diff --git a/Conversions/BinaryToDecimal.js b/Conversions/BinaryToDecimal.js
index 907468470..4a391cf84 100644
--- a/Conversions/BinaryToDecimal.js
+++ b/Conversions/BinaryToDecimal.js
@@ -1,11 +1,14 @@
-function binaryToDeicmal (binaryNumber) {
+const binaryToDecimal = (binaryString) => {
   let decimalNumber = 0
-  const binaryDigits = binaryNumber.split('').reverse() // Splits the binary number into reversed single digits
+  const binaryDigits = binaryString.split('').reverse() // Splits the binary number into reversed single digits
   binaryDigits.forEach((binaryDigit, index) => {
     decimalNumber += binaryDigit * (Math.pow(2, index)) // Summation of all the decimal converted digits
   })
-  console.log(`Decimal of ${binaryNumber} is ${decimalNumber}`)
+  console.log(`Decimal of ${binaryString} is ${decimalNumber}`)
+  return decimalNumber
 }
 
-binaryToDeicmal('111001')
-binaryToDeicmal('101')
+(() => {
+  binaryToDecimal('111001')
+  binaryToDecimal('101')
+})()
diff --git a/DIRECTORY.md b/DIRECTORY.md
index e447facda..475f2bac3 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -34,7 +34,9 @@
     * [MaxHeap](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Heap/MaxHeap.js)
     * [MinPriorityQueue](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Heap/MinPriorityQueue.js)
   * Linked-List
+    * [CycleDetection](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/CycleDetection.js)
     * [DoublyLinkedList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/DoublyLinkedList.js)
+    * [SingleCircularLinkedList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/SingleCircularLinkedList.js.js)
     * [SinglyLinkList](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Linked-List/SinglyLinkList.js)
   * Queue
     * [Queue](https://github.com/TheAlgorithms/Javascript/blob/master/Data-Structures/Queue/Queue.js)
@@ -50,6 +52,7 @@
   * [ClimbingStairs](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/ClimbingStairs.js)
   * [CoinChange](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/CoinChange.js)
   * [EditDistance](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/EditDistance.js)
+  * [FibonacciNumber](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/FibonacciNumber.js)
   * [KadaneAlgo](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/KadaneAlgo.js)
   * [LevenshteinDistance](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/LevenshteinDistance.js)
   * [LongestCommonSubsequence](https://github.com/TheAlgorithms/Javascript/blob/master/Dynamic-Programming/LongestCommonSubsequence.js)
@@ -87,6 +90,7 @@
   * [FindHcf](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/FindHcf.js)
   * [FindLcm](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/FindLcm.js)
   * [GridGet](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/GridGet.js)
+  * [MeanSquareError](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/MeanSquareError.js)
   * [ModularBinaryExponentiationRecursive](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/ModularBinaryExponentiationRecursive.js)
   * [Palindrome](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/Palindrome.js)
   * [PascalTriangle](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/PascalTriangle.js)
@@ -101,6 +105,7 @@
 
 ## Recursive
   * [EucledianGCD](https://github.com/TheAlgorithms/Javascript/blob/master/Recursive/EucledianGCD.js)
+  * [FibonacciNumberRecursive](https://github.com/TheAlgorithms/Javascript/blob/master/Recursive/FibonacciNumberRecursive.js)
   * [TowerOfHanoi](https://github.com/TheAlgorithms/Javascript/blob/master/Recursive/TowerOfHanoi.js)
 
 ## Search
@@ -130,6 +135,7 @@
   * [QuickSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/QuickSort.js)
   * [RadixSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/RadixSort.js)
   * [SelectionSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/SelectionSort.js)
+  * [SelectionSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/SelectionSort.test.js)
   * [ShellSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/ShellSort.js)
   * [TimSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/TimSort.js)
   * [TopologicalSort](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/TopologicalSort.js)
@@ -141,6 +147,8 @@
   * [CheckPalindrome](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckPalindrome.js)
   * [CheckPalindrome](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckPalindrome.test.js)
   * [CheckRearrangePalindrome](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckRearrangePalindrome.js)
+  * [CheckWordOccurrence](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckWordOccurrence.js)
+  * [CheckWordOcurrence](https://github.com/TheAlgorithms/Javascript/blob/master/String/CheckWordOcurrence.test.js)
   * [PatternMatching](https://github.com/TheAlgorithms/Javascript/blob/master/String/PatternMatching.js)
   * [PatternMatching](https://github.com/TheAlgorithms/Javascript/blob/master/String/PatternMatching.test.js)
   * [ReverseString](https://github.com/TheAlgorithms/Javascript/blob/master/String/ReverseString.js)
diff --git a/Data-Structures/Linked-List/CycleDetection.js b/Data-Structures/Linked-List/CycleDetection.js
new file mode 100644
index 000000000..910fe2214
--- /dev/null
+++ b/Data-Structures/Linked-List/CycleDetection.js
@@ -0,0 +1,32 @@
+/**
+ * A LinkedList based solution for Detect a Cycle in a list
+ * https://en.wikipedia.org/wiki/Cycle_detection
+ */
+
+function main () {
+  /*
+  Problem Statement:
+  Given head, the head of a linked list, determine if the linked list has a cycle in it.
