What are the steps for performing projection in TypeScript?

Question

What are the steps for performing projection in TypeScript?

Looking to fill up the orders array, which consists of objects of type Order. The desired output is

orders=[{id:1,qt:4},{id:2, qt:2},{id:3,qt:2}]

. How can I achieve this using TypeScript? I am new to this language.

export class Product {
  constructor(public id: number, public name: string, public price: number) {}
}
export interface Order {
  id: number;
  qt: number;
}

export const products: Product[] = [
  new Product(1, 'Apple', 2.1),
  new Product(2, 'Banana', 2.2),
  new Product(3, 'Chocolate', 2.3),
  new Product(4, 'Dessert', 2.4),
];

export const cart: Product[] = [
  products[0],
  products[0],
  products[2],
  products[1],
  products[2],
  products[0],
  products[1],
  products[0],
];

export const orders: Order[] = [];

Edit

For those wondering how

orders=[{id:1,qt:4},{id:2, qt:2},{id:3,qt:2}]

is achieved.

In the cart:

the quantity of apples (id:1) is qt:4
the quantity of bananas (id:2) is qt:2
the quantity of chocolates (id:3) is qt:2

Therefore, by utilizing the cart, the objective is to obtain

orders=[{id:1,qt:4},{id:2, qt:2},{id:3,qt:2}]

. This should clarify things.

javascript typescript

Answer 1

Answer №1

If you're searching for a solution similar to LINQ, then you may want to consider using higher order functions like map, filter, and reduce.

While LINQ projections may not directly solve your problem, they are essentially representations of map (Select), concatMap/flatMap (SelectMany), and zip (Zip). Your task revolves around counting the occurrences of each id within the entire array.

Most data manipulation challenges can be tackled with higher order folds such as reduce in JavaScript or Aggregate in C#. In this case, you start by counting the occurrences of each id and create a counter object as explained here.

cart.reduce((acc, { id }) => {
    acc[id] = (acc[id] ?? 0) + 1;
    return acc;
}, {} as Record<number, number>);

Essentially, you initiate the fold operation with an empty object and increment the count for each id in the array. If the id is not present, the nullish coalescing operator (acc[id] ?? 0) assigns and increments the count using 0.

This process yields-

{ '1': 4, '2': 2, '3': 2 }

Next, you need to transform this into-

[ { id: 1, qt: 4 }, { id: 2, qt: 2 }, { id: 3, qt: 2 } ]

By applying Object.entries to the fold result, you get-

> Object.entries({ '1': 4, '2': 2, '3': 2 })
[ [ '1', 4 ], [ '2', 2 ], [ '3', 2 ] ]

Finally, a simple map operation is sufficient-

Object.entries(...).map(([id, qt]) => ({ id: Number(id), qt }))

By combining all these steps, you get-

export const orders: Order[] = Object.entries(
    cart.reduce((acc, { id }) => {
        acc[id] = (acc[id] ?? 0) + 1;
        return acc;
    }, {} as Record<number, number>)
).map(([id, qt]) => ({ id: Number(id), qt }));

It's worth noting that Object.entries might not be the most efficient as it generates an array instead of an iterator. If efficiency is a concern, consider implementing an iterator version of Object.entries using generator functions-

function* objEntries<T>(x: Record<string, T>): IterableIterator<[string, T]> {
  for (const k in x) {
    yield [k, x[k]];
  }
}

Answer 2

If you're searching for a solution similar to LINQ, then you may want to consider using higher order functions like map, filter, and reduce.

While LINQ projections may not directly solve your problem, they are essentially representations of map (Select), concatMap/flatMap (SelectMany), and zip (Zip). Your task revolves around counting the occurrences of each id within the entire array.

Most data manipulation challenges can be tackled with higher order folds such as reduce in JavaScript or Aggregate in C#. In this case, you start by counting the occurrences of each id and create a counter object as explained here.

cart.reduce((acc, { id }) => {
    acc[id] = (acc[id] ?? 0) + 1;
    return acc;
}, {} as Record<number, number>);

Essentially, you initiate the fold operation with an empty object and increment the count for each id in the array. If the id is not present, the nullish coalescing operator (acc[id] ?? 0) assigns and increments the count using 0.

This process yields-

{ '1': 4, '2': 2, '3': 2 }

Next, you need to transform this into-

[ { id: 1, qt: 4 }, { id: 2, qt: 2 }, { id: 3, qt: 2 } ]

By applying Object.entries to the fold result, you get-

> Object.entries({ '1': 4, '2': 2, '3': 2 })
[ [ '1', 4 ], [ '2', 2 ], [ '3', 2 ] ]

Finally, a simple map operation is sufficient-

Object.entries(...).map(([id, qt]) => ({ id: Number(id), qt }))

By combining all these steps, you get-

export const orders: Order[] = Object.entries(
    cart.reduce((acc, { id }) => {
        acc[id] = (acc[id] ?? 0) + 1;
        return acc;
    }, {} as Record<number, number>)
).map(([id, qt]) => ({ id: Number(id), qt }));

It's worth noting that Object.entries might not be the most efficient as it generates an array instead of an iterator. If efficiency is a concern, consider implementing an iterator version of Object.entries using generator functions-

function* objEntries<T>(x: Record<string, T>): IterableIterator<[string, T]> {
  for (const k in x) {
    yield [k, x[k]];
  }
}

What are the steps for performing projection in TypeScript?

Edit

Answer №1

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