捕获页面中全局Javascript异常

捕获页面中全局Javascript异常主题UglifyJS一个流量巨大的前端页面面临的浏览器环境是非常复杂的，尤其是移动端页面(Android的碎片化所致)。面对如此多样的浏览器环境，常规的测试是无法完全覆盖的，我们需要一种页面脚本异常监控机制作为补充，保证能够发现前端页面脚本异常的原因。有很多种情况会导致Javascript抛出异常，包括网络失效、语法错误、运行时错误等。我们希望在页面上有异常发生时，能够获得脚本错误的基本信息、文件url、行号。接下来我们探讨几种实现方式。1使用window.onError浏览器提供了全局的onError函数，我们可以使用它搜集页面上的错误window.onerror=function(message,source,lineno,colno,error){...}其中mesage为异常基本信息，source为发生异常Javascript文件url，lineno为发生错误的行号，我们可以通过error.stack获取异常的堆栈信息。下面是chrome中通过window.onError捕获的错误例子：message:UncaughtReferenceError:testisnotdefinedsource:/release/attach.jslineno:16144colno:6error:ReferenceError:testisnotdefinedat/release/attach.js:16144:6atHTMLDocument.<anonymous>(/release/vendor.js:654:71)这种方式看似完美，其实有一个致命的问题。有些浏览器为了安全方面的考虑，对于不同域的Javascript文件，通过window.onError无法获取有效的错误信息。比如firefox的错误消息只有Scripterror，而且无法获得确切的行号，更没有错误堆栈信息：message:Scripterror.source:"/release/attach.jslineno:0colno:0error:null为了使得浏览器针对window.onError的跨域保护失效,我们可以在静态资源服务器或者CDN的HTTP头中加上如下允许跨域提示：Access-Control-Allow-Origin:*并在引用Javascript脚本是加上crossorigin属性:<scriptcrossoriginsrc=""></script>完成上述两步后，我们就可以方便的使用window.onError进行全局异常捕获，并获取丰富的异常信息了。但是有时对于第三方的CDN，我们无法添加跨域相关的头信息，下面我们就讨论针这种情况的全局Javascript异常捕获方法。2使用AST为所有函数加上trycatch上文中提到了使用window.onError进行浏览器全局异常捕获，但是当我们无法添加跨域相关头信息时，window.onError就失效了。针对这种情况，我们可以对每一个函数添加trycatch来捕获函数内的异常，但是一个大型项目的函数太多，对每一个函数都手动添加trycatch无疑是一个巨大的工作量。本文我们借助AST（抽象语法树）技术，对源文件进行预处理，对每个函数自动的添加trycatch。语法树是对源代码最精确的表示，通过遍历和操作语法树，我们能够精确的控制源代码。生成JavaScript的AST是一件非常复杂的工作，本文暂时不打算涉及，好在UglifyJS已经有了完整的实现。比如如下代码：functiontest(){vara=1;varb=2;console.log(a+b);}可以用语法树表示：通过使用Uglify提供的操作AST(抽象语法树)的API，我们可以对每个函数添加trycatch代码块，并在catch中捕获该函数的一切异常，下面是我的实现（请参考我的github：try-catch-global.js）：varfs=require('fs');var_=require('lodash');varUglifyJS=require('uglify-js');varisASTFunctionNode=function(node){returnnodeinstanceofUglifyJS.AST_Defun||nodeinstanceofUglifyJS.AST_Function;}varglobalFuncTryCatch=function(inputCode,errorHandler){if(!_.isFunction(errorHandler)){throw'errorHandlershouldbeavalidfunction';}varerrorHandlerSource=errorHandler.toString();varerrorHandlerAST=UglifyJS.parse('('+errorHandlerSource+')(error);');vartryCatchAST=UglifyJS.parse('try{}catch(error){}');varinputAST=UglifyJS.parse(inputCode);vartopFuncScope=[];//将错误处理函数包裹进入catch中tryCatchAST.body[0].bcatch.body[0]=errorHandlerAST;//搜集所有函数varwalker=newUglifyJS.TreeWalker(function(node){if(isASTFunctionNode(node)){topFuncScope.push(node);}});inputAST.walk(walker);//对函数进行变换,添加trycatch语句vartransfer=newUglifyJS.TreeTransformer(null,function(node){if(isASTFunctionNode(node)&&_.includes(topFuncScope,node)){//函数内部代码搜集varstream=UglifyJS.OutputStream();for(vari=0;i<node.body.length;i++){node.body[i].print(stream)}varinnerFuncCode=stream.toString();//清除trycatch中定义的多余语句tryCatchAST.body[0].body.splice(0,tryCatchAST.body[0].body.length);//用trycatch包裹函数代码varinnerTyrCatchNode=UglifyJS.parse(innerFuncCode,{toplevel:tryCatchAST.body[0]});//获取函数壳node.body.splice(0,node.body.length);//生成有trycatch的函数returnUglifyJS.parse(innerTyrCatchNode.print_to_string(),{toplevel:node});}});inputAST.transform(transfer);varoutputCode=inputAST.print_to_string({beautify:true});returnoutputCode;}module.exports.globalFuncTryCatch=globalFuncTryCatch;借助于globalFuncTryCatch，我们对每个函数进行自动化地添加trycatch语句，并使用自定义的错误处理函数：globalFuncTryCatch(inputCode,function(error){//此处是异常处理代码，可以上报并记录日志console.log(error);});通过将globalFuncTryCatch功能集成到构建工具中，我们就可以对目标Javascript文件进行trycatch处理。