），如果VariableEnvironment中不存在，則添加，隨后賦值。如果VariableEnvironment中不存在arguments，則添加并賦值。依次遍歷源碼中每個(gè)VariableDeclaration（變量聲明），對每一項(xiàng)（變量），如果VariableEnvironment中不存在，則添加并賦值為undefined。定義綁定初始化

（DeclarationBindingInstantiation）functionformat(template,data){varregex=/\{(\w+)\:(\w+)\}/g;functionreplacer(match,name,type){varvalue=data[name];switch(type){case'boolean':value=!!value;break;case'html':value=encodeHTML(value);break;}returnvalue;}varhtml=template.replace(regex,replacer);returnhtml;}VariableEnvironmentarguments消化一下還有完沒完了！NO變量查找GetIdentifierReference(lex,name)從給定的LexicalEnvironment中查找是否存在該變量如果不存在，則從LexicalEnvironment的OuterEnvironment中查找依次進(jìn)行，直到OuterEnvironment為null，則返回undefined返回一個(gè)Reference對象，通過GetValue進(jìn)一步獲取變量的值變量查找functionGetIdentifierReference(lex,name){if(lex==null){returnnewReference(name,undefined);}varenvRec=lex.environmentRecords;if(envRec.hasBinding(name)){returnnewReference(name/*name*/,envRec/*base*/);}returnGetIdentifierReference(lex.outerEnvironment,name);}functionGetValue(reference){varenvRec=reference.base;returnenvRec.getValue();}大串燒進(jìn)入全局環(huán)境創(chuàng)建全局執(zhí)行環(huán)境并入棧創(chuàng)建全局環(huán)境對象全局的詞法環(huán)境對象指向該對象全局的變量環(huán)境對象指向該對象在變量環(huán)境中添加outer綁定并賦值在變量環(huán)境中添加prefix綁定在變量環(huán)境中添加inner綁定//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒創(chuàng)建outer函數(shù)創(chuàng)建一個(gè)對象設(shè)置[[Call]]、[[Construct]]、[[HasInstance]]等設(shè)置[[Scope]]為當(dāng)前詞法環(huán)境–全局環(huán)境設(shè)置[[Code]]、[[FormalParameterList]]等設(shè)置length、prototype等//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalEnvironmentouter:{[[Scope]]}inner:undefinedprefix:undefinedGlobalExecutionContextVariableEnvironmentLexicalEnvironment大串燒為prefix變量賦值在全局環(huán)境中尋找name綁定–找到得到上一步返回的Reference的base

–即全局環(huán)境的環(huán)境數(shù)據(jù)對象調(diào)用其setBinding(‘prefix’,‘Hello’)//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalEnvironmentouter:{[[Scope]]}inner:undefinedprefix:‘Hello‘GlobalExecutionContextVariableEnvironmentLexicalEnvironment大串燒執(zhí)行outer函數(shù)創(chuàng)建執(zhí)行環(huán)境并入棧創(chuàng)建一個(gè)詞法環(huán)境–DeclarativeEnvironmentouter的詞法環(huán)境對象指向該對象outer的變量環(huán)境對象指向該對象在變量環(huán)境中添加say綁定并賦值在變量環(huán)境中添加name綁定//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒創(chuàng)建say函數(shù)創(chuàng)建一個(gè)對象設(shè)置[[Call]]、[[Construct]]、[[HasInstance]]等設(shè)置[[Scope]]為當(dāng)前詞法環(huán)境–outer的詞法環(huán)境設(shè)置[[Code]]、[[FormalParameterList]]等設(shè)置length、prototype等//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;

functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalExecutionContextGlobalEnvironmentouter:{[[Scope]]}inner:undefinedprefix:‘Hello‘VariableEnvironmentLexicalEnvironmentOuterEnvironmentsay:{[[Scope]]}name:undefinedOuterExecutionContextVariableEnvironmentLexicalEnvironment大串燒為name變量賦值在outer的詞法環(huán)境中尋找name綁定–找到得到上一步返回的Reference的base

