node.js - How does exactly Express.js handle errors? -

so, trivial thing. know express has built-in default error handler expect 4 arguments (err, req, res, next) handle "synchronous exceptions" referenceerror, typeerror, etc:

update: question express-specific, not how catch unhandled exceptions/etc. want know how in first code block express can handle user-defined exception. second example async. exception doesn't directly belongs question.

const express = require('express')();  express.all('*', (req, res) => {   throw new error('my own error');   res      .send('okay?'); });  express.use((err, req, res, next) => {   console.error(err.stack);   res      .status(503)     .send('express still , running'); }).listen(3000); 

but not this:

const express = require('express')();  express.all('*', (req, res) => {   process.nexttick(() => {     throw new error('my own error');   });   res      .send('okay?'); });  express.use((err, req, res, next) => {   console.error(err.stack);   res      .status(503)     .send('won\'t executed'); }).listen(3000); 

but i'm curious realization of handler.

i can't find

process.on('uncaughtexception'... or domains/promises/cluster.

maybe miss something.

can clarify?

there catch-all kind of block in express's router gets executed:

try {   fn(req, res, next); } catch (err) {   next(err); } 

so in block exceptions catched , translated next() function calls error parameter set match exception.

in more detail, when request arrives express:

• app.handle() called, determines router , calls router.handle()
• router.handle() processes through layers , matches handler registered express.all('*', ...) signature
• handle_request (inside router's internal layer module) calls handler function specified in above try-catch block, catching possible exceptions , translating them next calls.