Is it possible to terminate script on a certain condition with a specific error message? Exceptions are not an option, as they can be catched, thus avoiding termination.
Update:
I'd prefer the universal approach, but if there's none, then it's for the in-browser JavaScript. Error should be silent, and the error message should only go into the browser error log.
If your code is inside a try statement, and you are looking for a way to stop it for certain errors, then I think you are looking for the finally block:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Statements?redirectlocale=en-US&redirectslug=JavaScript%2FGuide%2FStatements#The_finally_Block
From that site (and editing the annoying alert() for console.log()) you can find this example:
function f() {
try {
console.log(0);
throw "bogus";
} catch(e) {
console.log(1);
return true; // this return statement is suspended until finally block has completed
console.log(2); // not reachable
} finally {
console.log(3);
return false; // overwrites the previous "return"
console.log(4); // not reachable
}
// "return false" is executed now
console.log(5); // not reachable
}
f(); // alerts 0, 1, 3; returns false
Unfortunately, I only managed to accomplish this through the following ugly workaround:
var failWithFatalError; // A non-existent function
console.error (message + ' in ' + homebrewStackTraceFunction());
failWithFatalError();
Related
I would like to know the main goal to use Try & Catch in Javascript, below is the example i am achieving
Should I need to let the program stop in case of error?
Why In programming I need to my application to continue running
however there is an error?
try {
if (typeof a != "number") {
throw new ReferenceError("The First argument is not a number");
} else if (typeof b != "number") {
throw new ReferenceError("The Second argument is not a number");
} else {
console.log(a + b);
}
} catch (error) {
console.log("Error:", error.message);
}
}
addTwoNums("10", 100);
console.log("It still works");
The code in the try block is executed first, and if it throws an exception, the code in the catch block will be executed. The code in the finally block will always be executed before control flow exits the entire construct
There are some things in java that require try and catch statements to function, for instance if you wanted to read a file java will prevent you from doing so unless error catching is used.
In your example whilst there is no harm in using try, it is simply unnecessary as there is no variability in the success of your function. Also, since there is no need to stop the program if the arguments are invalid, instead of throwing errors it might be preferable to instead halt the operation and return a warning.
Is there such a thing as blocking vs non blocking Javascript errors?
What I mean is that I've seen my site report some errors, but the process involved still completes successfully (as far as I can tell). These errors include:
TypeError: undefined is not an object (evaluating 'e.nodeName.toLowerCase')
TypeError: e.nodeName is undefined
(these are always from my general JQuery library file)
These are what I mean by 'non-blocking' errors, since the scripts involved seem to complete successfully.
If there is such a thing as blocking v non-blocking, what is the correct terminology for it, and how can I tell the difference between the 2 (eg so that I can prioritise investigating blocking errors)?
Thanks
Whenever
throw Error;
The exception bubles up the function stack. If it reaches the end of it, it throws an uncaught (=unexpected) exception. This halts the complete program as its unlikely to work properly anymore.
However it can be catched (=expected exception):
try{
someerrorproducingfunc();
//continue after success
}catch(e){
//continue after error
console.error(e);
}
//continue after error+success
So the code proceeds ( leaving out the continue after sucess part). However, if you use console.error it logs sth that looks like an error.
ressource: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/throw
Note that Error is just an object constructor in js:
new Error("test");
While throw throws an exception...
No. All un-caught error are blocking, they will end the execution of the entire call stack. However, an error that happens after various operation will not revert back to a situation like before. Making an AJAX call, and then having an undefined error will not cancel the call for example. Demo:
(function () {
"use strict";
let a = 5;
console.log(a); // 5
setTimeout(function () {
console.log(a); // 10
// will execute because it is a fresh stack
// the variable a is 10 despite and not 12 because it happened after
}, 250);
const withError2 = function () {
a = 10;
a.x.z = 2; // Error
a = 12; // never executes because it comes after the error
}
withError2();
console.log(a); // never executes because it comes after the error
}());
I would ask about java script error, is there type of error like php
or others,
example: In php we have notice, and Parse Error ..etc notice will not
be stop php execute, but parse will be stop execute php code
directly..
now is there js error like this, or what is the js classification
error .. I know we can handle error by try, catch ..,but is there
error in js was stooped script and others will not stop execute script
thank you
is there error in js was stooped script and others will not stop execute script
Not except for parsing/syntax errors, no.
JavaScript has exceptions. An exception exits the code in which it is thrown, and the code that called that, and so on until it's caught. If it isn't caught, then all currently-running functions are terminated and the error is logged to the web console.
So an exception (either one you throw explicitly or one that happens as a by-product of something you do) will either terminate all running functions (if not caught) or only terminate some code (if caught).
For example:
function foo() {
try {
bar(0);
}
catch (e) {
console.log("Caught exception");
}
}
function bar(a) {
if (a <= 0) {
throw new Error("'a' cannot be <= 0");
}
console.log("bar: a = " + a);
}
foo();
There, the code in bar following the exception is not run (we don't see "bar: a = 0") because an exception was throw, terminating bar. But foo's code continues, in the catch block, because foo caught the exception.
