Checkpoint: Study Receipt

This checkpoint is a small independent task. Budget about twenty minutes: five to plan, ten to implement and check, and five to explain and change the inputs. Close earlier solutions first. A hint is a normal part of learning, but try to name the obstacle before opening it.

Your receipt has a topic, a number of sessions, a duration per session, and a completion flag. These are ordinary JavaScript values. TypeScript helps preserve their roles as you change the program.

Write down the expected message before opening the editor. Decide which variables will change and use let only for those. You may infer obvious local types. For this checkpoint, practise explicit annotations on at least two values too.

After the first successful run, deliberately introduce a type mismatch. Read the compiler message out loud, then restore the program. A passing check and a correct output answer different questions; record evidence for both.

The tiny demonstration below is a reminder, not the completed receipt. Your task uses different data and a full sentence.

Predict, check, then run

Before running, write down the output and explain which information the checker knows. Click Check types first, then Check & run.

const topic: string = "Arrays";
const minutes = 15;

console.log(`${topic}: ${minutes}`);

Arrays: 15

Acceptance cases before you code

Write a small test plan before implementing typed calculation rather than literal output. These cases extend the brief; they are not a second worked solution. Predict the expected behavior independently, then test through the public function. A good ordinary example and a good boundary example should catch different possible mistakes.

After the core task passes, deliberately break one rule and identify the case that notices it. Restore the implementation and explain the failure in terms of the contract, not the number of assertions. If a case requires a concept you have not recalled, revisit the relevant lesson before opening the solution.

Your turn

// Plan four values, calculate the total, and format the receipt.

const topic: string = "Functions";
const sessions: number = 3;
const minutesPerSession = 20;
const completed = false;
const totalMinutes = sessions * minutesPerSession;

console.log(
  `${topic}: ${sessions} sessions, ${totalMinutes} minutes, done=${completed}`
);

Functions: 3 sessions, 60 minutes, done=false

Change it and explain it

Test zero sessions and a different topic. Would the checker reject negative sessions? Explain why not. Finish by writing a two-sentence explanation of inference and of what the compiler does before JavaScript runs. If you needed the full solution, redo the receipt tomorrow using reading sessions instead.