Reports & Tests

A report answers a question about data. Our report needs total planned minutes, completed minutes, and remaining minutes. Decide what each number means before naming the variables. A session marked done contributes its whole duration; partial completion is outside this project’s model.

Use a function returning an object of totals. A second function can turn that object into text. This separation lets a webpage display the same numbers without parsing a formatted string. Test the numbers directly, including an empty plan.

When a result is wrong, inspect the input and each stage. Did filtering select the right records? Did the sum start at zero? Did a property name match the records? More clever syntax will not repair an unclear definition.

Predict before running

Only one session is completed. Which minutes count toward planned, and which toward completed?

function summarize(sessions) {
  let planned = 0;
  let completed = 0;

  for (const session of sessions) {
    planned = planned + session.minutes;

    if (session.done) {
      completed = completed + session.minutes;
    }
  }

  return { planned, completed, remaining: planned - completed };
}

const summary = summarize([
  { topic: "Loops", minutes: 25, done: true },
  { topic: "Arrays", minutes: 20, done: false }
]);

console.log(summary.planned);
console.log(summary.completed);
console.log(summary.remaining);

45
25
20

Make report rules explicit

A report depends on definitions. Does total mean all planned minutes, only completed minutes, or minutes still remaining? Choose names that make this visible. Empty input is not an exceptional situation: a new learner has no records yet. Avoid dividing by zero when calculating an average.

function averageMinutes(sessions) {
  if (sessions.length === 0) {
    return 0;
  }

  let total = 0;

  for (const session of sessions) {
    total += session.minutes;
  }

  return total / sessions.length;
}

console.log(averageMinutes([]));
console.log(averageMinutes([{ minutes: 20 }, { minutes: 40 }]));

0
30

Calculate each total from its stated rule

A report derives values from source records; it should not change those records while adding them up. Planned minutes include every session. Completed minutes include only sessions marked done. Remaining minutes are their difference, not the number of unfinished records.

Use a tiny fixture where the units are obvious. Two sessions lasting 10 and 40 minutes are useful because confusing a count with a duration produces visibly different answers. Include an empty list and an all-complete list before trusting a report on realistic data.

const sessions = [
  { minutes: 10, done: true },
  { minutes: 40, done: false }
];
let count = 0;
let minutes = 0;

for (const session of sessions) {
  if (session.done) {
    count += 1;
    minutes += session.minutes;
  }
}

console.log(count);
console.log(minutes);

1
10

The same condition updates both accumulators, but their units differ. A variable named completed without a unit can hide this distinction. Prefer completedCount or completedMinutes.

const sessions = [
  { minutes: 10, done: true },
  { minutes: 20, done: true }
];
let planned = 0;
let completed = 0;

for (const session of sessions) {
  planned += session.minutes;

  if (session.done) {
    completed += session.minutes;
  }
}

console.log(planned - completed);
console.log(sessions[0].done);

0
true

A legitimate zero result must not be replaced with a default message by truthiness. The source flag is still true because calculating the report did not need to mutate it.

Try the rule on new inputs

Check empty, none-done, some-done and all-done plans. Choose different durations so a count-based mistake cannot accidentally pass. Explain why toggling a session should change completed minutes but not planned minutes.

Your turn

// Write completedMinutes, then throw an error if any expectation fails.

function completedMinutes(sessions) {
  let total = 0;

  for (const session of sessions) {
    if (session.done) {
      total = total + session.minutes;
    }
  }

  return total;
}

if (completedMinutes([]) !== 0) {
  throw new Error("empty");
}

if (completedMinutes([{ minutes: 20, done: false }]) !== 0) {
  throw new Error("incomplete");
}

if (completedMinutes([{ minutes: 25, done: true }]) !== 25) {
  throw new Error("completed");
}

console.log("3 checks passed");

3 checks passed

What you should be able to explain

• What does each number in your report mean?
• What should an empty report display?
• Does your reporting function change any records?