Building complex async applications is really hard. Whether you use callbacks, Promises, or EventEmitters, Error objects should have a place in your utility belt. They are indispensable when it comes to managing work flows in a highly asynchronous environment. This talk covers patterns for using JavaScript Error (with a capital E) objects to build resilient applications, and introduce some modules that can be used to build errors with an elegant history of stack traces even through multiple asynchronous operations. Try/catch, callbacks, and other error handling mechanisms will be examined, revealing some potential deficiencies in the JavaScript language for dealing with errors. Video: https://www.youtube.com/watch?v=PyCHbi_EqPs

1. To Err Is Human
Alex Liu
@stinkydofu
aliu@netflix.com

5. ☐ avoidance
☐ acceptance

6. ☐ avoidance
☐ acceptance☑

7. The Road To Happiness
‣ Handling Errors and Exceptions
‣ Preserving History
‣ The Problem With Catch

8. Handling Errors and
Exceptions
1

9. Error
> new Error();

10. kinds of errors
two

11. Expected

12. Expected

13. Expected
‣ invalid user input
‣ bad JSON input
‣ file not found
‣ request timeout
‣ request 500

14. Unexpected

15. Unexpected

16. Unexpected
‣ programmer typos
‣ read property of
undefined
‣ undefined is not a
function

17. How do I
communicate
expected errors?

18. Exception
> throw new Error();

19. an exception is a
thrown error

20. You could throw anything
> throw { foo: bar };
> throw 0;
> throw 'my error';

22. > 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.<anonymous> (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)

23. index.js

24. index.js data.js get.js

25. index.js data.js get.js
throw!

26. index.js data.js get.js
throw!
catch?

27. index.js data.js get.js
throw!catch?

28. index.js data.js get.js
throw!catch?

29. catch (e) {
// run!
}
index.js data.js get.js
throw!

30. CRASH! throw!
index.js data.js get.js

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

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

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

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

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

36. Bubble up if you can’t handle it
function drawStockPrice(callback) {
getStockPrice('NFLX', function(err, price) {
return callback(err);
});
}

37. index.js

38. index.js data.js get.js
cb(err)

39. index.js data.js get.js
cb(err) cb(err)

40. if (err) {
...
}
index.js data.js get.js
cb(err) cb(err)

41. SWALLOWED! cb(err)cb(err)
index.js data.js get.js

42. errbacks are a
channel
for expected errors

43. > throw err;
> callback(err);
sync
async

45. nope
nope
nope
nope
nope
nope
nopenope
nope
nopenope
nope
nope
nope
nope
nope
nope

46. throw / try / catch
when in async env
sanely
you can’t

47. favor
errbacks
because of
consistency

48. How do I
communicate
unexpected errors?

49. unexpected errors are
thrown
automatically

50. process.on('uncaughtException', function(err) {
...
});
Catch ‘em all, even async

51. process.on('uncaughtException', function(err) {
...
});
Catch ‘em all, even async
window.onerror = function(err) {
...
});

52. process.on('uncaughtException', function(err) {
...
});
Catch ‘em all, even async
window.onerror = function(err) {
...
});
_BAD IDEA_

53. 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]();
}
}

54. 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]();
}
}
<----------------------------------- (1)

55. 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]();
}
}
<----------------------------------- (1)
<------------------------------- (2)

56. 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]();
}
}
<----------------------------------- (1)
<------------------------------- (2)
<-------------------------------------------------------- (3)

57. Catch and do what…?
for (var i = 0; i < doThings.length; i++) {
try {
doThings[i]();
} catch (e) {
...
}
}

58. Catch and do what…?
for (var i = 0; i < doThings.length; i++) {
try {
doThings[i]();
} catch (e) {
...
}
}
<--------------------- what goes here?

59. programmer err : rollback
::

60. programmer err : rollback
::
rollback err : ?

61. you can’t fix unexpected
errors because their
existence is
unexpected

63. in node.js,
abort
on unexpected errors

64. goal is to
the blast radius
(not to write bug free code)
minimize

65. continuing after an
unhandled exception is a
trade off

67. _BROKEN_

68. program error : program crash
::

69. program error : program crash
::
OS error : ?

