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I want to write a common error handler which will catch custom errors thrown on purpose at any instance of the code.
When I did throw new Error('sample')
like in the following code
try {
throw new Error({'hehe':'haha'});
// throw new Error('hehe');
} catch(e) {
alert(e);
console.log(e);
}
Log shows in Firefox as Error: [object Object]
and I couldn’t parse the object.
For the second throw
the log shows as: Error: hehe
Whereas when I did
try {
throw ({'hehe':'haha'});
} catch(e) {
alert(e);
console.log(e);
}
the console showed as: Object { hehe="haha"}
in which I was able to access the error properties.
What is the difference?
Is the difference as seen in the code? Like string will be just passed as string and object as objects but the syntax will be different?
I haven’t explored throwing error object… I had done only throwing strings.
Is there any other way than the above two mentioned methods?
I want to write a common error handler which will catch custom errors thrown on purpose at any instance of the code.
When I did throw new Error('sample')
like in the following code
try {
throw new Error({'hehe':'haha'});
// throw new Error('hehe');
} catch(e) {
alert(e);
console.log(e);
}
Log shows in Firefox as Error: [object Object]
and I couldn’t parse the object.
For the second throw
the log shows as: Error: hehe
Whereas when I did
try {
throw ({'hehe':'haha'});
} catch(e) {
alert(e);
console.log(e);
}
the console showed as: Object { hehe="haha"}
in which I was able to access the error properties.
What is the difference?
Is the difference as seen in the code? Like string will be just passed as string and object as objects but the syntax will be different?
I haven’t explored throwing error object… I had done only throwing strings.
Is there any other way than the above two mentioned methods?
Share Improve this question edited Sep 2, 2017 at 10:41 Sebastian Simon 19.5k8 gold badges60 silver badges84 bronze badges asked Feb 6, 2012 at 6:07 Jayapal ChandranJayapal Chandran 11.1k14 gold badges67 silver badges91 bronze badges 3 |14 Answers
Reset to default 346The difference between throw new Error(something)
and throw something
in javascript is that throw new Error(something)
wraps the error passed to it in the following format:
{name:'Error', message:something}
The throw something
will throw the string/object as is.
A benefit of throw new Error(something)
is that when you console.log
the caught error object, you'll get a stack trace, like this:
Error: <something>
at foo (<anonymous>:3:11)
at <anonymous>:9:1
Both approaches will not allow any further code execution from the try block.
Here is a good explanation about The Error object and throwing your own errors
The Error Object
Just what we can extract from it in an event of an error? The Error object in all browsers support the following two properties:
name: The name of the error, or more specifically, the name of the constructor function the error belongs to.
message: A description of the error, with this description varying depending on the browser.
Six possible values can be returned by the name property, which as mentioned correspond to the names of the error's constructors. They are:
Error Name Description
EvalError An error in the eval() function has occurred.
RangeError Out of range number value has occurred.
ReferenceError An illegal reference has occurred.
SyntaxError A syntax error within code inside the eval() function has occurred.
All other syntax errors are not caught by try/catch/finally, and will
trigger the default browser error message associated with the error.
To catch actual syntax errors, you may use the onerror event.
TypeError An error in the expected variable type has occurred.
URIError An error when encoding or decoding the URI has occurred
(ie: when calling encodeURI()).
Throwing your own errors (exceptions)
Instead of waiting for one of the 6 types of errors to occur before control is automatically transferred from the try block to the catch block, you can also explicitly throw your own exceptions to force that to happen on demand. This is great for creating your own definitions of what an error is and when control should be transferred to catch.
throw "I'm Evil"
throw
will terminate the further execution & expose message string on catch the error.
try {
throw "I'm Evil"
console.log("You'll never reach to me", 123465)
} catch (e) {
console.log(e); // I'm Evil
}
Console after throw will never be reached cause of termination.
throw new Error("I'm Evil")
throw new Error
exposes an error event with two params name & message. It also terminate further execution
try {
throw new Error("I'm Evil")
console.log("You'll never reach to me", 123465)
} catch (e) {
console.log(e.name, e.message); // Error I'm Evil
}
throw Error("I'm Evil")
And just for completeness, this works also, though is not technically the correct way to do it -
try {
throw Error("I'm Evil")
console.log("You'll never reach to me", 123465)
} catch (e) {
console.log(e.name, e.message); // Error I'm Evil
}
console.log(typeof(new Error("hello"))) // object
console.log(typeof(Error)) // function
The following article perhaps goes into some more detail as to which is a better choice; throw 'An error'
or throw new Error('An error')
:
http://www.nczonline.net/blog/2009/03/10/the-art-of-throwing-javascript-errors-part-2/
It suggests that the latter (new Error()
) is more reliable, since browsers like Internet Explorer and Safari (unsure of versions) don't correctly report the message when using the former.
Doing so will cause an error to be thrown, but not all browsers respond the way you’d expect. Firefox, Opera, and Chrome each display an “uncaught exception” message and then include the message string. Safari and Internet Explorer simply throw an “uncaught exception” error and don’t provide the message string at all. Clearly, this is suboptimal from a debugging point of view.
TLDR: they are equivalent Error(x) === new Error(x)
.
// this:
const x = Error('I was created using a function call!');
// has the same functionality as this:
const y = new Error('I was constructed via the "new" keyword!');
source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
throw
and throw Error
are functionally equivalent. But when you catch them and serialize them to console.log
they are not serialized exactly the same way:
throw 'Parameter is not a number!';
throw new Error('Parameter is not a number!');
throw Error('Parameter is not a number!');
Console.log(e)
of the above will produce 2 different results:
Parameter is not a number!
Error: Parameter is not a number!
Error: Parameter is not a number!
You first mention this code:
throw new Error('sample')
and then in your first example you write:
throw new Error({'hehe':'haha'})
The first Error object would actually be useful, because it is expecting a string value, in this case 'sample'. The second would not because you are trying to pass an object in, and it is expecting a string, and would not display a helpful error.
The error object would have the "message" property, which would be 'sample'.
The Error
constructor is used to create an error object. Error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions.
User-defined Errors are thrown via the throw
statement. program control will be passed to the first catch
block in the call stack.
The difference between throwing an error with and without Error object:
throw {'hehe':'haha'};
In chrome devtools looks like this:
Chrome tells us that we have an uncaught error which just is a JS object. The object itself could have information regarding the error but we still don't know immediately where it came from. Not very useful when we are working on our code and debugging it.
throw new Error({'hehe':'haha'});
In chrome devtools looks like this:
An error thrown with the Error object gives us a stack trace when we expand it. This gives us valuable information where the error precisely came from which is often valuable information when debugging your code. Further note that the error says [object Object]
, this is because the Error
constructor expects a message string as a first argument. When it receives a object it will coerce it into a string.
The Error
class includes debugging information, such as the error's call stack, as properties of its instances. JS interpreters know how to serialise Error
instances into an informative error message string, and their structure can also be consumed by debugging software - such as browser dev tools - to construct a more informative GUI representation of the error. This is why it's generally more useful to throw an instance of the Error
class rather than simply throwing, for example, a string describing the error, or a number representing an error code.
Using custom errors
It's particularly useful to make your own subclasses of Error
, which allow you to uniquely identify different types of error with a descriptive name and machine-readable...
- clues for debugging,
- information for better user-facing error messages, or
- information to help recover from the error.
Then, when you're handling errors, you can use the nice and clean instanceof
operator to check what kind of error occurred. e.g.:
class DangerousWaterCurrent extends Error {
constructor(waterSpeed){
super(`These waters are moving at ${waterSpeed} metres per second - too fast to cross!`) // Provide a `message` argument to the Error() constructor
this.waterSpeed = waterSpeed // This passes some context about why/how the error occurred back to whichever function is going to catch & handle it
}
}
// ...later...
try {
swimAcrossRiver(river)
} catch (thrownValue) {
if (thrownValue instanceof DangerousWaterCurrent) {
if (thrownValue.waterSpeed <= 3){
paddleKayak(river)
} else {
constructBridge(river)
}
} else {
throw thrownValue // "Re-throw" the error back up the execution chain, for someone else to handle
}
}
new Error()
vs Error()
There is a "convenient" shorthand way to make an instance of Error
: by calling Error(message)
, instead of new Error(message)
, the way you'd make an instance of a normal class. This is a deliberate exception, inserted by the language designers, to the rule. There are similar shorthands for other in-language classes, like Number()
and String()
. They also let you call these classes with ()
as if they were functions, not classes. JS doesn't allow normal classes to do this, even though they're all actually functions under the syntactical sugar of "classes". Try in a REPL:
> class E extends Error {}
> Error(); "a value"
"a value"
> E(); "a value"
Uncaught TypeError: Class constructor E cannot be invoked without 'new'
at <anonymous>:2:1
A broader opinion on design: Personally, I think this design decision was a mistake, as it adds more exceptions to the rules of JavaScript - which means more to learn for programmers, and more for language translators/interpreters to account for. Instead of C++/Java's new
keyword, simply calling a class as if it were a function (as in Number("abc123")
) should have the properties that the new
keyword currently has: the class's constructor
function should be executed. Inside that function, this
should be bound to the instance and then returned implicitly. The new
keyword could then be discarded from the language. This is how Python's syntax works, and it's simpler, more readable, and more convenient.
you can throw
as object
throw ({message: 'This Failed'})
then for example in your try/catch
try {
//
} catch(e) {
console.log(e); //{message: 'This Failed'}
console.log(e.message); //This Failed
}
or just throw a string error
throw ('Your error')
try {
//
} catch(e) {
console.log(e); //Your error
}
throw new Error //only accept a string
TLDR
throw new Error('problem')
captures a number of properties of the place where the error happened.
throw 'problem'
does not
new Error('message')
captures the execution stack + others
Using an Error object allows you to capture the execution stack at the point where you throw the error. So when the error gets passed up the error handling tree, so does this stack snapshot.