+
+  Note:
+  * While Solving the problem in given link below, don't use main() function.
+  * Just use only the code inside main() function.
+  * The purpose of using main() function here is to aviod global variables.
+
+  Link for the Problem: https://leetcode.com/problems/linked-list-cycle/
+  */
+  const head = '' // Reference to head is given in the problem. So please ignore this line
+  let fast = head
+  let slow = head
+
+  while (fast != null && fast.next != null && slow != null) {
+    fast = fast.next.next
+    slow = slow.next
+    if (fast === slow) {
+      return true
+    }
+  }
+  return false
+}
+
+main()
diff --git a/Data-Structures/Linked-List/SingleCircularLinkedList.js.js b/Data-Structures/Linked-List/SingleCircularLinkedList.js.js
new file mode 100644
index 000000000..fd50253ad
--- /dev/null
+++ b/Data-Structures/Linked-List/SingleCircularLinkedList.js.js
@@ -0,0 +1,97 @@
+class Node {
+  constructor (data, next = null) {
+    this.data = data
+    this.next = next
+  }
+}
+
+class SinglyCircularLinkedList {
+  constructor () {
+    this.head = null
+    this.size = 0
+  }
+
+  insert (data) {
+    const node = new Node(data)
+
+    if (!this.head) {
+      node.next = node
+      this.head = node
+      this.size++
+    } else {
+      node.next = this.head
+
+      let current = this.head
+
+      while (current.next.data !== this.head.data) {
+        current = current.next
+      }
+
+      current.next = node
+      this.size++
+    }
+  }
+
+  insertAt (index, data) {
+    const node = new Node(data)
+
+    if (index < 0 || index > this.size) return
+
+    if (index === 0) {
+      this.head = node
+      this.size = 1
+      return
+    }
+
+    let previous
+    let count = 0
+    let current = this.head
+
+    while (count < index) {
+      previous = current
+      current = current.next
+      count++
+    }
+
+    node.next = current
+    previous.next = node
+    this.size++
+  }
+
+  remove () {
+    if (!this.head) return
+
+    let prev
+    let current = this.head
+
+    while (current.next !== this.head) {
+      prev = current
+      current = current.next
+    }
+
+    prev.next = this.head
+    this.size--
+  }
+
+  printData () {
+    let count = 0
+    let current = this.head
+
+    while (current !== null && count !== this.size) {
+      console.log(current.data + '\n')
+      current = current.next
+      count++
+    }
+  }
+}
+
+const ll = new SinglyCircularLinkedList()
+
+ll.insert(10)
+ll.insert(20)
+ll.insert(30)
+ll.insert(40)
+ll.insert(50)
+ll.insertAt(5, 60)
+ll.remove(5)
+ll.printData()
diff --git a/Dynamic-Programming/FibonacciNumber.js b/Dynamic-Programming/FibonacciNumber.js
new file mode 100644
index 000000000..a39912f18
--- /dev/null
+++ b/Dynamic-Programming/FibonacciNumber.js
@@ -0,0 +1,18 @@
+//  https://en.wikipedia.org/wiki/Fibonacci_number
+
+const fibonacci = (N) => {
+  // creating array to store values
+  const memo = new Array(N + 1)
+  memo[0] = 0
+  memo[1] = 1
+  for (let i = 2; i <= N; i++) {
+    memo[i] = memo[i - 1] + memo[i - 2]
+  }
+  return memo[N]
+}
+
+// testing
+(() => {
+  const number = 5
+  console.log(number + 'th Fibonacci number is ' + fibonacci(number))
+})()
diff --git a/Maths/MeanSquareError.js b/Maths/MeanSquareError.js
new file mode 100644
index 000000000..de9dcfd7f
--- /dev/null
+++ b/Maths/MeanSquareError.js
@@ -0,0 +1,26 @@
+// Wikipedia: https://en.wikipedia.org/wiki/Mean_squared_error
+
+const meanSquaredError = (predicted, expected) => {
+  if (!Array.isArray(predicted) || !Array.isArray(expected)) {
+    throw new TypeError('Argument must be an Array')
+  }
+
+  if (predicted.length !== expected.length) {
+    throw new TypeError('The two lists must be of equal length')
+  }
+
+  let err = 0
+
+  for (let i = 0; i < expected.length; i++) {
+    err += (expected[i] - predicted[i]) ** 2
+  }
+
+  return err / expected.length
+}
