algorithm: count letters (#1164)

String/CountLetters.js

@@ -0,0 +1,33 @@
+/**
+ * @function countLetters
+ * @description Given a string, count the number of each letter.
+ * @param {String} str - The input string
+ * @return {Object} - Object with letters and number of times
+ * @example countLetters("hello") => {h: 1, e: 1, l: 2, o: 1}
+ */
+
+const countLetters = (str) => {
+  const specialChars = /\W/g
+
+  if (typeof str !== 'string') {
+    throw new TypeError('Input should be a string')
+  }
+
+  if (specialChars.test(str)) {
+    throw new TypeError('Input must not contain special characters')
+  }
+
+  if (/\d/.test(str)) {
+    throw new TypeError('Input must not contain numbers')
+  }
+
+  const obj = {}
+  for (let i = 0; i < str.toLowerCase().length; i++) {
+    const char = str.toLowerCase().charAt(i)
+    obj[char] = (obj[char] || 0) + 1
+  }
+
+  return obj
+}
+
+export { countLetters }