JavaScript - lesson 5 | HackIndia Skills

Key point

Choose by result

Do not start with the method name. Start with the result you need.

Use map to change every item. Use filter to keep some items. Use find to get one item. Use some to check if at least one item matches. Use reduce when you need one final value from many items.

Why this matters

Hackathon projects usually get data as arrays of objects: events, expenses, tasks, colleges, products, users, or API results.

If your code uses clear array methods, a reviewer can read your logic fast. That helps when your GitHub repo is checked for internship shortlists.

Your to-do

Pick the method

Do each one yourself, then tap it to tick it off. The ticks are only a checklist for you: they are not marked or scored.

Use map when the output array has the same length as the input array.
Use filter when the output array may be shorter.
Use find when you need the first matching object, not a list.
Use some when you need a true or false answer.
Use reduce when you are building one value, like a total or grouped object.

0 of 5 done

Example

Common project data

Imagine a simple student event planner. Your project may fetch this later. For now, practise on local data.

const events = [
  { id: 1, title: "AI Meetup", city: "Bengaluru", fee: 0, seats: 12 },
  { id: 2, title: "JS Workshop", city: "Pune", fee: 299, seats: 0 },
  { id: 3, title: "Design Jam", city: "Bengaluru", fee: 149, seats: 5 }
];

const eventCards = events.map((event) => ({
  id: event.id,
  label: `${event.title} - ₹${event.fee}`,
  available: event.seats > 0
}));

const freeEvents = events.filter((event) => event.fee === 0);

const puneEvent = events.find((event) => event.city === "Pune");

const hasSoldOutEvent = events.some((event) => event.seats === 0);

const totalSeats = events.reduce((sum, event) => sum + event.seats, 0);

console.log(eventCards);
console.log(freeEvents);
console.log(puneEvent);
console.log(hasSoldOutEvent);
console.log(totalSeats);

Common mistake

Do not mutate blindly

Methods like map and filter return new arrays. That is usually safer for project code because your original data stays unchanged.

sort changes the original array. If you need sorting, copy first.

const sortedByFee = [...events].sort((a, b) => a.fee - b.fee);

console.log(sortedByFee);
console.log(events); // original order is still safe

Tip

Name the callback

A good callback name makes your code read like English. This is better than writing short names everywhere.

If AI gives you code with x, y, item1, or data2, rename them before you commit.

const bengaluruEvents = events.filter((event) => event.city === "Bengaluru");

const titles = bengaluruEvents.map((event) => event.title);

Check AI output

You can ask an AI tool to suggest array methods for a task. Do not paste the answer directly.

Check three things: does the method return the shape you need, does it mutate the original array, and are edge cases handled, like an empty array or no match from find.