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.
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.
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.
Catching an exception is also known as handling an exception.
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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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.
catch
blocks only execute if an exception is thrown
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.
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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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...
Check Your Understanding
What statement do we use if we want to attempt to run code, but think an exception might be thrown?
catch
try
throw
finally
How do you handle an exception that is thrown?
- With code placed within the
try
block. - With code placed within the
catch
block. - With code placed within a
throw
statement. - With code placed within the
finally
block.
What statement do you use to ensure a code block is executed regardless if an exception was thrown?
throw
catch
try
finally