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1. .map – "Apply discounts and format prices"

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Goal: Use Array.prototype.map to apply a 20% discount to every price, round each result to two decimal places, then chain a second .map to format each number as a currency string (prefix with "$").

Requirements:

1. Create a new array discounted where each price is reduced by 20% (price * 0.8).
2. Round each discounted price to two decimals.
3. Chain a second .map to convert numbers to strings like "$9.60".
4. Log the final array and ensure original prices is unchanged.

Example input:

const prices = [12, 45, 23, 8, 90];

Expected final output (example):

["$9.60", "$36.00", "$18.40", "$6.40", "$72.00"]

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2. .filter – "Keep the weekend"

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Goal: Use Array.prototype.filter to keep only days that start with the letter 'S', then map those names to uppercase.

Requirements:

1. Use .filter to select days whose first character is 'S' (case-sensitive for this exercise).
2. Chain .map to convert the filtered names to uppercase.
3. Return or log the final array of uppercase weekend days.

Example input:

const days = ['Mon','Tue','Wed','Thu','Fri','Sat','Sun'];

Expected final output:

['SAT', 'SUN']

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3. .find – "First above average"

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Goal: Compute the average of the scores, then use .find to locate the first score strictly greater than the average. Also return the index of that score using .findIndex.

Requirements:

1. Compute the average from scores.
2. Use .find to get the first score > average.
3. Use .findIndex (or .indexOf) to get that score's position in the original array.
4. Log both the found score and its index. If none found, explicitly log undefined (or -1 for index).

Example input:

const scores = [67, 42, 89, 55, 73];

Expected example output:

First above average: 89
Index of first above average: 2

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4. "Mega-chain" – map → filter → map

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Goal: Chain several array methods to produce greeting strings for users who have paid and are over 18. Then return only the first valid greeting.

Requirements:

1. From users, filter to those with paid: true and age > 18.
2. Map the filtered users to greeting strings like: "Hello Ada — thanks for your purchase!".
3. Part A: Return the full array of greetings.
4. Part B: Modify to return only the first greeting (use .find or chain + [0] safely).

Example input:

const users = [
  {name: 'Ada',  age: 31, paid: true},
  {name: 'Ben',  age: 17, paid: false},
  {name: 'Cara', age: 25, paid: true},
  {name: 'Dan',  age: 42, paid: false}
];

Expected example outputs: Part A:

["Hello Ada — thanks for your purchase!", "Hello Cara — thanks for your purchase!"]

Part B:

"Hello Ada — thanks for your purchase!"

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5. DIY groupBy – without .reduce

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Goal: Implement a groupBy function that groups items by a provided key selector (no use of .reduce). After grouping, filter each customer's orders to keep only those with qty >= 3.

Requirements:

1. Write groupBy(array, keyFn) that returns an object whose keys are results of keyFn(item) and values are arrays of items.
2. After grouping orders by customer, transform each group's array to only include orders where qty >= 3.
3. Return or log the filtered grouped result.

Example input:

const orders = [
  { customer: 'A', product: 'Book',  qty: 2 },
  { customer: 'B', product: 'Game',  qty: 1 },
  { customer: 'A', product: 'Game',  qty: 3 },
  { customer: 'B', product: 'Book',  qty: 4 }
];

Expected final output:

{
  A: [{ customer: 'A', product: 'Game', qty: 3 }],
  B: [{ customer: 'B', product: 'Book', qty: 4 }]
}

Starter skeleton (fill in implementation):

function groupBy(array, keyFn) {
  const result = {};
  // iterate array and push items into result[key]
  return result;
}

// usage: groupBy(orders, o => o.customer)