diff --git a/2309.greatest-english-letter-in-upper-and-lower-case.js b/2309.greatest-english-letter-in-upper-and-lower-case.js
new file mode 100644
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+++ b/2309.greatest-english-letter-in-upper-and-lower-case.js
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+/* URL of this problem
+ * https://leetcode.com/problems/greatest-english-letter-in-upper-and-lower-case/description/
+ *
+ * @param {string} s
+ * @return {string}
+ */
+
+var greatestLetter = function(s) {
+    const Chars = s.split("");
+    const UpperLetters = Chars.filter(isUpperCaseLetter);
+    const LowerLetters = Chars.filter(isLowerCaseLetter);
+    const UpperLowerLetters = UpperLetters.filter(char => {
+        const LowerLetter = char.toLowerCase();
+
+        return LowerLetters.includes(LowerLetter);
+    });
+    let GreatestLetter;
+
+    // Sort UpperLowerLetters in an ascending order by UTF-16 code
+    // Initialize GreatestLetter to the letter with the greatest UTF-16 code
+    // Initialize it to an empty string if the greatest code does not exist
+    UpperLowerLetters.sort();
+    GreatestLetter = UpperLowerLetters[UpperLowerLetters.length - 1] ?? "";
+
+    return GreatestLetter;
+};
+
+function isUpperCaseLetter(char) {
+    const Code = char.charCodeAt(0);
+
+    // The UTF-16 code range of uppercase English letter is 65 to 90
+    return 65 <= Code && Code <= 90;
+}
+
+function isLowerCaseLetter(char) {
+    const Code = char.charCodeAt(0);
+
+    // The UTF-16 code range of lowercase English letter is 97 to 122
+    return 97 <= Code && Code <= 122;
+}
+
+module.exports = greatestLetter;
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diff --git a/greatestLetter.test.js b/greatestLetter.test.js
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+++ b/greatestLetter.test.js
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+const greatestLetter = require("./2309.greatest-english-letter-in-upper-and-lower-case");
+
+test("Return the greatest English letter which occurs as both a lowercase and uppercase letter in s", () => {
+    expect(greatestLetter("lEeTcOdE")).toBe("E");
+});
+
+test("Return an empty string if theres is not an English letter which occurs as both a lowercase and uppercase letter in s", () => {
+    expect(greatestLetter("AbCdEfGhIjK")).toBe("");
+});
+
+test("Return an empty string if the input s is emtpy", () => {
+    expect(greatestLetter("")).toBe("");
+});
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