When node encounters an caught exception, it prints the line the error occurred on, followed by the stack trace, then exits:
/example.js:1
throw new Error()
^
Error
at ...
I would like to catch an exception, print exactly the same thing, and continue. Basically:
try {
something();
} catch (e) {
// what goes here?
console.error(e);
}
Note that the above example already prints the stack trace. The missing part is the first four lines, including the file/line reference, line text, and column indicator.
You can use the stack property of Error:
try {
something();
} catch (e) {
console.err(e.stack);
}
Related
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
When I try the following in the chrome console, i get 'undefined' as the value of 'e':
try{
var test=somethingInvalid();
}
catch(e){
console.log(e);
}
How do I access the details of e?
What properties does it have?
Thanks.
Doh. Doing console.log('E is: '+e) seams to work. Sorry!
Well, it will return undefined, because the last line console.log doesn't return a value.
try{
false = true;
}
catch(e){
console.log(e);
}
but with the above you will also see an object > Reference Error followed by the next line of undefined
According to MDN, it is the value on the throw statement.
https://developer.mozilla.org/en/JavaScript/Reference/Statements/try...catch
The exception identifier
When an exception is thrown in the try block, exception_var (e.g.
the e in catch (e)) holds the value specified by the throw statement.
You can use this identifier to get information about the exception
that was thrown.
This identifier is local to the catch clause. That is, it is created
when the catch clause is entered, and after the catch clause finishes
executing, the identifier is no longer available.
As far as I can see, Chrome does output the details of 'e'. I just did this:
var err;
try { /* what you did */ }
catch (e) { err = e; }
upon which Chrome shows you the details of 'e'.
In this case, you can perform calls to see the contents of the following:
err.name
err.arguments
err.type
err.stack
Good luck!
although all the answers provided were very helpful, what we were infact looking for was:
try{
var test=somethingThatDoesntExist();
}
catch(e){
console.log(e.message);
}
e.message is the variable we want to log, as it (to the best of our knowledge) contains a string representation of the error that was caught.
thanks!
I have a custom assert() function that pulls a stack trace of functions (in all modern browsers, except IE):
if (!condition) {
try {
throw Error();
} catch (e) {
// Browsers Firefox, Chrome, Opera provide additional stack property;
// Check if it does not exist (IE);
stackTrace = (!e.stack) ? "No stack available" : e.stack;
self.reportError(errorType, stackTrace, message);
}
}
Obviously, there is a lot more going on than what I'm showing, but the idea is there.
Can someone explain to me why an "Uncaught SyntaxError" isn't handled by a try/catch? Is it possible to gracefully handle this type of error so the rest of the JS runs?
For example:
try { response.write(;); }
catch(e) { console.log(e); }
This code throws a "Uncaught SyntaxError: Unexpected token ;" [Chrome] browser error instead of jumping to the catch and logging the error object. You get similar results in Firefox as well; a thrown error instead of handling it with the catch.
A syntax error is not a run-time exception. In order for your program to be able to throw and catch exceptions, it needs to be able to run. In order to run, it needs to be javascript. The above example is not valid javascript. The interpreter gives up and prints an error when it sees the first invalid line and it never even gets to the "catch" line.
Well, since this is a syntax exception, it means that javascript parser failed to parse you code. Since browser failed to parse your code, it can not execute it. It does not even know about your try/catch block. try/catch block can only catch exception thrown as a result of code executing in context of the block.
You could use a workaround for this.
try {
Function('response.write(;);')
} catch(e) {
console.log(e);
}
Could be used for determining es6 template string support this way:
var isEs6Template;
try {
Function('``');
isEs6Template = true;
} catch(e) {
isEs6Template = false;
}
console.log('es6 template is', isEs6Template ? 'supported' : 'unsupported');