to err is human

To Err Is HumanAlex Liu @stinkydofu [email protected]

avoidance acceptance

The Road To Happiness

Handling Errors and Exceptions Preserving History The Problem With Catch

Handling Errors and Exceptions

1

Error

> new Error();

kinds of errorstwo

Expected

Expected

invalid user input bad JSON input file not found request timeout request 500

Unexpected

Unexpected

programmer typos read property of undefined

undefined is not a function

How do I communicate expected errors?

Exception

> throw new Error();

an exception is a

thrown error

You could throw anything

> throw { foo: bar }; > throw 0; > throw 'my error';

> throw 0; undefined > throw 'something happened'; something happened > throw new Error('something happened'); Error: something happened at repl:1:7 at REPLServer.defaultEval (repl.js:132:27) at bound (domain.js:254:14) at REPLServer.runBound [as eval] (domain.js:267:12) at REPLServer. (repl.js:279:12) at REPLServer.emit (events.js:107:17) at REPLServer.Interface._onLine (readline.js:214:10) at REPLServer.Interface._line (readline.js:553:8) at REPLServer.Interface._ttyWrite (readline.js:830:14)

index.js

index.js data.js get.js


throw!


throw!catch?

catch (e) {// run!

}


throw!

CRASH! throw!


throw / try / catch

try { var price = getStockPrice('NFLX'); document.write('NFLX: ' + price); } catch(err) { // err instanceof Error => true document.write('NFLX: unavailable'); }

Can we catch?

try { getStockPrice('NFLX', function(err) { if (err) { throw (err); } /* continue as normal */ }); } catch(err) { /* error handling */ }

Error first callbacks (errbacks)

getStockPrice('NFLX', function(err, price) { if (err) { /* handle err! */ } document.write('NFLX: ' + price); });

Error first callbacks (errbacks)

getStockPrice('NFLX', function(err, price) { if (err) { document.write('NFLX: unavailable'); return; } document.write('NFLX: ' + price); });

Bubble up if you cant handle it

function drawStockPrice(callback) { getStockPrice('NFLX', function(err, price) { return callback(err); }); }

index.js


cb(err)


cb(err) cb(err)

if (err) {...

}


cb(err) cb(err)

SWALLOWED! cb(err)cb(err)


errbacks are a

channelfor expected errors

> throw err;

> callback(err);

sync

async

nopenope

nopenope

nopenope

nopenope

nope

nopenope

nope

nope

nopenope

nope

nope

throw / try / catchwhen in async env

sanelyyou cant

favor errbacks because of

consistency

How do I communicate unexpected errors?

unexpected errors are thrown

automatically

process.on('uncaughtException', function(err) { ... });

Catch em all, even async



window.onerror = function(err) { ... });



window.onerror = function(err) { ... });_BAD

IDEA_

Can you recover? function dispatch(doThings) { if (!Array.isArray(doThings)) { throw new TypeError('not an array'); } for (var i = 0; i < doThings.length; i++) { if (typeof doThings[i] !== 'function') { throw new TypeError('not a function'); } } for (var i = 0; i < doThings.length; i++) { doThings[i](); } }

Catch and do what?

for (var i = 0; i < doThings.length; i++) { try { doThings[i](); } catch (e) { ... } }

programmer err : rollback ::

programmer err : rollback ::

rollback err : ?

you cant fix unexpected errors because their existence is

unexpected

in node.js,

aborton unexpected errors

goal is to the blast radius

(not to write bug free code)

minimize

continuing after an

unhandled exception is a

trade off

_BROKEN_

program error : program crash ::


OS error : ?


OS error : OS crash

in the browser,

reportall unexpected errors

avoidhandling errors in global exception handlers

Preserving History 2

Working backward

function getEpicStory(cb) { database.get('iliad', function(err, data) { if (err) { return cb(err); } return cb(null, data); }); }

We can do better

database.get('iliad', function(err, data) { if (err) { err.cause = 'db error!'; err.timestamp = new Date(); return cb(err); } return cb(null, data); });

VError: we can do even better

database.get('iliad', function(err, data) { if (err) { return cb(new VError(err, 'db error!')); } return cb(null, data); });

VError: we can do even better

// redis error [Error: missing key] // our verror { [VError: db error: missing key] jse_shortmsg: 'db error', jse_summary: 'db error: missing key', jse_cause: [Error: missing key], message: 'db error: missing key' }

