I'm trying to get the error code when I catch an error, but it's always undefined.
How do I get the error code?
Here are two examples:
try {
const f = 4;
f = 9;
} catch (err) {
console.log(err.code);
console.log(err.message);
}
try {
d = 12;
} catch (err) {
console.log(err.code);
console.log(err.message);
}
Console:
undefined
Assignment to constant variable.
undefined
d is not defined
I work on Node.js v14.17.1.
As the others commented try console.log(err) to see what the object looks like before trying to select properties from it.
I misunderstood the question with my original answer. To my understanding there is no .code property on the error object returned within a try catch block. Check out this documentation https://javascript.info/try-catch#error-object and try error.name.
A standard Javascript/Ecmascript error has no code property. See the Error docs for details.
The link you provide, from Node's Errors documentation pertains only to custom errors thrown by Node.js — Javascript errors thrown by, say, V8 will adhere to Javascript standard referenced in the 1st link above.
try {
throw new Error('yikes!');
} catch (e) {
throw e;
} finally {
console.log('caught error!')
}
Prints out:
caught error!
Error: yikes!
So is the finally block running before the throw statement?
It only looks like the finally block ran first, but we can see this isn't actually the case:
let failed;
try {
throw new Error('yikes!');
failed = false;
} catch (e) {
failed = true;
throw e;
} finally {
console.log(`caught error! failed: ${failed}`)
}
prints
caught error! failed: true
Error: yikes!
So why is the printing of the error out of band? is there some asynchronous behaviour here that I'm not seeing?
Try runs - Your try runs and throws the error
Catch runs - Your catch catches the error and simply re-throws it.
Finally runs - You print out your string
The rethrown error is now uncaught - And your browser logs details about the error
The finally clause runs before the overall try block finishes. The error that's logged happens when the runtime intercepts the exception, which is after the try block finishes. (Note that your catch code does not console.log() anything. It throws a new exception, but the finally clause will still run before the rest of the world sees that.)
All languages with try catch finally that I know of behave in exactly the same way. The point of finally blocks is to provide a place to put code that is guaranteed to run whether the try succeeds or fails.
The throw statement doesn't print anything. The error is printed later when the browser catches the thrown error.
First off the catch clause isn't being executed. But I am getting "SyntaxError: missing exponent" error in the browser. I'm wondering if it's because of load time? Or my setup is not right.
I want to make a custom error for incorrect variable name.
try {
var 1ele = 1;
} catch (error) {
if (error instanceOf SyntaxError) {
throw new SyntaxError("There is a syntax error!");
}
}
Thanks for the help.
You have introduced a Syntax error in your code var 1ele = 1; and intend to catch the error to display a custom error message. However this won't work in the way you have written now since the Syntax error will cause the JavaScript parser to stop soon after encountering the Syntax error. The catch part of your code will never be reached.
However, you can implement the same using an eval statement. Try the code below.
try {
eval("var 1ele = 1");
} catch (error) {
if (error instanceof SyntaxError) {
throw new SyntaxError("There is a syntax error!");
}
}
Here, I have wrapped the Syntax error causing variable declaration inside an eval function. This will cause the syntax error only in the environment where the eval statement is executed and not your main code. You will be able to catch the error and display your custom message.
NOTE 1: You are getting the "SyntaxError: missing exponent" error message because your variable name starts with 1e which makes the JavaScript parser think that it is a number in exponential format.
NOTE 2: You have used instanceOf in your code. It is actually instanceof (with a lower case o). Ref
Is there a preferred way to include inner exceptions when throwing exceptions in JavaScript?
I'm relatively new to JavaScript coming from a C# background. In C#, you can do the following:
try
{
// Do stuff
}
catch (Exception ex)
{
throw new Exception("This is a more detailed message.", ex);
}
In the samples I've seen in JavaScript, I haven't been able to find how to catch an exception, add a new message, and re-throw the new exception while still passing the original exception.
You can throw any object you want:
try {
var x = 1/0;
}
catch (e) {
throw new MyException("There is no joy in Mudville", e);
}
function MyException(text, internal_exception) {
this.text = text;
this.internal_exception = internal_exception;
}
Then an error will be thrown of type MyException with properties text and internal_exception.
An inner error may be available in the Error.cause property. You may provide one via the options parameter of the Error constructor. Example:
try {
divide(dividend, divisor);
} catch (err) {
throw new Error(`Devision by ${divisor} failed. See cause for details.`, { cause: err });
}
When such an error is thrown and printed, it will show the inner errors recursivly, just as you'd expect. Running throw new Error("Outer", { cause: new Error("Inner") }); in ts-node REPL produces:
Uncaught Error: Outer
at <repl>.ts:1:7
at Script.runInThisContext (node:vm:129:12)
... 7 lines matching cause stack trace ...
at bound (node:domain:433:15) {
[cause]: Error: Inner
at <repl>.ts:1:35
at Script.runInThisContext (node:vm:129:12)
at runInContext (/usr/local/lib/node_modules/ts-node/src/repl.ts:665:19)
at Object.execCommand (/usr/local/lib/node_modules/ts-node/src/repl.ts:631:28)
at /usr/local/lib/node_modules/ts-node/src/repl.ts:653:47
at Array.reduce (<anonymous>)
at appendCompileAndEvalInput (/usr/local/lib/node_modules/ts-node/src/repl.ts:653:23)
at evalCodeInternal (/usr/local/lib/node_modules/ts-node/src/repl.ts:221:12)
at REPLServer.nodeEval (/usr/local/lib/node_modules/ts-node/src/repl.ts:243:26)
at bound (node:domain:433:15)
Note that it's only a convention to put the inner error inside cause (but a strong one, as seen from ts-node truncating the outer stack trace due to 7 lines matching cause stack trace). So anything may be inside the cause property, so check it before you consume it! MDN gives an example of putting additional data after the example of using it for an inner error:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause
I have a number of functions which either return something or throw an error. In a main function, I call each of these, and would like to return the value returned by each function, or go on to the second function if the first functions throws an error.
