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Errors Examples

  • System errors
  • Standard JavaScript errors
    • SyntaxError
    • ReferenceError
    • TypeError
  • Assertion Errors
  • Custom errors

System Errors

Different HTTP servers attempting to use same port

Create and run script:

const http = require('http');

const port = 3000;

const serverOne = http.createServer();
const serverTwo = http.createServer();

serverOne.listen(port, () => {
    console.log(`Server One running at http://localhost:${port}/`);
});

serverTwo.listen(port, () => {
    console.log(`Server Two running at http://localhost:${port}/`);
});

Output with error:

Server One running at http://localhost:3000/
events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE :::3000
    at Object.exports._errnoException (util.js:870:11)
    at exports._exceptionWithHostPort (util.js:893:20)
    at Server._listen2 (net.js:1237:14)
    at listen (net.js:1273:10)
    at Server.listen (net.js:1369:5)
    at Object.<anonymous> (/home/achyzh/server.js:12:11)
    at Module._compile (module.js:410:26)
    at Object.Module._extensions..js (module.js:417:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)

Attempting to open a file that does not exist

Create and run script:

const fs = require('fs');

fs.readFile('/tmp/1234567890', (err, data) => {
    if (err) throw err;
    console.log(data);
});

Output with error:

/home/user/file.js:4
    if (err) throw err;
             ^

Error: ENOENT: no such file or directory, open '/tmp/1234567890'
    at Error (native)

Standard JavaScript errors

SyntaxError

Thrown in response to improper JavaScript language syntax.

Create and run script:

constant http = require('http');

Output with error:

/home/user/server.js:1
(function (exports, require, module, __filename, __dirname) { constant http = require('http');
                                                                       ^^^^

SyntaxError: Unexpected identifier
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:374:25)
    at Object.Module._extensions..js (module.js:417:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:442:10)
    at startup (node.js:136:18)
    at node.js:966:3

ReferenceError

Thrown when using undefined variables

Create and run script:

const result = 1000;
console.log(result + lastStep);

Output with error:

/home/user/counter.js:2
console.log(result + lastStep);
                     ^

ReferenceError: lastStep is not defined
    at Object.<anonymous> (/home/achyzh/server.js:3:22)
    at Module._compile (module.js:410:26)
    at Object.Module._extensions..js (module.js:417:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:442:10)
    at startup (node.js:136:18)
    at node.js:966:3

TypeError

Thrown when passing arguments of the wrong type

Create and run script:

const helper = null;
const result = helper.get();

Output with error:

/home/user/helper.js:2
const result = helper.get();
                     ^

TypeError: Cannot read property 'get' of null
    at Object.<anonymous> (/home/achyzh/server.js:2:22)
    at Module._compile (module.js:410:26)
    at Object.Module._extensions..js (module.js:417:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:442:10)
    at startup (node.js:136:18)
    at node.js:966:3

Assertion Errors

Special class of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur.

Set not valid parameter to require()

Create and run script:

const lib = require({ location: '/etc/passwd' });

Output with error:

assert.js:89
  throw new assert.AssertionError({
  ^
AssertionError: path must be a string
    at Module.require (module.js:353:3)
    at require (internal/module.js:12:17)
    at Object.<anonymous> (/home/achyzh/server.js:1:75)
    at Module._compile (module.js:410:26)
    at Object.Module._extensions..js (module.js:417:10)
    at Module.load (module.js:344:32)
    at Function.Module._load (module.js:301:12)
    at Function.Module.runMain (module.js:442:10)
    at startup (node.js:136:18)
    at node.js:966:3