Recursion in JavaScript – What Is a Recursive Function?
Recursion is the method by which a problem gets solved through iteration.
In other words, a recursive function is a function that repeatedly invokes itself infinitely (or until something stops it).
Important Stuff to Know About Recursive Functions
Keep these two essential pieces of info in mind whenever you choose to use recursive functions.
1. Recursion is not an IIFE
A recursive function is different from an Immediately Invoking function Expression (IIFE).
An IIFE automatically invokes itself once.
However, a recursive function automatically invokes itself repeatedly for an unlimited amount of time or until something stops its re-invocation.
2. A recursive function needs a base case
The code written to discontinue the re-invocation of a recursive function is called a base case.
It is always important to define a base case when creating a recursive function—so that the function will not run endlessly, thereby crashing the browser.
Example of a Recursive Function
Below is a JavaScript code that outputs a concatenation of all the values returned through the countDown()
function’s recursive invocation.
In the recursive algorithm above, the countDown(num - 1)
code makes the whole function a recursion because it is the code that makes countDown()
recall itself repeatedly.
Events Behind the Scenes
When we invoked the countDown
function and passed in 2
(that is, countDown(2)
), the algorithm started running as follows:
Step 1: Check if 2
is less than 0
The computer checked if the value 2
—that we passed to the num
parameter of the countDown
function—is less than 0
.
Since 2
is not less than 0
, the computer did not execute the if
statement’s code. Instead, it skipped to the next code after the if
statement—which is the recursion code.
Step 2: Execute the return
statement
After skipping the if
statement, the computer executed the return num + " " + countDown(num - 1)
code—but substituted the num
parameter with the parameter’s value (that is, 2
) like so:
Step 3: Execute only the recursive statement
In step 2’s code above, notice that the return
command cannot return any value because the return
statement includes a recursive code (countDown(1)
) recalling the countDown
function.
Therefore, while retaining the other parts of the return
statement (that is, 2 + ", " +
), the computer will execute only the recursion code (countDown(1)
).
In other words, the countDown(1)
code will automatically invoke the countDown
function while passing in 1
. Then, the algorithm will start running again by checking if 1
is less than 0
.
Since 1
is not less than 0
, the computer skipped to the recursion code like so:
Step 4: Invoke only the recursion code
Again, notice that the return
command (in step 3) cannot return any value because the return
statement includes a recursion code (countDown(0)
) that recalls the countDown
function.
Therefore, while retaining the other parts of the return
statement (that is, 2 + ", " + 1 + ", " +
), the computer will execute only the recursion code (countDown(0)
). So, the countDown(0)
code will automatically invoke the countDown
function while passing in 0
.
Then, the function will start running again by checking if 0
is less than 0
.
Since 0
is not less than 0
, the computer skipped to the recursion code like so:
Step 5: Execute only the recursion code
Yet again, the return
command (in step 4) cannot return
any value because the return
statement includes a recursion code (countDown(-1)
) recalling the countDown
function.
Therefore, while retaining the other parts of the return
statement (that is, 2 + ", " + 1 + ", " + 0 + ", " +
), the computer will execute only the recursion code (countDown(-1)
). So, the countDown(-1)
code will automatically invoke the countDown
function while passing in -1
.
Then, the function will start running again by checking if -1
is less than 0
.
At this point, -1
is less than 0
. Therefore, the computer will execute the code of the if
statement by returning the value "Recursion Stopped!"
like so:
At last, the return
statement now has values it can validly concatenate and return. Therefore, the returned value from countDown
will be: