JavaScript Error Statements

The try Statement

In JavaScript, the try statement is used to handle errors (also called exceptions) that may occur during code execution - without stopping the entire program.

The try statement works together with catch.

Sometimes it works with finally.

And sometimes it works with throw.

The Try Block

The try block contains the code that might throw an error.

If no error occurs, the catch block is skipped.

Syntax

try {

  // Code that may cause an error

} catch (error) {

  // Code to handle the error

}

The Catch Block

The catch block executes only if an error occurs in the try block.

The error object provides details about what went wrong.

Syntax

try {

  // Code that may cause an error

} catch (error) {

  // Code to handle the error

}

The Finally Block (Optional)

The finally block executes after the try and catch blocks, whether an error occurred or not.

It is commonly used for cleanup tasks (e.g., closing files, stopping loaders, etc.).

Syntax

try {

  // Code that may cause an error

} catch (error) {

  // Code to handle the error

} finally {

  // Code that always runs, no matter what

}

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).

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:

Example

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:

Example

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   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:

Example

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

Finally Example

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

```javascript 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 See Also: > JavaScript Errors > JavaScript Silent Errors > JavaScript Error Object > JavaScript Debugging