So inserting throw "test error"
somewhere in my codebase results in:
Whereas throw new Error('test error')
results in:
You can see that the native Error object captures the stack at the point I throw the error and makes it available to whatever captures the error. That makes it easier for me to trace the problem when I'm debugging it.
In addition to that it also captures properties such as fileName
, lineNumber
and columnNumber
.
If you use the stack trace it's there for exception trackers to log for you
In this case the stack is being printed into the browser console but if you're using Javascript error logging tools like Appsignal or Bugsnag then that stack will also be available in them too. If you inspect the error object you can access the stack snapshot directly:
err = new Error('test')
err.stack
The heuristic I use for deciding which format to use
When I don't plan to catch the exception I use new Error('problem')
When I'm throwing an error because something unexpected or out-of-bounds has happened in the application, let's say the local datastore is corrupted, I might be in a situation where I don't want to handle it, but I do want to flag it. In this case I'll use the Error object so I have that stack snapshot.
By using throw new Error('Datastore is corrupted')
it's easier to trace my way back to what's happened.
When I plan to catch the exception I use throw 'problem'
Edit - on re-reading this I think the next part needs some caution. It's generally a good idea to be very specific about which error you choose to catch otherwise you can end up catching things that you really wanted to bubble all the way up. In general it's probably better to create a specific error type and catch that specific error (or message string). This allows errors you didn't anticipate to bubble up to the surface."
If the error is an expected error that I plan to catch and handle then I'm not going to get much use out of the stack snapshot.
So, let's say I use an http service and it returns a 500 HTTP code. I may treat this as an error which I throw "responseCode=500"
and then subsequently catch and handle.
React behavior
Apart from the rest of the answers, I would like to show one difference in React.
If I throw a new Error()
and I am in development mode, I will get an error screen and a console log. If I throw a string literal, I will only see it in the console and possibly miss it, if I am not watching the console log.
Example
Throwing an error logs into the console and shows an error screen while in development mode (the screen won't be visible in production).
throw new Error("The application could not authenticate.");
Whereas the following code only logs into the console:
throw "The application could not authenticate.";
This is quite old but hopefully, anyone searching this can still learn from this:
First and famous, in javascript, we have something called Primitive Wrapper; a primitive wrapper takes primitive data and represents it in an object format by simply using the "constructor pattern". Still yet, in primitive wrappers, you can decide to have your data returned as an object type or you can have it returned as its primitive type(in this situation, you are now given a go-ahead command for javascript to extract the primitive value and in this case, you don't use the new keyword).
In Summary:
throw "My error": this creates an Error object and returns the primitive data extracted from the constructor "this" object. And if you try checking the typeof in the catch block, it tells you its a primitive typeof "string"
throw new Error("My error"): this returns you an object where you can access the error value from the message property. What simply happens here is that the "new keyword" constructs a "this" object and assign "{name:"Error",message:"..."}" to it and returns it. And when you try to check the typeof from the catch block, you will see a typeof "object".
Note: in a situation where you explicitly pass in a custom object to the throw, it will behave as if you invoked the constructor using the new keyword and therefore, the catch block will return you the custom object and not the message property value. For example: throw {name:"RangeError",message:"range is out of scope",environment:"Happened in testing function"}.
In conclusion, use whatever suits you ones you know what you are doing. But for me if i don't need much data but rather just the error, then I go for Primitive returner.
throw something
works with both object and strings.But it is less supported than the other method.throw new Error("")
Will only work with strings and turns objects into useless [Object obj] in the catch block.
The Error constructor basically does something like this:
Error (message) {
if(!this instanceof Error)
return new Error(message);
...
}
If a constructor is called with new, it gets the pointer of the newly created object as its this
pointer. If it is called as a function, this
is set to windows
.
However, it isn't as easy as it should to implement your own error types.
Since you don't want your constructor being in the stacktrace, you'll need to call Error.captureStackTrace(this, MyErrorConstructor);
in your constructor function, This will eliminate the constructor from the stack trace.
However, if you want to use above trick (new vs. function call) in your Subclass, it WILL show your constructor in the stack trace (as it appears twice on the stack). Handling this is quite tricky. You can instantiate a new Error and, replace its prototype with your subclass prototype or you can create a subclass of your Error, so the nested constructor of this sub-subclass is different and can be eliminated by captureStackTrace.
throw new Error()
is good for throwing a specified error. But if you want to do custom error handling, it's better to use throw { example: 'error' }
.
That is, if you want to know the specified error, use throw new Error("example string")
, if you want to handle the error in custom, use throw
.
function makeErrorResponse(err = {}, httpStatus, status, message, message) {
const error = new Error();
error.httpStatus = httpStatus;
error.status = status;
error.message = message;
error.err = err;
return error;
}
throw makeErrorResponse({}, 500, 500, 'server error');
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Error
s – Bergi Commented Jun 9, 2016 at 17:28creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
Spec in tc39.es/ecma262/#sec-error-constructor – kitimenpolku Commented Dec 2, 2020 at 8:47