CFP - Docs - passThrough

passThrough

(value: any, fnArray: function[]) => Result

Pass a value through fnArray.

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

Try It

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

passThrough(1, [increment, double, toAnswer])
// is 'answer: 4'

passThrough('a', [])
// is 'a'
const increment = (n: number) => n + 1
const double = (n: number) => n * 2
const toAnswer = (n: number) => 'answer: ' + n

passThrough(1, [increment, double, toAnswer])
// is 'answer: 4'

passThrough('a', [])
// is 'a'

Try It

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

passThrough(1, [increment, double, toAnswer])
// is 'answer: 4'

passThrough('a', [])
// is 'a'
const increment = (n: number) => n + 1
const double = (n: number) => n * 2
const toAnswer = (n: number) => 'answer: ' + n

passThrough(1, [increment, double, toAnswer])
// is 'answer: 4'

passThrough('a', [])
// is 'a'

Just like compose, passThrough gives us a flexible and readable way to combine logic. It's one of the big reasons I find functional programming to be helpful.

Sometimes we want to pass a value through some functions to get a result. Below, we have the five-day weather forecast and want to calculate the average low. Let's pass the forecast through some functions to do this.

Try It

const forecast = [
  { high: 68, low: 54 },
  { high: 62, low: 50 },
  { high: 63, low: 52 },
  { high: 55, low: 43 },
  { high: 58, low: 49 },
]

const avgLow = passThrough(forecast, [
  mapValues(get('low')),
  getAverageValue,
  roundToNearest('0.1'),
])

console.log(avgLow)
// is 49.6
type Forecast = {
  high: number
  low: number
}
const forecast: Forecast[] = [
  { high: 68, low: 54 },
  { high: 62, low: 50 },
  { high: 63, low: 52 },
  { high: 55, low: 43 },
  { high: 58, low: 49 },
]

const getLow = get('low')<Forecast>
const avgLow = passThrough(forecast, [
  mapValues(getLow)<Forecast[]>,
  getAverageValue,
  roundToNearest('0.1'),
])

console.log(avgLow)
// is 49.6

Try It

const forecast = [
  { high: 68, low: 54 },
  { high: 62, low: 50 },
  { high: 63, low: 52 },
  { high: 55, low: 43 },
  { high: 58, low: 49 },
]

const avgLow = passThrough(forecast, [
  mapValues(get('low')),
  getAverageValue,
  roundToNearest('0.1'),
])

console.log(avgLow)
// is 49.6
type Forecast = {
  high: number
  low: number
}
const forecast: Forecast[] = [
  { high: 68, low: 54 },
  { high: 62, low: 50 },
  { high: 63, low: 52 },
  { high: 55, low: 43 },
  { high: 58, low: 49 },
]

const getLow = get('low')<Forecast>
const avgLow = passThrough(forecast, [
  mapValues(getLow)<Forecast[]>,
  getAverageValue,
  roundToNearest('0.1'),
])

console.log(avgLow)
// is 49.6

When to use passThrough vs. compose?
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 passThrough vs. compose?