JAVASCRIPT ERROR HANDLING FROM A TALK BY @XJAMUNDX AT #MWRC

TRY/CATCH • Use for JSON.parse • Don’t use like a Java programmer

DON’T THROW • Only as a last resort • Nothing else you can do

THROW - TJ FONTAINE • throw for programmer errors (eg, missing required parameter) • https://us- east.manta.joyent.com/ dap/public/drop/er2.htm

ALWAYS USE ERROR OBJECTS • Not simply strings • Errors contains stack traces • Will provide debug value

CREATE CUSTOM ERRORS • Extend Error - still of type Error • New, specific type • Can attach other helpful properties (eg, data associated at time of error) • Other standard properties in Appendix: https://us- east.manta.joyent.com/dap/public/drop/er2.htm

CREATE CUSTOM ERRORS (2) ! function SpecialError(message, specialInfo) { Error.captureStackTrace(this, arguments.callee) this.message = message this.name = ‘SpecialError' this.specialInfo = specialInfo } ! SpecialError.prototype = Object.create(Error.prototype)

CALLBACK W/ ERROR • Common pattern in Node • If action creates error, pass as first parameter in callback

CALLBACK W/ ERROR (2) function doManyThings(done) { doAsync(function (err, data) { if (err) return done(err) // … done(null, data) }) }

NAME ANONYMOUS FUNCTIONS • Provides name in stack trace

NAME ANONYMOUS FUNCTIONS (2) doSomethingWithCallback(function veryCallback() { // if I throw an error, stack trace <3 })

DOMAINS AREN’T COOL • core team is not pushing the concept • https:// nodefirm.hackpad.com/ Node-Error-Handling- Summit-uXFi4FUg8Td

MAKE ERRORS TO CLIENTS CONSISTENT • Pass all errors through a common error serializer • Client can also handle consistently

MAKE ERRORS TO CLIENTS CONSISTENT (2) res.json(400, formatError(err))

USE EXPRESS DEFAULT ERROR HANDLER • Good catch-all • Avoid potential infinite loop bug in Express

USE EXPRESS DEFAULT ERROR HANDLER (2) app.use(function (err, req, res, next) { res.json(500, formatError(err)) })

ON(‘UNCAUGHTEXCEPTION’) SHOULD ALWAYS EXIT • Call process.exit() • Otherwise, stuff hangs • Long-running process might never complete for client; wait for timeout • Resources can be leaked (eg, db connections)

ON(‘UNCAUGHTEXCEPTION’) SHOULD ALWAYS EXIT (2) process.on('uncaughtException', function(err) { hurryAndWriteYourWill(err) process.exit() });

RESOURCES • MWRC SLIDES FROM @XJAMUNDX 
 HTTPS://CLOUDUP.COM/IHRJZBVDIFZ ! • CORE TEAM NOTES
 HTTPS://NODEFIRM.HACKPAD.COM/NODE-ERROR-HANDLING-SUMMIT-UXFI4FUG8TD ! • TJ FONTAINE ERROR HANDLING
 HTTPS://US-EAST.MANTA.JOYENT.COM/DAP/PUBLIC/DROP/ER2.HTM

https://speakerdeck.com/jaketrent/javascript-error-handling