–即outer的詞法環(huán)境的環(huán)境數(shù)據(jù)對象調(diào)用其setBinding(‘name’,‘GrayZhang’)//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalEnvironmentouter:{[[Scope]]}inner:{[[Scope]]}prefix:‘Hello‘OuterEnvironmentsay:{[[Scope]]}name:‘GrayZhang’GlobalExecutionContextVariableEnvironmentLexicalEnvironmentOuterExecutionContextVariableEnvironmentLexicalEnvironment大串燒返回并賦值給inner變量將outer的ExecutionContext出棧在全局環(huán)境下尋找inner綁定–找到得到上一步返回的Reference的base–即全局環(huán)境的環(huán)境數(shù)據(jù)對象調(diào)用其setBinding(‘inner’,&say);//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalEnvironmentouter:{[[Scope]]}inner:{[[Scope]]}prefix:‘Hello‘OuterEnvironmentsay:{[[Scope]]}name:‘GrayZhang’GlobalExecutionContextVariableEnvironmentLexicalEnvironment大串燒執(zhí)行inner函數(shù)創(chuàng)建執(zhí)行環(huán)境并入棧創(chuàng)建一個(gè)詞法環(huán)境–DeclarativeEnvironmentinner的詞法環(huán)境對象指向該對象inner的變量環(huán)境對象指向該對象在變量環(huán)境中添加message綁定//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalEnvironmentouter:{[[Scope]]}inner:{[[Scope]]}prefix:‘Hello‘OuterEnvironmentsay:{[[Scope]]}name:‘GrayZhang’InnerEnvironmentmessage:undefinedGlobalExecutionContextVariableEnvironmentLexicalEnvironmentInnerExecutionContextVariableEnvironmentLexicalEnvironment大串燒為message變量賦值查找prefix變量的值在inner的詞法環(huán)境中尋找prefix綁定–沒有在outer的詞法環(huán)境中尋找prefix綁定–沒有在全局環(huán)境中尋找prefix綁定–找到取得prefix的值查找name變量的值…在inner的詞法環(huán)境中尋找message給message綁定賦值//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();大串燒GlobalEnvironmentouter:{[[Scope]]}inner:{[[Scope]]}prefix:‘Hello‘OuterEnvironmentsay:{[[Scope]]}name:‘GrayZhang’InnerEnvironmentmessage:‘HelloGrayZhang’GlobalExecutionContextVariableEnvironmentLexicalEnvironmentInnerExecutionContextVariableEnvironmentLexicalEnvironment大串燒獲取inner的值在inner的詞法環(huán)境中尋找message綁定–找到得到上一步返回的Reference的base

–即inner的詞法環(huán)境的環(huán)境數(shù)據(jù)對象調(diào)用該對象的getValue(‘message’)獲取alert的值…將inner作為參數(shù)，調(diào)用alert函數(shù)//script…varprefix=‘Hello‘;functionouter(){varname=‘GrayZhang’;functionsay(){varmessage=prefix+name;alert(message);}returnsay;}varinner=outer();//inner.[[Scope]]inner();alert從何而來？大串燒functionborn(){varname='unknown';varage=1;return{setName:function(value){name=value;},grow:function(){age++;},print:function(){varparts=[name,age];varjoint='isnow';alert(parts.join(joint));}};}vargod=born();god.setName(‘leeight’);god.grow();god.grow();god.print();總結(jié)相關(guān)概念可執(zhí)行代碼–ExecutableCode執(zhí)行環(huán)境–ExecutionContext詞法環(huán)境–LexicalEnvironment變量環(huán)境–VariableEnvironment環(huán)境數(shù)據(jù)–EnvironmentRecords總結(jié)過程創(chuàng)建函數(shù)–[[Scope]][[Scope]]在創(chuàng)建時(shí)決定且不會變化進(jìn)入函數(shù)–執(zhí)行環(huán)境+詞法環(huán)境+變量環(huán)境執(zhí)行時(shí)在最內(nèi)層增加詞法環(huán)境定義綁定初始化–參數(shù)+函數(shù)聲明+變量聲明變量環(huán)境和詞法環(huán)境是同一個(gè)對象變量查找–GetIdentifierReference延詞法環(huán)境自內(nèi)向外查找繼續(xù)消化我以為我懂了，直到……HowwithworksHowcatchworksHowletworksWhencodemeetsevalWhencodemeetsnewFunctionWhenthereisstrictmode從代碼說起functionouter(){varo=LargetObject.fromSize('400MB');returnfunction(){console.log('inner');};}varinner=outer();//對象圖此時(shí)對象之間的引用關(guān)系？GlobalFunctionLexicalEnvironmentEnvironmentRecordsBindingObjectLargeObjectinner[[Scope]]environmentRecordsbindingObjectoGlobal和o有間接引用，無法回收o但是事實(shí)上……functionouter(){vari=3;