综上所述：当静态资源服务器可以添加Access-Control-Allow-Origin:*时，我们可以直接使用window.onError进行全局异常捕获；当静态资源服务器不受控制，window.onError失效，我们可以借助AST技术，自动化地对全部目标Javascript函数添加trycatch来捕获所有异常。参考文档：CaptureandreportJavaScripterrorswithwindow.onerroronerrorisaspecialbrowsereventthatfireswheneveranuncaughtJavaScripterrorhasbeenthrown.It’soneoftheeasiestwaystologclient-sideerrorsandreportthemtoyourservers.It’salsooneofthemajormechanismsbywhichSentry’sclientJavaScriptintegration(raven-js)works.Youlistentotheonerroreventbyassigningafunctiontowindow.onerror:window.onerror=function(msg,url,lineNo,columnNo,error){//...handleerror...returnfalse;}Whenanerroristhrown,thefollowingargumentsarepassedtothefunction:msg–Themessageassociatedwiththeerror,e.g.“UncaughtReferenceError:fooisnotdefined”url–TheURLofthescriptordocumentassociatedwiththeerror,e.g.“/dist/app.js”lineNo–Thelinenumber(ifavailable)columnNo–Thecolumnnumber(ifavailable)error–TheErrorobjectassociatedwiththiserror(ifavailable)Thefirstfourargumentstellyouinwhichscript,line,andcolumntheerroroccurred.Thefinalargument,Errorobject,isperhapsthemostvaluable.Let’slearnwhy.TheErrorobjectanderror.stackAtfirstglancetheErrorobjectisn’tveryspecial.Itcontains3standardizedproperties:message,fileName,andlineNumber.Redundantvaluesthatalreadyprovidedtoyouviawindow.onerror.Thevaluablepartisanon-standardproperty:Etotype.stack.Thisstackpropertytellsyouatwhatsourcelocationeachframeoftheprogramwaswhentheerroroccurred.Thestacktracecanbeacriticalpartofdebugginganerror.Anddespitebeingnon-standard,thispropertyisavailableineverymodernbrowser.Here’sanexampleoftheErrorobject’sstackpropertyinChrome46:"Error:foobar\natnewbar(<anonymous>:241:11)\natfoo(<anonymous>:245:5)\nat<anonymous>:250:5\nat<anonymous>:251:3\nat<anonymous>:267:4\natcallFunction(<anonymous>:229:33)\nat<anonymous>:239:23\nat<anonymous>:240:3\natObject.InjectedScript._evaluateOn(<anonymous>:875:140)\natObject.InjectedScript._evaluateAndWrap(<anonymous>:808:34)"Hardtoread,right?Thestackpropertyisactuallyjustanunformattedstring.Here’swhatitlookslikeformatted:Error:foobaratnewbar(<anonymous>:241:11)atfoo(<anonymous>:245:5)atcallFunction(<anonymous>:229:33)atObject.InjectedScript._evaluateOn(<anonymous>:875:140)atObject.InjectedScript._evaluateAndWrap(<anonymous>:808:34)Onceit’sbeenformatted,it’seasytoseehowthestackpropertycanbecriticalinhelpingtodebuganerror.There’sjustonesnag:thestackpropertyisnon-standard,anditsimplementationdiffersamongbrowsers.Forexample,here’sthesamestacktracefromInternetExplorer11:Error:foobaratbar(Unknownscriptcode:2:5)atfoo(Unknownscriptcode:6:5)atAnonymousfunction(Unknownscriptcode:11:5)atAnonymousfunction(Unknownscriptcode:10:2)atAnonymousfunction(Unknownscriptcode:1:73)Notonlyistheformatofeachframedifferent,theframesalsohavelessdetail.Forexample,Chromeidentifiesthatthenewkeywordhasbeenused,andhasgreaterinsightintoevalinvocations.AndthisisjustIE11vsChrome–otherbrowserssimilarhavevaryingformatsanddetail.Luckily,therearetoolsouttherethatnormalizestackpropertiessothatitisconsistentacrossbrowsers.Forexample,raven-jsusesTraceKittonormalizeerrorstrings.There’salsostacktrace.jsandafewotherprojects.Browsercompatibilitywindow.onerrorhasbeenavailableinbrowsersforsometime–you’llfinditinbrowsersasoldasIE6andFirefox2.Theproblemisthateverybrowserimplementswindow.onerrordifferently.Particularly,inhowmanyargumentsaresenttototheonerrorlistener,andthestructureofthosearguments.Here’satableofwhichargumentsarepassedtoonerrorinmostbrowsers:Browser