Recursion is a mathematical concept that has many applications in daily life.
As website developers, we encounter recursive functions every day.
This tutorial will explore the pattern of problems, which can be solved using recursion.
Basic Concept
function recurse() {
// 2nd call to itself
recurse();
}
// 1st call
recurse();
Each recursive function must have a base case (also called termination condition), where it stops the recursion, or else it will continue calling itself indefinitely.
function recurse() {
if (terminate)
return; // stop calling recurse();
// continue recurse() if there is no termination
recurse();
}
recurse();
While Loop and Recursion Comparison
The recursion technique looks similar to the while loop.
Imagine that you need to multiply the desired number by themselves X times.
For example: 2 * 2 * 2 = 8
While Loop
function multiply(n, x) {
let i = 0;
let res = 1;
while (i < x) {
res = res * n;
i++;
}
return res;
}
How does the loop works?
multiply(2,3)
1. i = 0, res = (1) * 2 // 0 < 3 continue ...
2. i = 1; res = (2) * 2 // 1 < 3 continue ...
3. i = 2; res = (2 * 2) * 2 // 2 < 3 continue ...
4. i = 3; res = (2 * 2 * 2) // 3 < 3 (false) break and return 8
Recursion 🔁
function multiply(n, x) {
return x > 1 ? n * multiply(n, x - 1) : n;
}
How does the recursion works?
Examples
#1 (String URL Encode)
Let's imagine we need to URL encode the string <html> 5 times
The output should look like this:
%252525253Chtml%252525253E
Loop Solution
function encode(str, n) {
let i = 0;
while (i < n) {
str = encodeURI(str)
i++;
}
return str;
}
Recursion Solution 🔁
function encode(str, n) {
return n ? encode(encodeURI(str), n - 1) : str;
}
#2 (String URL Decode)
Let's imagine we need to decode an URL that has been encoded multiple times
For example let's take previous URL encoded string:
%252525253Chtml%252525253E
The output result will be: <html>
Loop Solution
function decode(str) {
while (str !== decodeURI(str)) {
str = decodeURI(str)
}
return str;
}
Recursion Solution 🔁
function decode(str) {
return str !== decodeURI(str) ? decode(decodeURI(str)) : str;
}
#3 (String Replace)
Imagine you need to replace bad tags, like <script>, from your HTML code
1st case: hello<script> world<script>
2nd case: hello<sc<script>ript>world
With the first case, we can easily do something like this:
let html_code = 'hello<script> world<script>';
let output = html_code.replaceAll('<script>','');
// output: hello world
But.. with the second case it will fail:
let html_code = 'hello<sc<script>ript> world';
let output = html_code.replaceAll('<script>','');
// output: hello<script> world
Here is where Recursion comes to the rescue
Recursion Solution 🔁
function clean_html(html, bad_tag) {
let c_html = html.replaceAll(bad_tag, '');
return html === c_html ? html : clean_html(c_html, bad_tag)
}
clean_html('hello<sc<script>ript> world', '<script>');
// output: hello world
#4 (Find Nested Elements)
In this example, we need to find category by ID in a deeply nested array
Our target is a category with ID number 5
let the_category_list = [
{"id" : 1, "name" : "fruits", "child_list" : [
{"id" : 2, "name" : "apple", "child_list" : [
{"id" : 4, "name" : "red apple", "child_list" : []},
{"id" : 5, "name" : "green apple", "child_list" : []}
]},
{"id" : 3, "name" : "banana", "child_list" : []}
]}
]
Recursion Solution 🔁
function find_cat_by_id(id, category_list) {
let found_category = false;
category_list.forEach(cat => {
if (cat.id === id)
found_category = cat ;
if (found_category === false && cat.child_list.length)
found_category = find_cat_by_id(id, cat.child_list)
});
return (found_category) ? found_category : false;
}
find_cat_by_id(5, the_category_list)
// Output: {id: 5, name: "green apple", child_list: Array(0)}
#5 (Factorial Using Recursion)
This example will show you how to write a factorial program in javascript using recursion
Let’s imagine we need a factorial of 5: 1 * 2 * 3 * 4 * 5 = 120
Recursion Solution 🔁
function factorial(x) {
return x ? x * factorial(x - 1) : 1;
}
#6 (Fibonacci Series Using Recursion)
In this example, you will learn how to write a program to print the Fibonacci series using recursion
Fibonacci sequence is written as: 0, 1, 1, 2, 3, 5, 8, 13, 21, ...
Recursion Solution 🔁
function fibonacci(num) {
return num < 2 ? num : fibonacci(num - 1) + fibonacci(num - 2);
}
function fibonacci_printer(numberOfTerms) {
let out = []; for(let i = 0; i < numberOfTerms; i++) {
out.push(fibonacci(i));
} console.log(out.join(', '));
}
To use this program, you need to call fibonacci_printer(5) and the output will be: 0, 1, 1, 2, 3
Thanks for reading!
More examples will be added later.
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Top comments (2)
Plot twist: recursion is just a loop in disguise.
Any recursive function can be represented as a loop and vice versa.
Yes :) I couldn't find an example where the desired task can be completed only using recursion and nothing else