diff --git a/String/PercentageOfLetters.js b/String/PercentageOfLetters.js
new file mode 100644
index 0000000000..fbd6a03be3
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+++ b/String/PercentageOfLetters.js
@@ -0,0 +1,27 @@
+/**
+ * @function percentageOfLetter
+ * @description Return the percentage of characters in 'str'
+ * that equal 'letter' rounded down to the nearest whole percent.
+ * More info: https://leetcode.com/problems/percentage-of-letter-in-string/
+ * @param {String} str
+ * @param {String} letter
+ * @returns {Number}
+ * @example
+ * const str = 'foobar', const letter = 'o'
+ * percentageOfLetter(str, letter) // ===> 33
+ */
+const percentageOfLetter = (str, letter) => {
+  if (typeof str !== 'string' || typeof letter !== 'string') {
+    throw new Error('Input data must be strings')
+  }
+  let letterCount = 0
+  // Iterate through the whole given text
+  for (let i = 0; i < str.length; i++) {
+    // Count how often the letter appears in the word
+    letterCount += str[i].toLowerCase() === letter.toLowerCase() ? 1 : 0
+  }
+  const percentage = Math.floor((100 * letterCount) / str.length)
+  return percentage
+}
+
+export { percentageOfLetter }
diff --git a/String/test/PercentageOfLetters.test.js b/String/test/PercentageOfLetters.test.js
new file mode 100644
index 0000000000..7a58dd0ecc
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+++ b/String/test/PercentageOfLetters.test.js
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+import { percentageOfLetter } from '../PercentageOfLetters'
+
+describe('Percentage of Letters in a String', () => {
+  test('Calculate percent for lower case', () => {
+    expect(percentageOfLetter('foobar', 'o')).toEqual(33)
+    expect(percentageOfLetter('aaabcd', 'a')).toEqual(50)
+  })
+  test('Calculate percent for upper case', () => {
+    expect(percentageOfLetter('foobar', 'o')).toEqual(33)
+    expect(percentageOfLetter('aAabcd', 'a')).toEqual(50)
+  })
+  test('Throwing an exception', () => {
+    expect(() => percentageOfLetter(100, 'string')).toThrow()
+    expect(() => percentageOfLetter('string', true)).toThrow()
+  })
+})