Finished

Test Name	Status
empty string	Pass
isogram with only lower case characters	Pass
word with one duplicated character	Pass
word with one duplicated character from the end of the alphabet	Pass
longest reported english isogram	Pass
word with duplicated character in mixed case	Pass
word with duplicated character in mixed case, lowercase first	Pass
hypothetical isogrammic word with hyphen	Pass
hypothetical word with duplicated character following hyphen	Pass
isogram with duplicated hyphen	Pass
made-up name that is an isogram	Pass
duplicated character in the middle	Pass
same first and last characters	Pass
word with duplicated character and with two hyphens	Pass

  * @returns {boolean}
  */
 export const isIsogram = (string) => {
+  throw new Error('Remove this line and implement the function');
+  // Edge Case: Empty string is considered an isogram
+  if (string.length === 0) {
+    return true;
+  }
+  // Convert to lowercase to handle case insensitivity
+  const lowerString = string.toLowerCase();
+  const seenLetters = new Set();
+  for (let i = 0; i < lowerString.length; i++) {
+    const char = lowerString[i];
+    // Edge Case: Skip spaces and hyphens as they are allowed to repeat
+    if (char === ' ' || char === '-') {
+      continue;
+    }
+    // Edge Case: Check for repeated characters (including non-letters)
+    if (seenLetters.has(char)) {
+      return false;
+    }
+    seenLetters.add(char);
+  }
+  return true;
 };
+// Handled Edge Cases: empty string, case sensitivity, spaces, hyphens, non-letter characters, repeated letters

Polyglot JavaScript - without test patch