Catch exception ex c asp net là gì năm 2024

Errors can be coding errors made by the programmer, errors due to wrong input, and other unforeseeable things.

Example

In this example we misspelled "alert" as "adddlert" to deliberately produce an error:

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JavaScript catches adddlert as an error, and executes the catch code to handle it.

JavaScript try and catch

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The JavaScript statements try and `catch`come in pairs:

try { Block of code to try } catch(err) { Block of code to handle errors }

JavaScript Throws Errors

When an error occurs, JavaScript will normally stop and generate an error message.

The technical term for this is: JavaScript will throw an exception (throw an error).

JavaScript will actually create an Error object with two properties: name and message.

The throw Statement

The throw statement allows you to create a custom error.

Technically you can throw an exception (throw an error).

The exception can be a JavaScript String, a `catch`0, a `catch`1 or an `catch`2:

throw "Too big"; // throw a text throw 500; // throw a number

If you use throw together with try and catch, you can control program flow and generate custom error messages.

Input Validation Example

This example examines input. If the value is wrong, an exception (err) is thrown.

The exception (err) is caught by the catch statement and a custom error message is displayed:

Please input a number between 5 and 10:

Test Input

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HTML Validation

The code above is just an example.

Modern browsers will often use a combination of JavaScript and built-in HTML validation, using predefined validation rules defined in HTML attributes:

You can read more about forms validation in a later chapter of this tutorial.

The finally Statement

The finally statement lets you execute code, after try and catch, regardless of the result:

Syntax

try { Block of code to try } catch(err) { Block of code to handle errors } finally { Block of code to be executed regardless of the try / catch result }

Example

function myFunction() { const message = document.getElementById("p01"); message.innerHTML = ""; let x = document.getElementById("demo").value; try { if(x.trim() == "") throw "is empty"; if(isNaN(x)) throw "is not a number"; x = Number(x); if(x > 10) throw "is too high"; if(x < 5) throw "is too low"; } catch(err) { message.innerHTML = "Error: " + err + "."; } finally { document.getElementById("demo").value = ""; } }

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The Error Object

JavaScript has a built in error object that provides error information when an error occurs.

The error object provides two useful properties: name and message.

Error Object Properties

PropertyDescription nameSets or returns an error name messageSets or returns an error message (a string)

Error Name Values

Six different values can be returned by the error name property:

Error NameDescription EvalErrorAn error has occurred in the eval() function RangeErrorA number "out of range" has occurred ReferenceErrorAn illegal reference has occurred SyntaxErrorA syntax error has occurred TypeErrorA type error has occurred URIErrorAn error in encodeURI() has occurred

The six different values are described below.

Eval Error

An `catch`7 indicates an error in the eval() function.

Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.

Range Error

A `catch`8 is thrown if you use a number that is outside the range of legal values.

For example: You cannot set the number of significant digits of a number to 500.

Example

let num = 1; try { num.toPrecision(500); // A number cannot have 500 significant digits } catch(err) { document.getElementById("demo").innerHTML = err.name; }

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Reference Error

A `catch`9 is thrown if you use (reference) a variable that has not been declared:

Example

let x = 5; try { x = y + 1; // y cannot be used (referenced) } catch(err) { document.getElementById("demo").innerHTML = err.name; }

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Syntax Error

A `finally`0 is thrown if you try to evaluate code with a syntax error.

Example

try { eval("alert('Hello)"); // Missing ' will produce an error } catch(err) { document.getElementById("demo").innerHTML = err.name; }

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Type Error

A `finally`1 is thrown if an operand or argument is incompatible with the type expected by an operator or function.