用于 Javascript 的类似 SQL 的内部联接

Question

 const products = [{ id: 5, productName: "Logitech Mouse", unitprice: 35 }, { id: 6, productName: "Logitech Keyboard", unitprice: 40 } ]; const cart = [{ id: 101, userId: 3, productId: 5, quantity: 2 }]; totals = cart.reduce((r, { productId: id, quantity }) => (r[id] = (r[id] || 0) + quantity, r), {}), result = products.map(({ id, productName, unitprice }) => ({ productName, unitprice, quantity: totals[id] })); console.log(result)

Currently, it prints two rows instead of one row. Second row should not be printed because there is only one row in cart . How do I the so-called inner join which return only one record which is for cart id 101?

Answer 1

If you want retain the cart with the "join", you can map the found product (without its id) and include the quantity.

 const products = [{ id: 5, productName: "Logitech Mouse", unitprice: 35 }, { id: 6, productName: "Logitech Keyboard", unitprice: 40 }]; const cart = [{ id: 101, userId: 3, productId: 5, quantity: 2 }]; let joined = cart.map(item => { let { id, ...rest} = products.find(p => p.id === item.productId); return { ...rest, 'quantity' : item.quantity }; }); console.log(joined);

 .as-console-wrapper { top: 0; max-height: 100% !important; }

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If you want the total price of all items in the cart you will need to:

1. Reduce the cart by the items within it
2. Locate the product by its id
3. Add to the total, the unit price of the product times the quantity in the cart

 const products = [{ id: 5, productName: "Logitech Mouse", unitprice: 35 }, { id: 6, productName: "Logitech Keyboard", unitprice: 40 }]; const cart = [{ id: 101, userId: 3, productId: 5, quantity: 2 }]; let total = cart.reduce((subtotal, item) => { let product = products.find(p => p.id === item.productId); return subtotal + product.unitprice * item.quantity; }, 0); console.log(total);

Short version:

let t = cart.reduce((s, i) => s + products.find(p => p.id === i.productId).unitprice * i.quantity, 0);

Answer 2

I recommend to use map function like this

 const products = [{ id: 5, productName: "Logitech Mouse", unitprice: 35 }, { id: 6, productName: "Logitech Keyboard", unitprice: 40 } ]; const cart = [{ id: 101, userId: 3, productId: 5, quantity: 2 }]; result = cart.map(item => { const product = products.find(product => item.productId === product.id); return { id: item.id, quantity: item.quantity, productName: product.productName, unitprice: product.unitprice } }); console.log(result);

Answer 3

While Mr. Polywhirl's answer is a cleaner and better solution, here is a simple edit to your code that solves your problem. The reason why your code returns two is because of the map . Replace it with filter and the condition on the presence of id to solve the issue.

 const products = [{ id: 5, productName: "Logitech Mouse", unitprice: 35 }, { id: 6, productName: "Logitech Keyboard", unitprice: 40 } ]; const cart = [{ id: 101, userId: 3, productId: 5, quantity: 2 }]; totals = cart.reduce((r, { productId: id, quantity }) => (r[id] = (r[id] || 0) + quantity, r), {}), result = products.filter(({ id, productName, unitprice }) => { if (totals[id]) return ({ productName, unitprice, quantity: totals[id] }) }); console.log(result)