CFP - Docs - pCompose

pCompose

(fnArray: async function[]) => async (...args: any[]) => Result

Compose an async function which passes the arguments through an array of async functions

Each function takes the result of the prior function as its parameter.

Try It

const increment = async n => n + 1
const double = async n => n * 2
const toAnswer = async n => 'answer: ' + n

const calculate = pCompose([increment, double, toAnswer])
await calculate(1) // is 'answer: 4'

const returnFirstArg = pCompose([])
await returnFirstArg('a', 'b') // is 'a'
const increment = async (n: number) => n + 1
const double = async (n: number) => n * 2
const toAnswer = async (n: number) => 'answer: ' + n

const calculate = pCompose([increment, double, toAnswer])
await calculate(1) // is 'answer: 4'

const returnFirstArg = pCompose([])
await returnFirstArg('a', 'b') // is 'a'

Try It

const increment = async n => n + 1
const double = async n => n * 2
const toAnswer = async n => 'answer: ' + n

const calculate = pCompose([
  increment,
  double,
  toAnswer,
])
await calculate(1) // is 'answer: 4'

const returnFirstArg = pCompose([])
await returnFirstArg('a', 'b') // is 'a'
const increment = async (n: number) => n + 1
const double = async (n: number) => n * 2
const toAnswer = async (n: number) =>
  'answer: ' + n

const calculate = pCompose([
  increment,
  double,
  toAnswer,
])
await calculate(1) // is 'answer: 4'

const returnFirstArg = pCompose([])
await returnFirstArg('a', 'b') // is 'a'

Sometimes we want to name a set of async functions. Below, we're coding the checkout stage to our online shop. To prep our order, we need to call an API to assign our shipping amount and then make another call to finalize the total. Let's compose a function that does this and call it 'prepForCheckout.'

Try It

const mockSalesTax = 0.08
const mockShipping = 10

const mockStoreApi = {
  assignShipping: async order => ({
    ...order,
    shipping: mockShipping,
  }),
  finalizeTotal: async order => ({
    ...order,
    salesTax: mockSalesTax,
    total: order.subtotal * (1 + mockSalesTax) + order.shipping,
  }),
}

const userLocation = 'cleveland'
const shoppingCart = ['broom', 'mop', 'detergent']
const order = {
  cart: shoppingCart,
  destination: userLocation,
  subtotal: 26,
}
const prepForCheckout = pCompose([
  mockStoreApi.assignShipping,
  mockStoreApi.finalizeTotal,
])
const updatedOrder = await prepForCheckout(order)

console.log(updatedOrder)
// prints
// {
//   cart: [broom, mop, detergent]
//   destination: cleveland
//   subtotal: 26
//   shipping: 10
//   salesTax: 0.08
//   total: 38.08
// }
type Order = {
  cart: string[]
  destination: string
  salesTax: number
  shipping: number
  subtotal: number
  total: number
}

const mockSalesTax = 0.08
const mockShipping = 10

const mockStoreApi = {
  assignShipping: async (order: Order): Promise<Order> => ({
    ...order,
    shipping: mockShipping,
  }),
  finalizeTotal: async (order: Order): Promise<Order> => ({
    ...order,
    salesTax: mockSalesTax,
    total: order.subtotal * (1 + mockSalesTax) + order.shipping,
  }),
}

const userLocation = 'cleveland'
const shoppingCart = ['broom', 'mop', 'detergent']
const order = {
  cart: shoppingCart,
  destination: userLocation,
  subtotal: 26,
} as Order
const prepForCheckout = pCompose([
  mockStoreApi.assignShipping,
  mockStoreApi.finalizeTotal,
])
const updatedOrder = await prepForCheckout(order)

console.log(updatedOrder)
// prints
// {
//   cart: [broom, mop, detergent]
//   destination: cleveland
//   subtotal: 26
//   shipping: 10
//   salesTax: 0.08
//   total: 38.08
// }

Try It

const mockSalesTax = 0.08
const mockShipping = 10

const mockStoreApi = {
  assignShipping: async order => ({
    ...order,
    shipping: mockShipping,
  }),
  finalizeTotal: async order => ({
    ...order,
    salesTax: mockSalesTax,
    total:
      order.subtotal * (1 + mockSalesTax) +
      order.shipping,
  }),
}

const userLocation = 'cleveland'
const shoppingCart = ['broom', 'mop', 'detergent']
const order = {
  cart: shoppingCart,
  destination: userLocation,
  subtotal: 26,
}
const prepForCheckout = pCompose([
  mockStoreApi.assignShipping,
  mockStoreApi.finalizeTotal,
])
const updatedOrder = await prepForCheckout(order)

console.log(updatedOrder)
// prints
// {
//   cart: [broom, mop, detergent]
//   destination: cleveland
//   subtotal: 26
//   shipping: 10
//   salesTax: 0.08
//   total: 38.08
// }
type Order = {
  cart: string[]
  destination: string
  salesTax: number
  shipping: number
  subtotal: number
  total: number
}

const mockSalesTax = 0.08
const mockShipping = 10

const mockStoreApi = {
  assignShipping: async (
    order: Order
  ): Promise<Order> => ({
    ...order,
    shipping: mockShipping,
  }),
  finalizeTotal: async (
    order: Order
  ): Promise<Order> => ({
    ...order,
    salesTax: mockSalesTax,
    total:
      order.subtotal * (1 + mockSalesTax) +
      order.shipping,
  }),
}

const userLocation = 'cleveland'
const shoppingCart = ['broom', 'mop', 'detergent']
const order = {
  cart: shoppingCart,
  destination: userLocation,
  subtotal: 26,
} as Order
const prepForCheckout = pCompose([
  mockStoreApi.assignShipping,
  mockStoreApi.finalizeTotal,
])
const updatedOrder = await prepForCheckout(order)

console.log(updatedOrder)
// prints
// {
//   cart: [broom, mop, detergent]
//   destination: cleveland
//   subtotal: 26
//   shipping: 10
//   salesTax: 0.08
//   total: 38.08
// }

When to use pPassThrough vs. pCompose?
They can often be used interchangeably. Compose allows you to name a group of functions, which can sometimes be helpful. Other times a name is unnecessary, and you just want to pass data through.

When to use pPassThrough vs. pCompose?