Message URL lineNo colNo errorObjFirefox42 Chrome46 AndroidBrowser4.4 Edge IE11 IE10 IE9,8 Safari9 iOS9 You’llnoticethatthelatestApplebrowsers–SafariandiOS–don’tsupporta5therrorobjectargument.AndwhilethefinalversionofInternetExplorer(11)supportstheerrorobject,Microsoft’slatestbrowser,Edge,doesnot.Withouttheerrorobject,thereisnostacktraceproperty.Thismeansthatthesebrowserscannotretrievevaluablestackinformationfromerrorscaughtbyonerror.Polyfillingwindow.onerrorwithtry/catchButthereisaworkaround–youcanwrapcodeinyourapplicationinsideatry/catchandcatchtheerroryourself.Thiserrorobjectwillcontainourcovetedstackpropertyineverymodernbrowser.Considerthefollowinghelpermethod,invoke,whichcallsafunctiononanobjectwithanarrayofarguments:functioninvoke(obj,method,args){returnobj[method].apply(this,args);}invoke(Math,'max',[1,2]);//returns2Here’sinvokeagain,thistimewrappedintry/catch,inordertocaptureanythrownerror:functioninvoke(obj,method,args){try{returnobj[method].apply(this,args);}catch(e){captureError(e);//reporttheerrorthrowe;//re-throwtheerror}}invoke(Math,'highest',[1,2]);//throwserror,nomethodMath.highestOfcourse,doingthismanuallyeverywhereisprettycumbersome.Youcanmakeiteasierbycreatingagenericwrapperutilityfunction:functionwrapErrors(fn){//don'twrapfunctionmorethanonceif(!fn.__wrapped__){fn.__wrapped__=function(){try{returnfn.apply(this,arguments);}catch(e){captureError(e);//reporttheerrorthrowe;//re-throwtheerror}};}returnfn.__wrapped__;}varinvoke=wrapErrors(function(obj,method,args){returnobj[method].apply(this,args);});invoke(Math,'highest',[1,2]);//nomethodMath.highestBecauseJavaScriptissinglethreaded,youdon’tneedtousewrapeverywhere–justatthebeginningofeverynewstack.Thatmeansyou’llneedtowrapfunctiondeclarations:Atthestartofyourapplication(e.g.in$(document).readyifyouusejQuery)Ineventhandlers,e.g.addEventListeneror$.fn.clickTimer-basedcallbacks,e.g.setTimeoutorrequestAnimationFrameForexample:$(wrapErrors(function(){//applicationstartdoSynchronousStuff1();//doesn'tneedtobewrappedsetTimeout(wrapErrors(function(){doSynchronousStuff2();//doesn'tneedtobewrapped});$('.foo').click(wrapErrors(function(){doSynchronousStuff3();//doesn'tneedtobewrapped});}));Ifthatseemslikeaheckofalotofwork,don’tworry!Mosterrorreportinglibrarieshavemechanismsforaugmentingbuilt-infunctionslikeaddEventListenerandsetTimeoutsothatyoudon’thavetocallawrappingutilityeverytimeyourself.Andyes,raven-jsdoesthistoo.TransmittingtheerrortoyourserversOkay,soyou’vedoneyourjob–you’vepluggedintowindow.onerror,andyou’readditionallywrappingfunctionsintry/catchinordertocatchasmucherrorinformationaspossible.There’sjustonelaststep:transmittingtheerrorinformationtoyourservers.Inorderforthistowork,you’llneedtosetupsomekindofreportingwebservicethatwillacceptyourerrordataoverHTTP,logittoafileand/storeitinadatabase.Ifthiswebserviceisonthesamedomainasyourwebapplication,thisisachievedeasilybyusingXMLHttpRequest.Intheexamplebelow,weusejQuery’sAJAXfunctiontotransmitthedatatoourservers:functioncaptureError(ex){varerrorData={name:,//e.g.ReferenceErrormessage:ex.line,//e.g.xisundefinedurl:document.location.href,stack:ex.stack//stacktracestring;remember,differentper-browser!