Instructions & Help

This is the hardest lab in the set, and the most "real" one: it's the same delimiter-matching logic that powers your code editor's bracket-checking. The list here isn't just storage - it's used as a stack: things get added and removed from the SAME end, in last-in-first-out order, which is exactly what nesting rules need. Fill in the missing code in code.js, one TODO at a time.

Try each TODO yourself first. If you get stuck, use the hint button for that spot — hints start vague and get more specific. The last hint is the literal answer.

⏱ Hints are on a shared 30-second timer. Asking for a hint anywhere locks every hint button on the page — there's no dodging the wait by jumping to a different blank.

Trace these character by character with pen and paper, tracking exactly what's in the delimiters list at each step. This is genuinely hard to do in your head - write it out. One attempt per question.

Your certificate below is based on your Predict & Check results. It updates automatically as you answer questions.

Once your code works and you've finished Predict & Check, try any of these:

• Add support for a 4th delimiter type, like angle brackets < and >.
• Instead of just reporting the FIRST error found, count and report the total number of errors in the string.
• Add a "line number" to the error message by counting how many newline characters appear before the problem character.
• Stretch: the current version stops at the first mismatch it finds. Change it to instead finish scanning the entire string and report the FULL final state of the stack, even after an error.