Throwing an error in node.js
This will help you !
var el = document.getElementById('el');
var log = function( val ){
el.innerHTML+= '<div><pre>' + val + '</pre></div>';
};
try {
throw Error('error in code');
} catch (e) {
log( e.message );
log( e.fileName );
log( e.lineNumber );
log( e.stack );
};<div id='el'></div>If you're using node-style callbacks, the convention is not to throw. Instead pass you error as the first argument to your callback -
// divide with callback
function div (x, y, done) {
if (y === 0)
return done (Error ('Cannot divide by zero'))
else
return done (null, x / y)
}
div (6, 3, function (err, result) {
// *always* check for err
if (err)
console.log ('error', err.message, err.stack)
else
console.log ('result', result)
})Kind of a stupid function to use a callback since it can be written in a purely synchronous way, but hopefully this illustrates the pattern
Your function might already be written in a synchronous way – don't worry tho, we can convert it to a node-style callback function using something like cps2 below -
// a "normal" synchronous function that throws an error
const div = (x, y) =>
{ if (y === 0)
throw Error ('cannot divide by zero')
else
return x / y
}
// convert it to a continuation passing style (cps) function
const cps2 = (f, x, y, k) =>
{ try
{ return k (null, f (x, y)) }
catch (err)
{ return k (err) }
}
// logging utility for demos below
const logger = (err, result) =>
{ if (err)
console.log ('error:', err.message, err.stack)
else
console.log ('result:', result)
}
cps2 (div, 6, 3, logger)
// result: 2
cps2 (div, 6, 0, logger)
// error: cannot divide by zeroAll of that said, most peoples are using Promises nowadays. Below we demonstrate how to turn a node-style callback function into one that returns a Promise. Note, node provides this function as util.promisify, though I've implemented it here for demonstration purposes -
// a conventional function with a node-style callback
const div = (x, y, done) =>
{ if (y === 0)
return done (Error ('cannot divide by zero'))
else
return done (null, x / y)
}
// convert a node-style callback function to a promise-returning function
const promisify = f => (...args) =>
new Promise
( (resolve, reject) =>
f ( ...args
, (err, result) =>
err
? reject (err)
: resolve (result)
)
)
// logging utility for demos below
const logger = p =>
p .then (console.log, console.error)
logger (promisify (div) (6, 3))
// 2
logger (promisify (div) (6, 0))
// Error: cannot divide by zeroContinuations are just functions tho so you can write this kind of thing in any way that you like – don't think you have to use node-style "callbacks" or Promises just because that's the only way you've seen it -
const cont = (...values) =>
k => k (...values)
const div = (x, y) =>
y === 0
? cont (Error ('cannot divide by zero'))
: cont (null, x / y)
const logger = (err, result) =>
err
? console .log ('error:', err.message)
: console .log ('result:', result)
div (6, 3) (logger)
// result: 2
div (6, 0) (logger)
// error: cannot divide by zero