Added JavaScript_Advance lesson on recursion

JavaScript_Advance/recursion.js

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+// Recursion is when a function calls itself.
+// Recursion gives us an interesting way to write algorithms that can solve complicated problems.
+
+// Take for example factorials which is just an integer times each of the integers below it: so 5 factorial (also written 5!) is just 5 * 4 * 3 * 2 * 1.
+// If we want to write an algorithm that can calculate the factorial of any given number without using recursion, we could do something like this:
+
+function calcuateFactorialWithoutRecursion(num) {
+  let total = num;
+  let nextNumber = num - 1
+  while(nextNumber >= 1) {
+    total = total * nextNumber; 
+    nextNumber-- // decrease the next number by 1
+  }
+  return total;
+}
+
+// calcuateFactorialWithoutRecursion(5) // 120
+// 5 * 4 * 3 * 2 * 1 = 120
+
+// We can write the same function this way using recursion:
+function calculateFactorialWithRecursion(num) {
+  if(num === 1) { // This step is critical. The num parameter will keep decreasing until it gets to 1. 
+    // Once the function gets called with 1 as a parameter, the function will return without calling itself.
+    return num;
+  }
+  return num * calculateFactorialWithRecursion(num - 1);
+}
+
+calculateFactorialWithRecursion(7) // 5040
+// 7 * 6 * 5 * 4 * 3 * 2 * 1 = 5040
+
+// See how clean recursion made this algorithm!