[2015-10-02T06:41:54.389Z] ERROR: test/11497 on lgml-aliu: db error: missing key VError: db error: missing key at Object. (/Users/aliu/Desktop/test/a.js:6:12) at Module._compile (module.js:434:26) at Object.Module._extensions..js (module.js:452:10) at Module.load (module.js:355:32) at Function.Module._load (module.js:310:12) at Function.Module.runMain (module.js:475:10) at startup (node.js:117:18) at node.js:951:3 Caused by: Error: missing key at Object. (/Users/aliu/Desktop/test/a.js:5:12) at Module._compile (module.js:434:26) at Object.Module._extensions..js (module.js:452:10) at Module.load (module.js:355:32) at Function.Module._load (module.js:310:12) at Function.Module.runMain (module.js:475:10) at startup (node.js:117:18) at node.js:951:3

restify-errors: the bees knees

var errs = require('restify-errors');

errs.makeConstructor('DatabaseError', { fooProp: 'someVal' });

var myErr = new errs.DatabaseError('db error!'); // myErr.fooProp => someVal

restify-errors: the bees knees

var myErr = new errs.DatabaseError('db error!'); myErr instanceof errs.DatabaseError // => true myErr instanceof VError // => true myErr instanceof Error // => true

Serving out the UI

request('/api/story', function(err, fpStoryData) { if (err instanceof HttpError) { return res.render('NetworkErrorPage'); } else if (err instanceof DatabaseError) { return res.render('UnavailablePage'); } else { return res.render('ErrorPage'); } res.render('iliad', fpStoryData); });

[2015-10-06T17:50:31.561Z] ERROR: test/64987 on lgml-aliu: rendering error! RenderError: rendering error page! at Object. (/Users/aliu/Desktop/test/test4.js:10:10) at Module._compile (module.js:460:26) at Object.Module._extensions..js (module.js:478:10) at Module.load (module.js:355:32) at Function.Module._load (module.js:310:12) at Function.Module.runMain (module.js:501:10) at startup (node.js:129:16) at node.js:814:3 Caused by: DataValidationError: data validation error at Object. (/Users/aliu/Desktop/test/test4.js:10:32) at Module._compile (module.js:460:26) at Object.Module._extensions..js (module.js:478:10) at Module.load (module.js:355:32) at Function.Module._load (module.js:310:12) at Function.Module.runMain (module.js:501:10) at startup (node.js:129:16) at node.js:814:3 Caused by: RequestError: request error at Object. (/Users/aliu/Desktop/test/test4.js:10:63) at Module._compile (module.js:460:26) at Object.Module._extensions..js (module.js:478:10) at Module.load (module.js:355:32) at Function.Module._load (module.js:310:12) at Function.Module.runMain (module.js:501:10) at startup (node.js:129:16) at node.js:814:3 Caused by: BadRequestError: http 400 at Object. (/Users/aliu/Desktop/test/test4.js:10:86) at Module._compile (module.js:460:26) at Object.Module._extensions..js (module.js:478:10)

[2015-10-06] ERROR: test/64987 on lgml-aliu: rendering error!

Error: something happened at repl:1:7 at REPLServer.defaultEval (repl.js:132:27) at bound (domain.js:254:14) at REPLServer.runBound [as eval] (domain.js:267:12) at REPLServer. (repl.js:279:12) at REPLServer.emit (events.js:107:17) at REPLServer.Interface._onLine (readline.js:214:10) at REPLServer.Interface._line (readline.js:553:8) at REPLServer.Interface._ttyWrite (readline.js:830:14) at Module.load (module.js:355:32) at Function.Module._load (module.js:310:12) at Function.Module.runMain (module.js:501:10) at startup (node.js:129:16) at node.js:814:3

use

for expected errors

typed errors

3The problem with catch

But {abstraction} solved this already!?

try: catch em all

try { if (Math.random() > 0.5) { JSON.parse(input); } else { var x = y + 1; } } catch (err) {...}

> [SyntaxError: Unexpected token b]

> [ReferenceError: y is not defined]

expected:

unexpected:

JSON.parse(input);

var x = y + 1;

Catching typed errors

catch (err) { if (err instanceof SyntaxError) { ... } }

Dont forget to rethrow!

catch (err) { if (err instanceof SyntaxError) { ... } else { throw err; } }

Centralize throwing

catch (err) { throwUnexpected(err); // handle err if not thrown }

expected vs unexpected?

catch (err) { if (err instanceof SyntaxError) { ... } else { throw err; } }

expected vs unexpected?

