Fix 3.js: convert cardNumber to string so slice works, add explanation

Author: lamlakamana-oss

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

@@ -1,9 +1,15 @@
-const cardNumber = 4533787178994213;
+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
+// PREDICTION: This code will result in a TypeError because cardNumber is a number,
+// and numbers don't have a .slice() method (it only exists on strings).
+//
+// ACTUAL ERROR: TypeError: cardNumber.slice is not a function (at 3.js:2:32)
+// Was my prediction correct? YES — it was indeed a TypeError caused by calling
+// a string method on a number.
+//
+// Fix: Make cardNumber a string by wrapping it in quotes: "4533787178994213".
+// (Card numbers should be strings in real life — they can have leading zeros
+// and are too large for the Number type.)
+
+console.log(last4Digits);