Add actual error output and deeper TDZ explanation

Author: lamlakamana-oss

Sprint-3/1-key-errors/0.js

@@ -1,12 +1,35 @@
 // Predict and explain first...
 //I think this code will show a SyntaxError. The function has two variables named str in the same scope.
+// ACTUAL ERROR (running the original code with `node 0.js`):
+// C:\Users\EarthHarvestAdmin\CYF2026 LAMLA KAMANA\Module-JavaScript-Fundamentals\Sprint-3\1-key-errors\0.js:8
+    let str = `${str[0].toUpperCase()}${str.slice(1)}`;
+        ^
 
+//SyntaxError: Identifier 'str' has already been declared
+
+// WHY IT IS ILLEGAL (DEEPER EXPLANATION):
+// 1. SCOPE COLLISION: The function parameter `str` and the variable `let str` 
+//    exist in the exact same function scope. In JavaScript, `let` and `const` 
+//    do NOT allow redeclaring an existing variable in the same scope.
+//
+// 2. TEMPORAL DEAD ZONE (TDZ): When the JavaScript engine parses this function, 
+//    it hoists the `let str` declaration to the top of the function scope. 
+//    However, unlike `var`, `let` variables remain in an uninitialized state 
+//    (the TDZ) until the exact line of declaration is executed.
+//
+// 3. FATAL PARSING ERROR: Because this is a SyntaxError (not a runtime error), 
+//    the JavaScript engine catches the illegal redeclaration during the 
+//    compilation/parsing phase, BEFORE any code executes. This is why the 
+//    `console.log` on line 12 never runs—the script never gets past the 
+//    parsing stage.
+//
+// FIX: use a different name for the new variable, e.g. `result`.
 // call the function capitalise with a string input
 // interpret the error message and figure out why an error is occurring
 
 function capitalise(str) {
-  let result = `${str[0].toUpperCase()}${str.slice(1)}`;
-  return result;
+    let str = `${str[0].toUpperCase()}${str.slice(1)}`;
+  return str;
 }
 console.log(capitalise("hello"));
 // str is used both as the function parameter (function capitalise(str)) and again in let str = ...