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JavaScript•Debugging, Projects, and Reference

JavaScript Error Statements

Flash cards

Review the key moves

1/4

Core idea

What is the main idea behind JavaScript Error Statements?

Lesson checks

Practice each idea before moving on

Short Mimo-style checks built from this lesson's code, terms, and sequence.

1Quick choice

Which statement best captures the main point of this lesson?

2Fill blank

Complete the missing token from the example code.

// ___ that may cause an error

3Order

Put the learning moves in the order that makes the concept easiest to apply.

The throw Statement

JavaScript Throws Errors

The Finally Block (Optional)

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) .

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:

Runnable example

Formatted code

<!DOCTYPE html>
<html>
<body>
<p>Please input a number between
5 and 10:</p>
<input id="demo" type="text">
<button type="button"
onclick="myFunction()">Test Input</button>
<p id="p01"></p>

<script>
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;
  }
}
</script>
</body>
</html>

Live preview

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:

<input id="demo" type="number" min="5" max="10" step="1">

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

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 = "";
}
}

JavaScript Error Statements

Flash cards

Review the key moves

1/4

Core idea

What is the main idea behind JavaScript Error Statements?

Lesson checks

Practice each idea before moving on

Short Mimo-style checks built from this lesson's code, terms, and sequence.

1Quick choice

Which statement best captures the main point of this lesson?

2Fill blank

Complete the missing token from the example code.

// ___ that may cause an error

3Order

Put the learning moves in the order that makes the concept easiest to apply.

The throw Statement

JavaScript Throws Errors

The Finally Block (Optional)

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) .

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:

Runnable example

Formatted code

<!DOCTYPE html>
<html>
<body>
<p>Please input a number between
5 and 10:</p>
<input id="demo" type="text">
<button type="button"
onclick="myFunction()">Test Input</button>
<p id="p01"></p>

<script>
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;
  }
}
</script>
</body>
</html>

Live preview

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:

<input id="demo" type="number" min="5" max="10" step="1">

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

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 = "";
}
}