NodeJS - ErrorsPEGGY2016-06-17

Four Categories Of Errors

1. Standard JavaScript errors such as: <EvalError> : thrown when a call to eval() fails. <SyntaxError> : thrown in response to improper JavaScript language syntax. <RangeError> : thrown when a value is not within an expected range <ReferenceError> : thrown when using undefined variables <TypeError> : thrown when passing arguments of the wrong type <URIError> : thrown when a global URI handling function is misused.

RangeError

A subclass of Error that indicates that a provided argument was not within the set or range of acceptable values for a function

require('net').connect(-1);D:\Framework\nodeJs_errors>node test.jsnet.js:954 throw new RangeError('"port" option should be >= 0 and < 65536: ' + port); ^

RangeError: "port" option should be >= 0 and < 65536: -1 at lookupAndConnect (net.js:954:13) at Socket.connect (net.js:929:5) at Object.exports.connect.exports.createConnection (net.js:70:35) at Object.<anonymous> (D:\Framework\nodeJs_errors\test.js:1:78) at Module._compile (module.js:541:32) at Object.Module._extensions..js (module.js:550:10) at Module.load (module.js:458:32) at tryModuleLoad (module.js:417:12) at Function.Module._load (module.js:409:3) at Function.Module.runMain (module.js:575:10)

ReferenceError

A subclass of Error that indicates that an attempt is being made to access a variable that is not defined.

D:\Framework\nodeJs_errors>node test.jsD:\Framework\nodeJs_errors\test.js:1(function (exports, require, module, __filename, __dirname) { test; ^

ReferenceError: test is not defined at Object.<anonymous> (D:\Framework\nodeJs_errors\test.js:1:63) at Module._compile (module.js:541:32) at Object.Module._extensions..js (module.js:550:10) at Module.load (module.js:458:32) at tryModuleLoad (module.js:417:12) at Function.Module._load (module.js:409:3) at Function.Module.runMain (module.js:575:10) at startup (node.js:160:18) at node.js:449:3

test;

SyntaxError

A subclass of Error that indicates that a program is not valid JavaScript.

D:\Framework\nodeJs_errors>node test.jsD:\Framework\nodeJs_errors\test.js:1(function (exports, require, module, __filename, __dirname) { var 77 = 88; ^^SyntaxError: Unexpected number at Object.exports.runInThisContext (vm.js:53:16) at Module._compile (module.js:513:28) at Object.Module._extensions..js (module.js:550:10) at Module.load (module.js:458:32) at tryModuleLoad (module.js:417:12) at Function.Module._load (module.js:409:3) at Function.Module.runMain (module.js:575:10) at startup (node.js:160:18) at node.js:449:3

var 77 = 88;

TypeError

A subclass of Error that indicates that a provided argument is not an allowable type.

D:\Framework\nodeJs_errors>node test.jsD:\Framework\nodeJs_errors\test.js:2temp();^

TypeError: temp is not a function at Object.<anonymous> (D:\Framework\nodeJs_errors\test.js:2:1) at Module._compile (module.js:541:32) at Object.Module._extensions..js (module.js:550:10) at Module.load (module.js:458:32) at tryModuleLoad (module.js:417:12) at Function.Module._load (module.js:409:3) at Function.Module.runMain (module.js:575:10) at startup (node.js:160:18) at node.js:449:3

var temp = "test"; temp();

Four Categories Of Errors

2. System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist, attempting to send data over a closed socket, etc

Four Categories Of Errors

3. User-specified errors triggered by application code.4. Assertion Errors are a special class of error that can be triggered

whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module.

Error Propagation and Interception

Try/Catch Callback EventEmitter

Try/Catch

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try / catch construct provided by the JavaScript language.

// Throws with a ReferenceError because z is undefined try {

const m = 1; const n = m + z;

} catch (err) { // Handle the error here.

}

Callback

const fs = require('fs'); fs.readFile('a file that does not exist', (err, data) => {

if (err) { console.error('There was an error reading the file!', err); return;

} // Otherwise handle the data

});

EventEmitter

const net = require('net'); const connection = net.connect('localhost'); // Adding an 'error' event handler to a stream: connection.on('error', (err) => {

// If the connection is reset by the server, or if it can't // connect at all, or on any sort of error encountered by // the connection, the error will be sent here. console.error(err);

});

Try / Catch with Asynchronous APIs

The JavaScript try / catch mechanism cannot be used to intercept errors generated by asynchronous APIs.

// THIS WILL NOT WORK: const fs = require('fs');

try { fs.readFile('/some/file/that/does-not-exist', (err, data) => {

// mistaken assumption: throwing here... if (err) { throw err; }

}); } catch(err) {

// This will not catch the throw!console.log(err);

}

Class: Error

const err = new Error('The message'); console.log(err.message); console.log(err.stack);

Throw Error

function makeError() { throw new Error('oh no!');

}

makeError(); // will throw: