17.3. Exceptions as Control Flow

Runtime errors occur as the program runs, and they are also called exceptions. Exceptions are caused by referencing undeclared variables and invalid or unexpected data.

17.3.1. Control Flow

The control flow of a program is the order in which the statements are executed. Normal control flow runs from top to bottom of a file. An exception breaks the normal flow and stops the program. A stopped program can no longer interact with the user. Luckily JavaScript provides a way to anticipate and handle exceptions.

17.3.2. Catching an Exception

JavaScript provides try and catch statements that allow us to keep our programs running even if there is an exception. We can tell JavaScript to try to run a block of code, and if an exception is thrown, to catch the exception and run a specific block of code. Anticipating and catching the exception makes the exception now part of the control flow.

Note

Catching an exception is also known as handling an exception.

Example

In this example there is an array of animals. The user is asked to enter the index for the animal they want to see. If the user enters an index that does NOT contain an animal, the code will throw a TypeError when name is referenced on an undefined value.

There is a try block around the code that will throw the TypeError. There is a catch block that catches the error and contains code to inform the user that they entered an invalid index.

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const input = require('readline-sync');

let animals = [{name: 'cat'}, {name: 'dog'}];
let index = Number(input.question("Enter index of animal:"));

try {
   console.log('animal at index:', animals[index].name);
} catch(err) {
   console.log("We caught a TypeError, but our program continues to run!");
   console.log("You tried to access an animal at index:", index);
}

console.log("the code goes on...");

repl.it

Console Output

If the user enters 9:

Enter index of animal: 9
We caught a TypeError, but our program continues to run!
You tried to access an animal at index: 9
the code goes on...

If the user enters 0:

Enter index of animal: 0
animal at index: cat
the code goes on...

On line 8 of the above code sample, a variable called err is passed to catch. err can be any type of error object in JavaScript. For the above example, that is a TypeError, but we could have had a ReferenceError or EvalError. If necessary, we could output different statements based on what kind of error err is.

Tip

catch blocks only execute if an exception is thrown

17.3.3. Finally

JavaScript also provides a finally block which can be used with try and catch blocks. A finally block code runs after the try and catch. What is special about finally is that finally code block ALWAYS runs, even if an exception is NOT thrown.

Example

Let's update the above example to print out the index the user entered. We want this message to be printed EVERY time the code runs. Notice the console.log statement on line 11.

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const input = require('readline-sync');

let animals = [{name: 'cat'}, {name: 'dog'}];
let index = Number(input.question("Enter index of animal:"));

try {
   console.log('animal at index:', animals[index].name);
} catch(err) {
   console.log("We caught a TypeError, but our program continues to run!");
} finally {
   console.log("You tried to access an animal at index:", index);
}

console.log("the code goes on...");

repl.it

Console Output

If the user enters 7:

Enter index of animal: 7
We caught a TypeError, but our program continues to run!
You tried to access an animal at index: 7
the code goes on...

If the user enters 1:

Enter index of animal: 1
animal at index: dog
You tried to access an animal at index: 1
the code goes on...

17.3.4. Check Your Understanding

Question

What statement do we use if we want to attempt to run code, but think an exception might be thrown?

  1. catch

  2. try

  3. throw

  4. finally

Question

How do you handle an exception that is thrown?

  1. With code placed within the try block.

  2. With code placed within the catch block.

  3. With code placed within a throw statement.

  4. With code placed within the finally block.

Question

What statement do you use to ensure a code block is executed regardless if an exception was thrown?

  1. throw

  2. catch

  3. try

  4. finally