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Error Handling in Javascript and Node.js

Error Handling in Javascript and Node.js

Todays Menu:
1. Error Object Definition
2. Error Best Practices
3. Extending the Error Object
4. Practical uses in Node.js
5. Using Errors at your API

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Thanos Polychronakis

January 21, 2016
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Transcript

  1. JS Error Handling JS Error Handling

  2. Todays Menu Todays Menu 1. Error Object Definition 2. Error

    Best Practices 3. Extending the Error Object 4. Practical uses in Node.js 5. Using Errors at your API
  3. The Error Constructor The Error Constructor new Error([message[, fileName[, lineNumber]]])

    function equals(a, b) { if (a !== b) { throw new Error('Values are not equal'); } }
  4. Error Properties Error Properties Standard Standard message The error message.

    name The error name. Mozilla Mozilla (node.js) (node.js) fileName Path to file that raised the error. lineNumber The line number in the file that raised the error. columnNumber The col number. stack The error execution stack.
  5. Error Methods Error Methods toString() Returns a string representing the

    Error Object, overrides the Object.prototype.toString() method.
  6. Error Types Error Types EvalError Creates an instance representing an

    error that occurs regarding the global function eval(). InternalError Creates an instance representing an error that occurs when an internal error in the JavaScript engine is thrown. E.g. "too much recursion". RangeError Creates an instance representing an error that occurs when a numeric variable or parameter is outside of its valid range. ReferenceError Creates an instance representing an error that occurs when de-referencing an invalid reference. SyntaxError Creates an instance representing a syntax error that occurs while parsing code in eval(). TypeError Creates an instance representing an error that occurs when a variable or parameter is not of a valid type. URIError Creates an instance representing an error that occurs when encodeURI() or decodeURI() are passed invalid parameters.
  7. Javascript Error Best Practises Javascript Error Best Practises General Advice

    Always be using Error Objects More Specifically In your [node.js] callbacks In error events you emit In your API (normalize) Throw in your Promises
  8. Extending Error Extending Error Your own Error Objects Your own

    Error Objects
  9. Classical Inheritance? Classical Inheritance? var util = require('util'); var CustomError

    = function() { Error.apply(this, arguments); this.customProp = true; }; util.inherits(CustomError, Error); console.log(customError.message); // undefined console.log(customError.stack); // empty response console.log(customError instanceof Error); // true console.log(Object.keys(customError)); // ['customProp']
  10. What happened here? What happened here? var util = require('util');

    var CustomError = function() { Error.apply(this, arguments); this.customProp = true; }; util.inherits(CustomError, Error); Error is a function that returns a new Error object and does not manipulate this in any way
  11. How to extend Error How to extend Error (node.js) (node.js)

    var util = require('util'); var CustomError = function(message) { Error.captureStackTrace(this, this.constructor); this.name = 'CustomError'; this.message = message; }; util.inherits(CustomError, Error); instanceof Error works Stack traces work as expected Can extend itself captureStackTrace() is v8 specific For web replace with local throw
  12. How to extend Error How to extend Error (web) (web)

    var util = require('util'); var CustomError = function(message) { // Use V8's native method if available, otherwise fallback if ('captureStackTrace' in Error) { Error.captureStackTrace(this, CustomError); } else { this.stack = (new Error()).stack; } this.name = 'CustomError'; this.message = message; }; util.inherits(CustomError, Error); instanceof Error works Stack traces have actual file on second line Can extend itself
  13. Errors in Node.js Errors in Node.js

  14. "Signing" Errors "Signing" Errors Errors in Node.js errors.DatabaseError = function(message)

    { Error.captureStackTrace(this, this.constructor); this.name = 'AcmeDatabaseError'; this.message = message; // custom props this.error = true; }; util.inherits(errors.DatabaseError, Error);
  15. Augmented Errors Augmented Errors Errors in Node.js var appError =

    require('app-errors'); // ... // User.prototype.hasPrivilege = function(userId, privilege, cb) { this.fetchUser(userId, function(err, udo) { if (!udo) { var err = new appError.EmptyResultsError('User not found'); err.userId = userId; cb(err); return; } // ... // }); };
  16. Catch Specific Errors Catch Specific Errors Errors in Node.js try

    { myroutine(); // may throw three types of exceptions } catch (ex) { if (ex instanceof TypeError) { } else if (ex instanceof RangeError) { } else if (ex instanceof EvalError) { } else { logMyErrors(ex); } }
  17. Using Errors on API Using Errors on API how to

    work with express how to work with express
  18. The Overview The Overview Using Errors on API Create custom

    Errors with an httpCode or similar attribute, set to 500 by default. Throw them or augment existing Error objects by defining the httpCode. Create an express error middleware handling those errors. Pipe all error handling to next
  19. Pipe Errors to next Pipe Errors to next Using Errors

    on API UserCtrl.prototype.fetchUser = function(req, res, next) { this.userEnt.readOne(req.param.userId) .then(res.json) .catch(next); };
  20. Express Error Handler Express Error Handler Using Errors on API

    var customErrorHandler = function (err, req, res, next) { var httpCode = 500; httpCode = err.httpCode || httpCode; res.status(httpCode); // figure out what the client accepts if (helpers.isRequestJson(req)) { res.json(error); } else { res.render('error/' + error.httpCode, {error: error}); } }; // Express error handler must always be last // in all of your middleware app.use(customErrorHandler);
  21. Thank you Thanasis Polychronakis @thanpolas https://speakerdeck.com/thanpolas

  22. Questions? Thanasis Polychronakis @thanpolas https://speakerdeck.com/thanpolas