+
+// testing
+(() => {
+  console.log(meanSquaredError([1, 2, 3, 4], [1, 2, 3, 4]) === 0)
+  console.log(meanSquaredError([4, 3, 2, 1], [1, 2, 3, 4]) === 5)
+  console.log(meanSquaredError([2, 0, 2, 0], [0, 0, 0, 0]) === 3)
+})()
diff --git a/Recursive/FibonacciNumberRecursive.js b/Recursive/FibonacciNumberRecursive.js
new file mode 100644
index 000000000..e5c5bb95b
--- /dev/null
+++ b/Recursive/FibonacciNumberRecursive.js
@@ -0,0 +1,13 @@
+//  https://en.wikipedia.org/wiki/Fibonacci_number
+
+const fibonacci = (N) => {
+  if (N === 0 || N === 1) return N
+
+  return fibonacci(N - 2) + fibonacci(N - 1)
+}
+
+// testing
+(() => {
+  const number = 5
+  console.log(number + 'th Fibonacci number is ' + fibonacci(number))
+})()
diff --git a/Sorts/SelectionSort.js b/Sorts/SelectionSort.js
index 38a4bfff2..c4a11e354 100644
--- a/Sorts/SelectionSort.js
+++ b/Sorts/SelectionSort.js
@@ -8,12 +8,19 @@
  *from the unsorted subarray is picked and moved to the sorted subarray.
  */
 
-function selectionSort (items) {
-  var length = items.length
-  for (var i = 0; i < length - 1; i++) {
+const selectionSort = (list) => {
+  if (!Array.isArray(list)) {
+    throw new TypeError('Given input is not an array')
+  }
+  const items = [...list] // We don't want to modify the original array
+  const length = items.length
+  for (let i = 0; i < length - 1; i++) {
+    if (typeof items[i] !== 'number') {
+      throw new TypeError('One of the items in your array is not a number')
+    }
     // Number of passes
-    var min = i // min holds the current minimum number position for each pass; i holds the Initial min number
-    for (var j = i + 1; j < length; j++) { // Note that j = i + 1 as we only need to go through unsorted array
+    let min = i // min holds the current minimum number position for each pass; i holds the Initial min number
+    for (let j = i + 1; j < length; j++) { // Note that j = i + 1 as we only need to go through unsorted array
       if (items[j] < items[min]) { // Compare the numbers
         min = j // Change the current min number position if a smaller num is found
       }
@@ -21,16 +28,23 @@ function selectionSort (items) {
     if (min !== i) {
       // After each pass, if the current min num != initial min num, exchange the position.
       // Swap the numbers
-      [items[i], items[min]] = [items[min], [items[i]]]
+      [items[i], items[min]] = [items[min], items[i]]
     }
   }
+  return items
 }
 
-// Implementation of Selection Sort
+/* Implementation of Selection Sort
 
-var ar = [5, 6, 7, 8, 1, 2, 12, 14]
-// Array before Sort
-console.log(ar)
-selectionSort(ar)
-// Array after sort
-console.log(ar)
+(() => {
+  let array = [5, 6, 7, 8, 1, 2, 12, 14]
+  // Array before Sort
+  console.log(array)
+  array = selectionSort(array)
+  // Array after sort
+  console.log(array)
+})()
+
+*/
+
+export { selectionSort }
diff --git a/Sorts/SelectionSort.test.js b/Sorts/SelectionSort.test.js
new file mode 100644
index 000000000..57ad1e8ec
--- /dev/null
+++ b/Sorts/SelectionSort.test.js
@@ -0,0 +1,22 @@
+import { selectionSort } from './SelectionSort'
+
+describe('selectionSort', () => {
+  it('expects to return the array sorted in ascending order', () => {
+    var toSort = [5, 6, 7, 8, 1, 2, 12, 14]
+    const expected = [1, 2, 5, 6, 7, 8, 12, 14]
+
+    expect(selectionSort(toSort)).toEqual(expected)
+  })
+
+  it('expects to throw if it is not a valid array', () => {
+    expect(() => selectionSort('abc')).toThrow('Given input is not an array')
+    expect(() => selectionSort(123)).toThrow('Given input is not an array')
+    expect(() => selectionSort({})).toThrow('Given input is not an array')
+    expect(() => selectionSort(null)).toThrow('Given input is not an array')
+    expect(() => selectionSort()).toThrow('Given input is not an array')
+  })
+
+  it('expects to throw if one of the elements in the array is not a number', () => {
