In programming, recursion is when a function calls itself. This can be useful for solving problems that can be divided into smaller, similar problems. The process of breaking down a problem into smaller pieces is called “divide and conquer.”
Functional programming is a programming paradigm (a way of organizing code) that emphasizes the use of functions. In functional programming, it is common to use recursion to solve problems.
Examples
Here are five examples of real-world problems that can be solved using the recursion pattern in JavaScript with functional-oriented programming:
Calculating the Factorial of a Number
The factorial of a number is the product of all the numbers from 1 to that number. For example, the factorial of 5 (written as 5!) is 1 * 2 * 3 * 4 * 5 = 120. Here is a recursive function that calculates the factorial of a number:
1 2 3 4 5 | <span class="token keyword">function</span> <span class="token function">factorial</span><span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>n <span class="token operator">===</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token keyword">return</span> n <span class="token operator">*</span> <span class="token function">factorial</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
Flattening an Array
Sometimes, you may have an array that contains other arrays as elements. You can use recursion to flatten the array and return a new array with all the elements in a single level. Here is a recursive function that flattens an array:
1 2 3 4 5 6 7 8 9 10 11 12 | <span class="token keyword">function</span> <span class="token function">flattenArray</span><span class="token punctuation">(</span><span class="token parameter">arr</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">let</span> flatArray <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> arr<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>arr<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> flatArray <span class="token operator">=</span> flatArray<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token function">flattenArray</span><span class="token punctuation">(</span>arr<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span> flatArray<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>arr<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token keyword">return</span> flatArray<span class="token punctuation">;</span> <span class="token punctuation">}</span> |
Finding an Element in an Array
You can use recursion to search for an element in an array. Here is a recursive function that searches for an element in an array and returns its index if found, or -1 if not found:
1 2 3 4 5 6 7 8 | <span class="token keyword">function</span> <span class="token function">findElement</span><span class="token punctuation">(</span><span class="token parameter">arr<span class="token punctuation">,</span> element</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>arr<span class="token punctuation">.</span>length <span class="token operator">===</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> element<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token function">findElement</span><span class="token punctuation">(</span>arr<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">,</span> element<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>index <span class="token operator">===</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token keyword">return</span> index <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
Reversing a String
You can use recursion to reverse a string by breaking it down into smaller strings and concatenating them in the opposite order. Here is a recursive function that reverses a string:
1 2 3 4 5 | <span class="token keyword">function</span> <span class="token function">reverseString</span><span class="token punctuation">(</span><span class="token parameter">str</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>str<span class="token punctuation">.</span>length <span class="token operator">===</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token string">""</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token function">reverseString</span><span class="token punctuation">(</span>str<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">+</span> str<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
Generating the Fibonacci Sequence
The Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones, usually starting with 0 and 1. You can use recursion to generate the Fibonacci sequence. Here is a recursive function that generates the Fibonacci sequence:
1 2 3 4 5 | <span class="token keyword">function</span> <span class="token function">fibonacci</span><span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>n <span class="token operator"><=</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">return</span> n<span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token function">fibonacci</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token function">fibonacci</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> |
Conclusion
The recursion pattern is a powerful technique for solving problems in JavaScript with functional-oriented programming. It allows you to break down a problem into smaller, similar problems and solve them recursively. In this article, we looked at five examples of real-world problems that can be solved using the recursion pattern. By understanding and mastering the recursion pattern, you can become a more effective and efficient programmer.
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