Mini project: Habit tracker

You've got arrays, objects, functions, conditionals, and loops. Time to wire them together into something that feels like a real program.

What you'll learn

• Model a small data structure with arrays and objects
• Split a program into small, testable functions
• Compose those functions inside main() and print a formatted summary

Instructions

Write a tiny habit tracker with three functions and a main() that uses them.

Each habit is an object: { name: "read", done: false }. A habit list is an array of those objects.

Write:

1. addHabit(habits, name) — returns the list with a new { name, done: false } appended.
2. markDone(habits, name) — returns the list with the matching habit's done flipped to true. If no habit has that name, return the list unchanged.
3. summary(habits) — returns a string like "1 of 3 habits done".
4. main() — builds up a list with three habits (read, walk, meditate), marks walk done, then prints each habit as "name: done" or "name: todo" (one per line), followed by the summary string.

Finally, call main() so your program actually runs.

Expected output from main():

read: todo
walk: done
meditate: todo
1 of 3 habits done

Key concepts

Small functions, composed

Each function does one thing. addHabit doesn't print. markDone doesn't add. summary doesn't mutate. main() is the only place that knows about the whole flow — it calls the other three in order.

This split is what makes a program testable: the unit tests check each function in isolation; main() is the integration.

Returning a modified list

You can either mutate and return:

function addHabit(habits, name) {
  habits.push({ name, done: false });
  return habits;
}

Or build a new one (no mutation):

function addHabit(habits, name) {
  return [...habits, { name, done: false }];
}

Either works for this project. The non-mutating style scales better in real apps; the mutating style is simpler to write first.

Formatting with template literals

` ${habit.name}: ${habit.done ? "done" : "todo"} handles the per-line format. done ? "done" : "todo" is a ternary — a short if/else` that returns a value.

Hints

1. Start with the three helper functions and get their unit tests passing before you touch main().
2. In main(), use console.log in a for...of loop to print each habit's line, then one more console.log for the summary.
3. habits.filter((h) => h.done).length gives you how many are done — useful for summary.