JavaScript is unusual in that you can throw anything, including a string, a number, etc. But if you want useful information, you usually throw Error:
throw new Error("Optional message here");
Since what you throw can be anything, you might be thinking there's a way to catch only certain things, but that's not the case. catch catches any exception that was thrown. So:
try {
throw "foo";
}
catch (e) {
}
try {
throw new Error();
}
catch (e)
}
try {
throw 42;
}
catch (e)
}
Note that those catch clauses are identical; they catch anything that was thrown. Of course, you can then inspect what you got and re-throw:
try {
// ...some code here that may throw any of several things...
}
catch (e)
if (typeof e === "string") {
// Handle it here
}
else {
throw e;
}
}
There we only handle exceptions that are strings, and not ones that are numbers, Error objects, etc.
You can create your own derived versions of Error if you like, although it's a bit more of a pain than it ought to be:
function MySpecificError(msg) {
this.message = msg;
try {
throw new Error();
}
catch (e) {
this.stack = e.stack;
}
}
MySpecificError.prototype = Object.create(Error.prototype);
MySpecificError.prototype.constructor = MySpecificError;
Then:
throw new MySpecificError("Something went wrong.");
Note that we had to fill in the code in MySpecificError to create the stack trace. (Also note that not all engines provide a stack trace, but if they do, this lets you use it.)
Some engines provide a few error types out of the box:
Error
RangeError (something was out of range)
ReferenceError (but usually that's something you'd let the engine throw)
TypeError (again)
SyntaxError (again)
Finally, it's worth noting that several things that would cause exceptions in other environments don't in JavaScript, mostly around math. For instance:
var result = 10 / 0;
In many non-JavaScript environments, that results in a runtime error (division by zero). In JavaScript, it doesn't; result gets the value Infinity.
Similarly:
var x = Number("I am not a number");
or
var x = parseInt("I am not a number", 10);
...doesn't throw a parsing error, it sets x to NaN ("not a number").
Yes. Javascript errors can have types, and there is a standard error type hierarchy. You can also write your code to "throw" things that are not error objects.
(In fact, since the catch clause in Javascript / ECMAScript does not discriminate based on the type of the exception, exception handling tends to be rather crude; i.e. "catch all errors" and then attempt to recover. Hence, to a first order, it doesn't matter what you throw.)
The ECMAScript 5.1 spec says that syntax errors are "early errors" and that they must be reported before the program is executed. An exception to this is syntax errors detected in code being run using eval. However, the spec doesn't say how early errors are to reported, or what happens afterwards. (At least, I don't think it does ...)
I believe that a common strategy for a Javascript parser/compiler/interpreter to skip to the enclosing block, and replace the affected code with code that throws an exception (e.g. SyntaxError) if it is run.
References:
http://www-archive.mozilla.org/js/language/js20-1999-02-18/error-recovery.html
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError
EcmaScript 5.1 - Errors
This is a very basic question. In Java I use the finally statement to close resources because "it's a good practice". I've been developing in Javascript and then in Node.js during some years and I've never used the finally statement. I know that in Node.js all of us follow the first parameter error handling pattern. Anyway, the 2 following snippets do the same:
try{
throw 123
}catch (e){
}finally{
console.log(1)
}
.
try{
throw 123
}catch (e){
}
console.log(1)
Both print 1.
Why is finally a keyword if it has no real benefit? The clean up code can be put inside the catch.
finally is useful for more than just exception handling — it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break.
Just a simple and straightforward example that shows the difference. There is a return that breaks the function completion, but the console.log in finally is called while the last console.log is skipped.
let letsTry = () => {
try {
// there is a SyntaxError
eval('alert("Hello world)');
} catch(error) {
console.error(error);
// break the function completion
return;
} finally {
console.log('finally')
}
// This line will never get executed
console.log('after try catch')
}
letsTry();
But try this:
try {
throw "foo"
} catch (e) {
throw "bar"
} finally {
console.log("baz")
}
console.log("quux")
If a second error is thrown from within the catch block, the code after the try...catch block will not run.
The finally block will always run, even if there is an error in the catch block.
Furthermore, the finally block runs even if a return or break statement stops the code in the try or catch block. return statements in the finally block override return statements in the try or catch block.
function foo() {
try {
return "bar";
} finally {
return "baz";
}
}
foo() // "baz"
oracle docs provide a good answer to this. Bottom line: finally gets called always! Even when you catch only one kind of exception (not the global catch), then finally gets called (after which your application probably breaks if there is no other catch)
the finally block is meant for a special purpose.
finally is useful for more than just exception handling — it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated.
Since it wont effect your business logic,Still it's compiler friendly,In memory aspects.
What if the try-block returns early or throws an exception that you don't handle? You would still want to free the resources you have allocated, right?
EDIT:
The answers to the question seem almost philosphical, there is some 'guessing' and basically 'we believe it should be useful, because it is there, so it should have a use', and 'even Oracle says so'. Or maybe it is there to help the programmer not 'to forget something' or 'accidently exit and not realize it'.