};$.post('/logger/js/',{data:errorData});}Notethatifyouhavetotransmityourerroracrossdifferentorigins,yourreportingendpointwillneedtosupportCORS(CrossOriginResourceSharing).SummaryIfyou’vemadeitthisfar,younowhaveallthetoolsyouneedtorollyourownbasicerrorreportinglibraryandintegrateitwithyourapplication:Howwindow.onerrorworks,andwhatbrowsersitsupportsHowtousetry/catchtocapturestacktraceswherewindow.onerrorislackingTransmittingerrordatatoyourserversOfcourse,ifyoudon’twanttobotherwithallofthis,thereareplentyofcommercialandopensourcetoolsthatdoalltheheavy-liftingofclient-sidereportingforyou.(Psst,youmightwanttotrySentry.)That’sit!Happyerrorhunting.SharethisonTwitterFacebookHackerNewsTheweb'scheckenginelight.Sentryprovidesreal-timecrashreportingforyourwebapps,mobileapps,andgames.TrySentryfreefor14daysGlobalEventHandlers.onerrorINTHISARTICLESyntaxwindow.onerrorelement.onerrorNotesSpecificationsBrowsercompatibilitySeealsoAneventhandlerfortheerrorevent.Erroreventsarefiredatvarioustargetsfordifferentkindsoferrors:WhenaJavaScriptruntimeerror(includingsyntaxerrors)occurs,anerroreventusinginterfaceErrorEventisfiredatwindowandwindow.onerror()isinvoked.Whenaresource(suchasan<img>or<script>)failstoload,anerroreventusinginterfaceEventisfiredattheelement,thatinitiatedtheload,andtheonerror()handlerontheelementisinvoked.Theseerroreventsdonotbubbleuptowindow,but(atleastinFirefox)canbehandledwithasinglecapturingwindow.addEventListener.Installingaglobalerroreventhandlerisusefulforautomatedcollectionoferrorreports.SyntaxForhistoricalreasons,differentargumentsarepassedtowindow.onerrorandelement.onerrorhandlers.window.onerrorwindow.onerror=function(message,source,lineno,colno,error){...}Functionparameters:message:errormessage(string).Availableasevent(sic!)inHTMLonerror=""handler.source:URLofthescriptwheretheerrorwasraised(string)lineno:Linenumberwhereerrorwasraised(number)colno:Columnnumberforthelinewheretheerroroccurred(number)error:ErrorObject(object)Whenthefunctionreturnstrue,thispreventsthefiringofthedefaulteventhandler.element.onerrorelement.onerror=function(event){...}element.onerroracceptsafunctionwithasingleargumentoftypeEvent.NotesWhenasyntax(?)erroroccursinascript,loadedfromadifferentorigin,thedetailsofthesyntaxerrorarenotreportedtopreventleakinginformation(seebug363897).Insteadtheerrorreportedissimply"Scripterror."Thisbehaviorcanbeoverrideninsomebrowsersusingthecrossoriginattributeon<script>andhavingtheserversendtheappropriateCORSHTTPresponseheaders.Aworkaroundistoisolate"Scripterror."andhandleitknowingthattheerrordetailisonlyviewableinthebrowserconsoleandnotaccessibleviaJavaScript.window.onerror=function(msg,url,lineNo,columnNo,error){varstring=msg.toLowerCase();varsubstring="scripterror";if(string.indexOf(substring)>-