Nodejs обработка ошибок
Safely "throwing" errors
deally we'd like to avoid uncaught errors as much as possible, as such, instead of literally throwing the error, we can instead safely "throw" the error using one of the following methods depending on our code architecture:
For synchronous code, if an error happens, return the error:
// Define divider as a syncrhonous function var divideSync = function(x,y) {
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by returning it
return new Error("Can't divide by zero");
}
else {
// no error occured, continue on
return x/y;
}
};
// Divide 4/2 var result; result = divideSync(4,2); // did an error occur? if ( result instanceof Error ) {
// handle the error safely
console.log('4/2=err', result);
} else {
// no error occured, continue on
console.log('4/2='+result);
}
// Divide 4/0 result = divideSync(4,0); // did an error occur? if ( result instanceof Error ) {
// handle the error safely
console.log('4/0=err', result);
} else {
// no error occured, continue on
console.log('4/0='+result);
} For callback-based (ie. asynchronous) code, the first argument of the callback is err, if an error happens err is the error, if an error doesn't happen then err is null. Any other arguments follow the err argument:
var divide = function(x,y,next) {
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by calling the completion callback
// with the first argument being the error
next(new Error("Can't divide by zero"));
}
else {
// no error occured, continue on
next(null, x/y);
}
};
divide(4,2,function(err,result){
// did an error occur?
if ( err ) {
// handle the error safely
console.log('4/2=err', err);
}
else {
// no error occured, continue on
console.log('4/2='+result);
}
});
divide(4,0,function(err,result){
// did an error occur?
if ( err ) {
// handle the error safely
console.log('4/0=err', err);
}
else {
// no error occured, continue on
console.log('4/0='+result);
}
}); For eventful code, where the error may happen anywhere, instead of throwing the error, fire the error event instead:
// Definite our Divider Event Emitter var events = require('events'); var Divider = function(){
events.EventEmitter.call(this);
}; require('util').inherits(Divider, events.EventEmitter);
// Add the divide function Divider.prototype.divide = function(x,y){
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by emitting it
var err = new Error("Can't divide by zero");
this.emit('error', err);
}
else {
// no error occured, continue on
this.emit('divided', x, y, x/y);
}
// Chain return this;
};
// Create our divider and listen for errors var divider = new Divider(); divider.on('error', function(err){
// handle the error safely console.log(err);
}); divider.on('divided', function(x,y,result){
console.log(x+'/'+y+'='+result);
});
// Divide divider.divide(4,2).divide(4,0);
