In this section, we will learn what the throw statement is and how to use it in JavaScript.

Note: we’re assuming you’re already read the try catch section.

What is throw Statement in JavaScript? (Throwing Errors in JavaScript)

JavaScript automatically catches and throws errors when they appear in the program. But sometime we want to manually throw an error when part of our program doesn’t follow the logic that is defined for it.

In situations like this, we can use the `throw` keyword to create a statement that produces an error. This error, then, can be handled by a `catch` block.

throw Statement Syntax:

throw errorMessage;

On the right side of the `throw` keyword, we put a value (like a String value) that will be passed to the target `catch` block. We use this value in the `catch` block to see the reason for the error.

For example:

throw "Wrong password";

Here the value `”Wrong password”` will be passed to the target `catch` block and inside this block we can for example directly send this value to the browser’s console as the reason of the error so that the users (including developers) can see it. Or we can store this error in a file, etc.

Example: using throw statement in JavaScript

const rand = Math.random();
try{
  if (rand < 0.5){
    throw "The value should be higher than 0.5";
  }
}catch(error){
  console.log("Inside the catch block");
  console.log(error);
}
console.log("End");

Output:

Inside the catch block

The value should be higher than 0.5

End

As you can see, the message that we set on the right side of the `throw` keyword is passed to the `catch` block and for this particular example, we just sent the value to the console.

Note: The `random()` method will generate a value that is in a range from 0 to 1. Please check the `Math.random()` section to learn more about this method.

JavaScript Error Constructor Function

Usually, when using the `throw` keyword, we create an object via the `Error` constructor function or other constructor functions that are linked (prototype wise) to this `Error` function and pass the object to the target `catch` block.

This constructor function takes one argument and that is the message (reason) of the error.

Example: using JavaScript Error Constructor function

const rand = Math.random();
try{
  if (rand < 0.5){
    throw new Error("The value should be higher than 0.5");
  }
}catch(error){
  console.log("Inside the catch block");
  console.log(error.name);
  console.log(error.message);
}
console.log("End");

Output:

Inside the catch block

Error

The value should be higher than 0.5

End

The other constructor functions that the JavaScript engine will automatically use to generate an error are:

Note: besides the engine, developers can use these constructor functions in their program if they need to.

JavaScript Rethrow an Exception

Rethrowing an exception basically means calling the `throw` operator in the body of a catch block that is handling an occurred error.

We rethrow an error because sometimes the catch handler that is currently handling an error might not do sufficient work and we have another enclosing handler that can run the rest of necessary tasks. So if we rethrow the error in the same form or another, then this error will reach to the enclosing handler and that can handle the rest of work.

Example: rethrowing an exception in JavaScript

function throwError(){
  try{
    console.log("Throwing an error");
    throw "This is a simple error";
  }catch(error){
    console.log("First handler catched the error but now rethrowing it!");
    throw error; 
  }
}

try{
  throwError();
}catch(error){
  console.log("The second handler catched the error");
  console.log(`Here's the message of the error: ${error}`);
}

Output:

Throwing an error
First handler catched the error but now rethrowing it!
The second handler catched the error
Here's the message of the error: This is a simple error

Reference book:

Professional JavaScript for Web Developers, 4^th Edition.

Author: Matt Frisbie

More to Read:

JavaScript finally block tutorial