Js throw learn javascript

JavaScript Errors

The try statement defines a code block to run (to try).

The catch statement defines a code block to handle any error.

The finally statement defines a code block to run regardless of the result.

The throw statement defines a custom error.

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:

try adddlert(«Welcome guest!»);
>
catch(err) document.getElementById(«demo»).innerHTML = err.message;
>

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:

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 :

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

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

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 = «»;
>
>

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

let 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

let x = 5;
try x = y + 1; // y cannot be used (referenced)
>
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

let 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 define 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.

Источник

throw

The throw statement throws a user-defined exception. Execution of the current function will stop (the statements after throw won’t be executed), and control will be passed to the first catch block in the call stack. If no catch block exists among caller functions, the program will terminate.

Try it

Syntax

Description

The throw statement is valid in all contexts where statements can be used. Its execution generates an exception that penetrates through the call stack. For more information on error bubbling and handling, see Control flow and error handling.

The throw keyword can be followed by any kind of expression, for example:

throw error; // Throws a previously defined value (e.g. within a catch block) throw new Error("Required"); // Throws a new Error object 

In practice, the exception you throw should always be an Error object or an instance of an Error subclass, such as RangeError . This is because code that catches the error may expect certain properties, such as message , to be present on the caught value. For example, web APIs typically throw DOMException instances, which inherit from Error.prototype .

Automatic semicolon insertion

The syntax forbids line terminators between the throw keyword and the expression to be thrown.

The code above is transformed by automatic semicolon insertion (ASI) into:

This is invalid code, because unlike return , throw must be followed by an expression.

To avoid this problem (to prevent ASI), you could use parentheses:

Examples

Throwing a user-defined error

This example defines a function that throws a TypeError if the input is not of the expected type.

function isNumeric(x)  return ["number", "bigint"].includes(typeof x); > function sum(. values)  if (!values.every(isNumeric))  throw new TypeError("Can only add numbers"); > return values.reduce((a, b) => a + b); > console.log(sum(1, 2, 3)); // 6 try  sum("1", "2"); > catch (e)  console.error(e); // TypeError: Can only add numbers > 

Throwing an existing object

This example calls a callback-based async function, and throws an error if the callback receives an error.

readFile("foo.txt", (err, data) =>  if (err)  throw err; > console.log(data); >); 

Errors thrown this way are not catchable by the caller and will cause the program to crash unless (a) the readFile function itself catches the error, or (b) the program is running in a context that catches top-level errors. You can handle errors more naturally by using the Promise() constructor.

function readFilePromise(path)  return new Promise((resolve, reject) =>  readFile(path, (err, data) =>  if (err)  reject(err); > resolve(data); >); >); > try  const data = await readFilePromise("foo.txt"); console.log(data); > catch (err)  console.error(err); > 

Specifications

Browser compatibility

BCD tables only load in the browser

See also

Found a content problem with this page?

This page was last modified on Jun 19, 2023 by MDN contributors.

Your blueprint for a better internet.

Источник