add: countSubstrings function implementation (TheAlgorithms#1091)

Author: ajinkyac03

String/CountSubstrings.js

@@ -0,0 +1,29 @@
+/**
+ * @function countSubstrings
+ * @description Given a string of words or phrases, count the occurrences of a substring
+ * @param {String} str - The input string
+ * @param {String} substring - The substring
+ * @return {Number} - The number of substring occurrences
+ * @example countSubstrings("This is a string", "is") => 2
+ * @example countSubstrings("Hello", "e") => 1
+ */
+
+const countSubstrings = (str, substring) => {
+  if (typeof str !== 'string' || typeof substring !== 'string') {
+    throw new TypeError('Argument should be string')
+  }
+
+  if (substring.length === 0) return str.length + 1
+
+  let count = 0
+  let position = str.indexOf(substring)
+
+  while (position > -1) {
+    count++
+    position = str.indexOf(substring, position + 1)
+  }
+
+  return count
+}
+
+export { countSubstrings }

String/test/CountSubstrings.test.js

@@ -0,0 +1,52 @@
+import { countSubstrings } from '../CountSubstrings'
+
+describe('CountSubstrings', () => {
+  it('count multiple occurrences of substring in a string', () => {
+    const str = 'This is a string'
+    const substring = 'is'
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(2)
+  })
+
+  it('should return 0 when input substring has no occurrences', () => {
+    const str = 'Jurassic Park'
+    const substring = 'World'
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(0)
+  })
+
+  it('should return 1 when input substring is of length 1 that is equal to string', () => {
+    const str = 's'
+    const substring = 's'
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(1)
+  })
+
+  it('should return the correct result when input string contains spaces', () => {
+    const str = 'ab cd ef  ghi'
+    const substring = ' '
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(4)
+  })
+
+  it('should return the correct result when input substring contains number or special characters', () => {
+    const str = 'abc1@2def1@2'
+    const substring = '1@2'
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(2)
+  })
+
+  it('should return string.length + 1 when the input substring is an empty string', () => {
+    const str = 'empty'
+    const substring = ''
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(6)
+  })
+
+  it('should return correct result when input is overlapping substring', () => {
+    const str = 'aaa'
+    const substring = 'aa'
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(2)
+  })
+})