JavaScript Errors
Table of contents
JavaScript Errors - Throw and Try to Catch The try statement lets you test a block of code for errors.
The catch statement lets you handle the error.
The throw statement lets you create custom errors.
The finally statement lets you execute code, after try and catch, regardless of the result.
Errors Will Happen! When executing JavaScript code, different errors can occur.
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:
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 Number, a Boolean or an Object:
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:
<!DOCTYPE html>
Please input a number between 5 and 10:
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() { var message, x; message = document.getElementById(“p01”); message.innerHTML = “”; x = document.getElementById(“demo”).value; try { if(x == “”) 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 = “”; } } 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 Property Description name Sets or returns an error name message Sets or returns an error message (a string) Error Name Values Six different values can be returned by the error name property:
Error Name Description EvalError An error has occurred in the eval() function RangeError A number “out of range” has occurred ReferenceError An illegal reference has occurred SyntaxError A syntax error has occurred TypeError A type error has occurred URIError An error in encodeURI() has occurred The six different values are described below.
Eval Error An EvalError indicates an error in the eval() function.
Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.
Range Error A RangeError 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 var num = 1; try { num.toPrecision(500); // A number cannot have 500 significant digits } catch(err) { document.getElementById(“demo”).innerHTML = err.name; } Reference Error A ReferenceError is thrown if you use (reference) a variable that has not been declared:
Example var x; try { x = y + 1; // y cannot be referenced (used) } catch(err) { document.getElementById(“demo”).innerHTML = err.name; } Syntax Error A SyntaxError 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; } Type Error A TypeError is thrown if you use a value that is outside the range of expected types:
Example var num = 1; try { num.toUpperCase(); // You cannot convert a number to upper case } catch(err) { document.getElementById(“demo”).innerHTML = err.name; } URI (Uniform Resource Identifier) Error A URIError is thrown if you use illegal characters in a URI function:
Example try { decodeURI(“%%%”); // You cannot URI decode percent signs } catch(err) { document.getElementById(“demo”).innerHTML = err.name; } Non-Standard Error Object Properties Mozilla and Microsoft defines some non-standard error object properties:
fileName (Mozilla) lineNumber (Mozilla) columnNumber (Mozilla) stack (Mozilla) description (Microsoft) number (Microsoft)
Do not use these properties in public web sites. They will not work in all browsers.
Complete Error Reference
For a complete reference of the Error object, go to our Complete JavaScript Error Reference