CFP - Docs - keepFirstWhile

keepFirstWhile

(predicate: Predicate) => (anArray: array) => array

Keep the first elements of an array that pass a condition

Try It

const lt3 = n => n < 3
const keepWhileLt3 = keepFirstWhile(lt3)

keepWhileLt3([1, 2, 3]) // is [1, 2]
keepWhileLt3([3, 1, 2]) // is []
const lt3 = (n: number) => n < 3
const keepWhileLt3 = keepFirstWhile(lt3)

keepWhileLt3([1, 2, 3]) // is [1, 2]
keepWhileLt3([3, 1, 2]) // is []

Try It

const lt3 = n => n < 3
const keepWhileLt3 = keepFirstWhile(lt3)

keepWhileLt3([1, 2, 3]) // is [1, 2]
keepWhileLt3([3, 1, 2]) // is []
const lt3 = (n: number) => n < 3
const keepWhileLt3 = keepFirstWhile(lt3)

keepWhileLt3([1, 2, 3]) // is [1, 2]
keepWhileLt3([3, 1, 2]) // is []

Sometimes we have ordered data and only care about the first elements. Below, we have an ordered list of 100-meter times. People who ran it in under 13 minutes qualify for the next round. Let's list the qualified runners.

Try It

const isSub13m = ({ time }) => time < 13
const keepSub13Times = keepFirstWhile(isSub13m)

const oneHundredMeterTimes = [
  { name: 'emma', time: 11.2 },
  { name: 'luke', time: 12.8 },
  { name: 'meg', time: 13.5 },
]

const getQualifiedRunners = compose([keepSub13Times, mapValues(get('name'))])

const qualifiedRunners = getQualifiedRunners(oneHundredMeterTimes)
console.log(qualifiedRunners)
// is [
//   emma
//   luke
// ]
type Runner = {
  name: string
  time: number
}

const isSub13m = ({ time }: Runner) => time < 13
const keepSub13Times = keepFirstWhile(isSub13m)

const oneHundredMeterTimes: Runner[] = [
  { name: 'emma', time: 11.2 },
  { name: 'luke', time: 12.8 },
  { name: 'meg', time: 13.5 },
]

const getName = get('name')<Runner>

const getQualifiedRunners = compose([
  keepSub13Times,
  mapValues(getName)<Runner[]>,
])

const qualifiedRunners = getQualifiedRunners(oneHundredMeterTimes)
console.log(qualifiedRunners)
// is [
//   emma
//   luke
// ]

Try It

const isSub13m = ({ time }) => time < 13
const keepSub13Times = keepFirstWhile(isSub13m)

const oneHundredMeterTimes = [
  { name: 'emma', time: 11.2 },
  { name: 'luke', time: 12.8 },
  { name: 'meg', time: 13.5 },
]

const getQualifiedRunners = compose([
  keepSub13Times,
  mapValues(get('name')),
])

const qualifiedRunners = getQualifiedRunners(
  oneHundredMeterTimes
)
console.log(qualifiedRunners)
// is [
//   emma
//   luke
// ]
type Runner = {
  name: string
  time: number
}

const isSub13m = ({ time }: Runner) => time < 13
const keepSub13Times = keepFirstWhile(isSub13m)

const oneHundredMeterTimes: Runner[] = [
  { name: 'emma', time: 11.2 },
  { name: 'luke', time: 12.8 },
  { name: 'meg', time: 13.5 },
]

const getName = get('name')<Runner>

const getQualifiedRunners = compose([
  keepSub13Times,
  mapValues(getName)<Runner[]>,
])

const qualifiedRunners = getQualifiedRunners(
  oneHundredMeterTimes
)
console.log(qualifiedRunners)
// is [
//   emma
//   luke
// ]