Update 3.js

Add comments explaining slice error and fix by converting number to string.

Author: HoussamLh

Sprint-1/2-mandatory-errors/3.js

@@ -1,9 +1,22 @@
 const cardNumber = 4533787178994213;
-const last4Digits = cardNumber.slice(-4);
-
-// The last4Digits variable should store the last 4 digits of cardNumber
-// However, the code isn't working
-// Before running the code, make and explain a prediction about why the code won't work
-// Then run the code and see what error it gives.
-// Consider: Why does it give this error? Is this what I predicted? If not, what's different?
-// Then try updating the expression last4Digits is assigned to, in order to get the correct value
+
+// We want last4Digits to store the last 4 digits of cardNumber
+
+// Prediction before running:
+// This will cause an error because cardNumber is a number,
+// and numbers don't have the slice() method. slice() works only on strings or arrays.
+
+// Running the code would give:
+// TypeError: cardNumber.slice is not a function
+
+// Why?
+// Because slice() is not defined for numbers in JavaScript.
+
+// Fix:
+// Convert cardNumber to a string first, so we can use slice() on it.
+// Then slice the last 4 characters to get the last 4 digits.
+
+const last4Digits = String(cardNumber).slice(-4);
+
+console.log(last4Digits);       // Output: 4213
+