+    expect(() => selectionSort([1, 'x', 2])).toThrow('One of the items in your array is not a number')
+  })
+})
diff --git a/String/CheckWordOccurrence.js b/String/CheckWordOccurrence.js
new file mode 100644
index 000000000..b86150c0b
--- /dev/null
+++ b/String/CheckWordOccurrence.js
@@ -0,0 +1,25 @@
+/**
+  * Check and count occurrence of each word in a string
+  * Inputs a String eg. Madonna and Boolean
+  **/
+
+const checkWordOccurrence = (str, isCaseSensitive = false) => {
+  if (typeof str !== 'string') {
+    throw new TypeError('The first param should be a string')
+  }
+  if (typeof isCaseSensitive !== 'boolean') {
+    throw new TypeError('The second param should be a boolean')
+  }
+
+  const result = {}
+  if (str.length > 0) {
+    for (let i = 0; i < str.length; i++) {
+      const word = isCaseSensitive ? str[i] : str[i].toUpperCase()
+      if (/\s/.test(word)) continue
+      result[word] = (!result[word]) ? 1 : result[word] + 1
+    }
+  }
+
+  return result
+}
+export { checkWordOccurrence }
diff --git a/String/CheckWordOcurrence.test.js b/String/CheckWordOcurrence.test.js
new file mode 100644
index 000000000..2b1a88782
--- /dev/null
+++ b/String/CheckWordOcurrence.test.js
@@ -0,0 +1,33 @@
+import { checkWordOccurrence } from './CheckWordOccurrence'
+describe('checkWordOccurrence', () => {
+  it('expects throw on insert wrong string', () => {
+    const value = 123
+    expect(() => checkWordOccurrence(value)).toThrow()
+  })
+  it('expect throw on insert wrong param for case sensitive', () => {
+    const value = 'hello'
+    expect(() => checkWordOccurrence(value, value)).toThrow()
+  })
+  it('check occurrence with case sensitive', () => {
+    const stringToTest = 'A Mad World'
+    const charsOccurrences = checkWordOccurrence(stringToTest, true)
+    const expectResult = { A: 1, M: 1, a: 1, d: 2, W: 1, l: 1, o: 1, r: 1 }
+    const occurrencesObjectKeys = Object.keys(charsOccurrences)
+    const expectObjectKeys = Object.keys(expectResult)
+    expect(occurrencesObjectKeys.length).toBe(expectObjectKeys.length)
+    expectObjectKeys.forEach(key => {
+      expect(expectResult[key]).toBe(charsOccurrences[key])
+    })
+  })
+  it('check occurrence with case insensitive', () => {
+    const stringToTest = 'A Mad World'
+    const charsOccurrences = checkWordOccurrence(stringToTest, false)
+    const expectResult = { A: 2, D: 2, L: 1, M: 1, O: 1, R: 1, W: 1 }
+    const occurrencesObjectKeys = Object.keys(charsOccurrences)
+    const expectObjectKeys = Object.keys(expectResult)
+    expect(occurrencesObjectKeys.length).toBe(expectObjectKeys.length)
+    expectObjectKeys.forEach(key => {
+      expect(expectResult[key]).toBe(charsOccurrences[key])
+    })
+  })
+})
diff --git a/package-lock.json b/package-lock.json
index 9bcf2855d..20ddcf663 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -3734,12 +3734,11 @@
       },
       "dependencies": {
         "ansi-styles": {
-          "version": "4.2.1",
-          "resolved": "https://registry.npmjs.org/ansi-styles/-/ansi-styles-4.2.1.tgz",
-          "integrity": "sha512-9VGjrMsG1vePxcSweQsN20KY/c4zN0h9fLjqAbwbPfahM3t+NL+M9HC8xeXG2I8pX5NoamTGNuomEUFI7fcUjA==",
+          "version": "4.3.0",
+          "resolved": "https://registry.npmjs.org/ansi-styles/-/ansi-styles-4.3.0.tgz",
+          "integrity": "sha512-zbB9rCJAT1rbjiVDb2hqKFHNYLxgtk8NURxZ3IZwD3F6NtxbXZQCnnSi1Lkx+IDohdPlFp222wVALIheZJQSEg==",
           "dev": true,
           "requires": {
-            "@types/color-name": "^1.1.1",
             "color-convert": "^2.0.1"
           }
         },
diff --git a/package.json b/package.json
index eea349d1b..450a58349 100644
--- a/package.json
+++ b/package.json
@@ -16,7 +16,9 @@
     "node-fetch": "2.6.1"
   },
   "standard": {
-    "env": [ "jest" ]
+    "env": [
+      "jest"
+    ]
   },
   "devDependencies": {
     "babel-jest": "^26.3.0",