returnfunction(){debugger;};}varinner=outer();inner();javascript引擎有能力回收i如果你是計(jì)算機(jī)……functionouter(){vari=3;varj=4;vark=5;functionprepare(){i=i+k;}functionhelp(){i=i+j;}

prepare();returnfunction(){help();console.log(i);};}varinner=outer();inner();i:不可回收j:不可回收k:可回收prepare:可回收help:不可回收人類的智商計(jì)算機(jī)的智商壓力好大~測試方法用斷點(diǎn)！Chrome/Firefox看內(nèi)存！IE/Opera一些基本結(jié)果IE6–8沒有回收閉包內(nèi)變量的機(jī)制Opera沒有回收閉包內(nèi)變量的機(jī)制Chrome回收閉包內(nèi)變量后，再次訪問該變量將拋出ReferenceErrorFirefox回收閉包內(nèi)變量后，再次訪問該變量會得到undefinedChrome、Firefox和IE9回收閉包內(nèi)變量的策略基本相同試問！有哪些因素可能導(dǎo)致變量無法回收？變量被返回的函數(shù)直接引用。變量被返回的函數(shù)間接引用（通過嵌套函數(shù)）。返回的函數(shù)中有eval。返回的函數(shù)在with表達(dá)式建立的作用域中。返回的函數(shù)在catch表達(dá)式中。只談結(jié)果，不談過程！直接引用EngineCollectableChrome–V8NOFirefox–SpiderMonkeyNOIE9-ChakraNOfunctionouter(){vari=3;returnfunction(){i;};}varinner=outer();間接引用EngineCollectableChrome–V8NOFirefox–SpiderMonkeyNOIE9-ChakraNOfunctionouter(){vari=3;functionhelp(){i;}returnfunction(){help();};}varinner=outer();嵌套函數(shù)的平衡functionouter(){vari=0;functionhelp(){i++;}help();returnfunction(){console.log('nothing');}}varinner=outer();functionouter(){vari=0;functionhelp(){i++;returninner();}functioninner(){returni>3?i:help();}returninner();}varinner=outer();需要圖的遍歷需要處理環(huán)引用高成本+低效嵌套函數(shù)的平衡EngineCollectableChrome–V8NOFirefox–SpiderMonkeyNOIE9-ChakraNOfunctionouter(){vari=3;functionhelp(){i;}returnfunction(){};}varinner=outer();大惡魔evalfunctionouter(){vari=3;returnfunction(){returneval(‘i’);}

}varinner=outer();varresult=inner();console.log(result);//3?由字符串從詞法環(huán)境中獲取對象的唯一途徑可變性特殊性大惡魔evalvarreference=eval(‘someObject’);字符串分析varreference=eval(‘some’+‘Object’);常量預(yù)計(jì)算vars=‘some’;varreference=eval(s+‘Object’);變量->常量替換vararray=[‘some’,‘ject’];varreference=eval(array.join(‘Ob’));大惡魔evalfunctionouter(){vari=3;returnfunction(variableName){returneval(variableName);}

}varinner=outer();varinput=document.getElementById(‘variable_name’);varname=input.value.trim();varresult=inner(name);console.log(result);//3囧TooSimple,SometimesNative.大惡魔evalEngineCollectableChrome–V8NOFirefox–SpiderMonkeyNOIE9-ChakraNOfunctionouter(){vari=3;returnfunction(){eval(‘’);//無論eval的內(nèi)容是什么};}varinner=outer();間接eval和newFunction間接evalwindow.eval(coe)|(1,eval)(code)|(true&&eval)(code)InEdition5,indirectcallstotheevalfunctionusetheglobalenvironmentasboththevariableenvironmentandlexicalenvironmentfortheevalcode.newFunctionReturnanewFunctionobjectcreatedasspecifiedin13.2passingPastheFormalParameterListandbodyastheFunctionBody.PassintheGlobalEnvironmentastheScopeparameterandstrictastheStrictflag.間接eval和newFunctionvari=3;functionouter(){vari=4;returnfunction(){returnwindow.eval('i');/**varfn=newFunction('returni;');*returnfn();*/}}varinner=outer();varresult=inner();console.log(result);//3X間接eval和newFunctionEngineCollectabl