These are almost all valid reasons, but there is also a technical reason.
It helps avoiding code duplication in the cases mentioned, where (a) either the try or one of the catch blocks returns or (b) if within the catch block a second exception is thrown.
In these cases, if some cleanup code or any other code that still needs to be executed after the return and after the second exception, could be placed into the finally block, if it is to be executed both after the try and after the catch block.
You could still do it without the finally block, but the code would have to be duplicated, which the finally block allows you to avoid. This is where you really need it.
So if you are sure you do not miss it as a case of (a) or (b) you could still put the 'finally' code after the try/catch block and omit the finally clause.
But what if the situation changes? When you or another person change the code at some later point it could be forgotten to check if the cleanup code is now skipped in some situation.
So why not always put the cleanup code inside the finally block? And this is what is recommended and what many JavaScript programmers do.
You use it when you want to be sure your code is executed at the end, even if there was an exception during execution :
InputStream is = new FileInputStream("C://test.txt");
try {
//code...
} catch (Exception e) {
//code...
} finally {
is.close();
}
This is a very good question.
There is little to no reason to use finally in javascript, but I can imagine situations where it could be of practical use.
Suppose you have a webpage where you show a certain div after some user action, e.g. button clicked.
The div shows some logging for instance for the action the user requested.
After the action is complete (error or no error), you want to be sure to hide the div again. For that you can use the finally clause.
function doSomething() {
var d = document.getElementById("log");
show(d);
try {
... execute action ...
} catch(e) {
log(e);
} finally {
hide(d);
}
}
In general, as you mentioned, exceptions are less and less used in JavaScript in favor of error callbacks.
So, one could as well ask, what good uses are for exceptions in JavaScript in general.
The problem is with your example. There are cases when you don't want to catch the exception.
try {
if (Math.random() > 0.5) throw 123
}
finally {
console.log(1)
}
In these cases all you could do is rethrowing the exception if you don't want to use finally.
try {
if (Math.random() > 0.5) throw 123
}
catch (e) {
console.log(1)
throw e
}
console.log(1)
or maybe
try {
if (Math.random() > 0.5) throw 123
console.log(1)
}
catch (e) {
console.log(1)
throw e
}
Both alternative solutions lead to code duplication, that's why you need the finally keyword. It is used most of the time to free up unused resources. Forgetting about it may lead to unwanted locks or connections or memory leaks. I guess in some cases even a smart GC cannot prevent it.
In Java, if there's an Exception thrown that is not matched by any of the catch-blocks execution will break and any open resources will be left open.
The finally block will always be executed, even if an uncaught exception occurs.
I am trying to test in browsermob if certain input field work or not. I am attempting to use a try...catch statement which I have never used before. I know that the form is:
try {
//some code
} catch (){
//some error code
};
What exactly is supposed to be put in the parenthesis after the catch statement?
When I try to use the statement it runs everything through the catch statement no matter if it is not an error. What am I doing wrong?
See the “try...catch statement” guide on MDN.
In short, try/catch is used to handle exceptions (which are "thrown" using the throw statement). The syntax for try/catch is:
try {
// Code
} catch (varName) { // Optional
// If exception thrown in try block,
// execute this block
} finally { // Optional
// Execute this block after
// try or after catch clause
// (i.e. this is *always* called)
}
varName is available to the scope of the catch block only. It refers to the exception object which was thrown (which could be any type of object, e.g. a String, but is usually an Error object).
The try catch statement is used to detected for exceptions/errors that are raised inside the try-block. In the catch block you can then react on this exceptional behavior and try to resolve it or get to a safe state.
You got the statement almost right:
try {
// code that may fail with error/exception
} catch (e) { // e represents the exception/error object
// react
}
Consider the following examples:
try {
var x = parseInt("xxx");
if(isNaN(x)){
throw new Error("Not a number");
}
} catch (e) { // e represents the exception/error object
alert(e);
}
try {
// some code
if(!condition){
throw new Error("Something went wrong!");
}
} catch (e) { // e represents the exception/error object
alert(e);
}
the stuff inside try {...} is what you want to execute. The stuff in catch() { ... } is what you want to execute if you get any javascript errors from anything executed in the try {...}
catch {...} only executes if there is a javascript error in the try {...} block. You can find out what the error is by doing for example this:
try {
// do something
} catch (err) {
alert(err);
}
According to ECMAScript specifications,
try {
// Code
} catch (varName) { // optional if 'finally' block is present.
if (condition) { // eg. (varName instanceof URIError)
// Condition (Type) specific error handling
}
else {
// Generic error handling
}
} finally { // Optional if 'catch' block is present.
// Execute this block after
// try or after catch clause
// (i.e. this is *always* called)
}
the code that is likely to throw an exception goes into try { }, The code to be run when an exception is thrown, comes into catch() { }. In catch() you can specify which exceptions you want to catch, and in which automatic variables to put it.
finally { } is always run, regardless whether exception was thrown or not.