> JSON.parse('bad input');

> vart x;

try/catch cannot

differentiatesource of errors

try: reduce surface area

try { JSON.parse(input); } catch (err) { ... }

var x = y + 1;

be very

targetedwith try / catch

promises: catch em all

foo.then(foo2) .then(foo3) .catch(function(err) { ... });

promises: unexpected behavior

foo.then(foo2) .then(fooError) .catch(function(err) { if (!(err instanceof ExpectedErr)) { throw err; } });

$ node promiseDemo.js Unhandled rejection ReferenceError: y is not defined at foo (/test/promiseDemo.js:14:13) at /test/promiseDemo.js:8:3 at processImmediate [as _immediateCallback] (timers.js:368:17) From previous event: at Object. (/test/promiseDemo.js:7:3) at Module._compile (module.js:435:26) at Object.Module._extensions..js (module.js:442:10) at Module.load (module.js:356:32) at Function.Module._load (module.js:311:12) at Function.Module.runMain (module.js:467:10) at startup (node.js:134:18) at node.js:961:3

$ echo $?

$ echo $?0

promises: unexpected behavior

foo.then(foo2) .then(fooError) .catch(function(err) { if (!(err instanceof ExpectedErr)) { throw err; } });

promises: call done()

foo.then(foo2) .then(fooError) .catch(function(err) { if (!(err instanceof ExpectedErr)) { throw err; } }) .done();

bluebird: catch only expected

foo.then(foo2) .then(fooError) .catch(ExpectedError, function(err) { ... }); // EVERYTHING else throws, exits 1!

bluebird: throw by default

Promise.onPossiblyUnhandledRejection(function(e) { throw e; });

surfacing unexpected errors should NOT be

opt-in

Making Catch Better

You don't need to sell me -- we had } catch (e if ...) { in SpiderMonkey and proposed it for ES3 (in 1998).

- Brendan Eich

try { ... } catch (e if ...) { ... }

Refutable Pattern Matching

GuardedPattern(refutable) ::= Pattern(refutable) "if" AssignmentExpression

Looking Forward

for rich error objects

5 npm install verror npm install restify-errors

Handle every error from every async API

4

Understand how your abstractions handle errors

3

(swallow? rethrow?)

Abort immediately on unexpected errors to minimize the blast radius

2

Design and build applications with error handling in mind

1

(exceptions are NOT exceptional!)

Error handling bible https://www.joyent.com/developers/node/

design/errors

https://www.joyent.com/developers/node/design/errors

Aborting on unexpected errors https://github.com/nodejs/node-v0.x-archive/

issues/5114 https://github.com/nodejs/node-v0.x-archive/

issues/5149

https://github.com/nodejs/node-v0.x-archive/issues/5114https://github.com/nodejs/node-v0.x-archive/issues/5149

Module up! verror

https://github.com/davepacheco/node-verror restify-errors

https://github.com/restify/errors

https://github.com/davepacheco/node-verrorhttps://github.com/restify/errors

Get involved! https://esdiscuss.org/topic/try-catch-conditional-

exceptions-in-light-of-generators http://wiki.ecmascript.org/doku.php?

id=strawman:pattern_matching

https://esdiscuss.org/topic/try-catch-conditional-exceptions-in-light-of-generatorshttp://wiki.ecmascript.org/doku.php?id=strawman:pattern_matching

FinAlex Liu @stinkydofu [email protected]

Image Credits

Image Credits http://lizclimo.tumblr.com/post/77531229510/youre-doing-it-wrong http://fantasio.deviantart.com/art/Godzilla-in-the-mountains-454304871 http://rockpapercynic.tumblr.com/post/85581052929/first-revealed-pagew-your-bad-

idea-illustration

http://lizclimo.tumblr.com/post/77531229510/youre-doing-it-wronghttp://fantasio.deviantart.com/art/Godzilla-in-the-mountains-454304871http://rockpapercynic.tumblr.com/post/85581052929/first-revealed-pagew-your-bad-idea-illustration

to err is human

Software