add: countSubstrings function implementation (#1091)

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 }