70. program error : program crash
::
OS error : OS crash

71. in the browser,
report
all unexpected errors

72. avoid
handling errors in global
exception handlers

73. Preserving History 2

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

80. 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);
});

81. 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);
});

82. 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' }

83. [2015-10-02T06:41:54.389Z] ERROR: test/11497 on lgml-aliu: db error: missing key
VError: db error: missing key
at Object.<anonymous> (/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.<anonymous> (/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

84. 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

85. 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

86. 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);
});

87. [2015-10-06T17:50:31.561Z] ERROR: test/64987 on lgml-aliu: rendering error!
RenderError: rendering error page!
at Object.<anonymous> (/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.<anonymous> (/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.<anonymous> (/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.<anonymous> (/Users/aliu/Desktop/test/test4.js:10:86)
at Module._compile (module.js:460:26)
at Object.Module._extensions..js (module.js:478:10)

88. [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.<anonymous> (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

89. use
for expected errors
typed errors

90. 3
The problem with catch

91. But
{abstraction}
solved this
already!?

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

93. > [SyntaxError: Unexpected token b]
> [ReferenceError: y is not defined]
expected:
unexpected:
JSON.parse(input);
var x = y + 1;

94. Catching typed errors
catch (err) {
if (err instanceof SyntaxError) {
...
}
}

95. Don’t forget to rethrow!
catch (err) {
if (err instanceof SyntaxError) {
...
} else {
throw err;
}
}

96. Centralize throwing
catch (err) {
throwUnexpected(err);
// handle err if not thrown
}

97. expected vs unexpected…?
catch (err) {
if (err instanceof SyntaxError) {
...
} else {
throw err;
}
}

98. expected vs unexpected…?
> JSON.parse('bad input');
> vart x;

99. try/catch cannot
differentiate
source of errors

100. try: reduce surface area
try {
JSON.parse(input);
} catch (err) {
...
}
var x = y + 1; <------------------- throws

101. be very
targeted
with try / catch

102. promises: catch ‘em all
foo.then(foo2)
.then(foo3)
.catch(function(err) {
...
});

103. promises: unexpected behavior
foo.then(foo2)
.then(fooError)
.catch(function(err) {
if (!(err instanceof ExpectedErr)) {
throw err;
}
});

104. $ 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.<anonymous> (/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

105. $

106. $ echo $?

107. $ echo $?
0

108. promises: unexpected behavior
foo.then(foo2)
.then(fooError)
.catch(function(err) {
if (!(err instanceof ExpectedErr)) {
throw err;
}
});

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

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

111. bluebird: throw by default
Promise.onPossiblyUnhandledRejection(function(e) {
throw e;
});

112. surfacing unexpected
errors should NOT be
opt-in

114. 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 ...) {
...
}

115. Refutable Pattern Matching
GuardedPattern(refutable) ::= Pattern(refutable) "if"
AssignmentExpression

116. Looking Forward

117. for rich error objects
5
npm install verror
npm install restify-errors

118. Handle every error from
every async API
4

119. Understand how your
abstractions handle errors
3
(swallow? rethrow?)

120. Abort immediately on
unexpected errors to
minimize the blast radius
2

121. Design and build applications
with error handling in mind
1
(exceptions are NOT exceptional!)

122. Error handling bible
‣ https://www.joyent.com/developers/node/
design/errors

123. Aborting on unexpected errors
‣ https://github.com/nodejs/node-v0.x-archive/
issues/5114
‣ https://github.com/nodejs/node-v0.x-archive/
issues/5149

124. Module up!
‣ verror
‣ https://github.com/davepacheco/node-verror
‣ restify-errors
‣ https://github.com/restify/errors

125. Get involved!
‣ https://esdiscuss.org/topic/try-catch-conditional-
exceptions-in-light-of-generators
‣ http://wiki.ecmascript.org/doku.php?
id=strawman:pattern_matching

126. Fin
Alex Liu
@stinkydofu
aliu@netflix.com

127. Image Credits

128. Image Credits

129. Image Credits

130. 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