So basically what I currently have is:
function testAll() {
try { return func1(); } catch(e) {}
try { return func2(); } catch(e) {} // If func1 throws error, try func2
try { return func3(); } catch(e) {} // If func2 throws error, try func3
}
But actually I'd like to only try to return it (i.e. if it doesn't throw an error). I do not need the catch block. However, code like try {} fails because it is missing an (unused) catch {} block.
I put an example on jsFiddle.
So, is there any way to have those catch blocks removed whilst achieving the same effect?
A try without a catch clause sends its error to the next higher catch, or the window, if there is no catch defined within that try.
If you do not have a catch, a try expression requires a finally clause.
try {
// whatever;
} finally {
// always runs
}
It's possible to have an empty catch block, without an error variable, starting with ES2019. This is called optional catch binding and was implemented in V8 v6.6, released in June 2018. The feature has been available since Node 10, Chrome 66, Firefox 58, Opera 53 and Safari 11.1.
The syntax is shown below:
try {
throw new Error("This won't show anything");
} catch { };
You still need a catch block, but it can be empty and you don't need to pass any variable. If you don't want a catch block at all, you can use the try/finally, but note that it won't swallow errors as an empty catch does.
try {
throw new Error("This WILL get logged");
} finally {
console.log("This syntax does not swallow errors");
}
Nope, catch (or finally) is try's friend and always there as part of try/catch.
However, it is perfectly valid to have them empty, like in your example.
In the comments in your example code (If func1 throws error, try func2), it would seem that what you really want to do is call the next function inside of the catch block of the previous.
I wouldn't recommend try-finally without the catch, because if both the try block and finally block throw errors, the error thrown in the finally clause gets bubbled up and the try block's error is ignored, in my own test:
try {
console.log('about to error, guys!');
throw new Error('eat me!');
} finally {
console.log ('finally, who cares');
throw new Error('finally error');
}
Result:
> about to error, guys!
> finally, who cares
> .../error.js:9
> throw new Error('finally error');
> ^
>
> Error: finally error
No, it is not possible to have try block without catch (or finally). As a workaround, I believe you might want to define a helper function such as this:
function tryIt(fn, ...args) {
try {
return fn(...args);
} catch {}
}
and use it like:
tryIt(function1, /* args if any */);
tryIt(function2, /* args if any */);
I've decide to look at the problem presented from a different angle.
I've been able to determine a way to to allow closely for the code pattern requested while in part addressing the un-handled error object listed by another commenter.
code can be seen # http://jsfiddle.net/Abyssoft/RC7Nw/4/
try:catch is placed within a for loop allowing graceful fall through. while being able to iterate through all the functions needed. when explicit error handling is needed additional function array is used. in the even of error and functional array with error handlers element is not a function, error is dumped to console.
Per requirements of stackoverflow here is the code inline [edited to make JSLint compliant (remove leading spaces to confirm), improve readability]
function func1() {"use strict"; throw "I don't return anything"; }
function func2() {"use strict"; return 123; }
function func3() {"use strict"; throw "I don't return anything"; }
// ctr = Code to Run <array>, values = values <array>,
// eh = error code can be blank.
// ctr and params should match 1 <-> 1
// Data validation not done here simple POC
function testAll(ctr, values, eh) {
"use strict";
var cb; // cb = code block counter
for (cb in ctr) {
if (ctr.hasOwnProperty(cb)) {
try {
return ctr[cb](values[cb]);
} catch (e) {
if (typeof eh[cb] === "function") {
eh[cb](e);
} else {
//error intentionally/accidentially ignored
console.log(e);
}
}
}
}
return false;
}
window.alert(testAll([func1, func2, func3], [], []));
If you only want functions 2 and 3 to fire if an error occurs why are you not putting them in the catch block?
function testAll() {
try {
return func1();
} catch(e) {
try {
return func2();
} catch(e) {
try {
return func3();
} catch(e) {
// LOG EVERYTHING FAILED
}
}
}
}
...is there any way to have those catch blocks removed whilst achieving the same effect? As it would seem, no; Javascript requires a try block be followed by either a catch or a finally block.
Having said that, there is a way to use those catch blocks to achieve the effect you want.
// If func1 throws error, try func2 The if throws error condition, is what the catch block is for.
Why remove them when their use is exactly what you are after?
try { return func1(); }
catch {
// if func1 throws error
try { return func2(); }
catch {
// if func2 throws error
try { return func3(); }
catch {
// if func3 throws error
}
}
}
I completely understand why you might not need a catch block, and would find it cleaner to be able to omit it entirely. But I don't think this is one of those situations.
They go together in every language that I know that has them (JavaScript, Java, C#, C++). Don't do it.
Since ES2019 you can easily use try {} without catch {}:
try {
parseResult = JSON.parse(potentiallyMalformedJSON);
} catch (unused) {}
For more info please reffer to Michael Ficcara's proposal
try & catch are like 2 side of one coin. so not possible without try.
No. You have to keep them.
This actually makes sense since errors shouldn't be silently ignored at all.