add: Example 4: Extract single character in Basic Usage substring()

Author: danieldanielecki

data/blog/software-development/web-development/frontend/javascript/slice-vs-substring-vs-substr-complete-javascript-string-methods-comparison.mdx

@@ -368,6 +368,10 @@ console.log(str.substring(4, 10));     // "Script"
 console.log(str.substring(4));         // "Script"
 // Explanation: When endIndex omitted, defaults to string.length
 // Equivalent to: str.substring(4, 10)
+
+// Example 4: Extract single character
+console.log(str.substring(0, 1));      // "J"
+// Explanation: Start at 0, end before 1, so only index 0 is included
 ```
 
 #### Negative Indices (Always Treated as 0)