CFP - Docs - roundToNearest

roundToNearest

(precision: Precision) => (aNumber: number) => number

Round a number to a precision

Try It

const roundToTenth = roundToNearest('0.1')
roundToTenth(1.55) // is 1.6
roundToTenth(1.54) // is 1.5

const roundToHundreds = roundToNearest('100')
roundToHundreds(1250) // is 1300
roundToHundreds(1249) // is 1200
const roundToTenth = roundToNearest('0.1')
roundToTenth(1.55) // is 1.6
roundToTenth(1.54) // is 1.5

const roundToHundreds = roundToNearest('100')
roundToHundreds(1250) // is 1300
roundToHundreds(1249) // is 1200

Try It

const roundToTenth = roundToNearest('0.1')
roundToTenth(1.55) // is 1.6
roundToTenth(1.54) // is 1.5

const roundToHundreds = roundToNearest('100')
roundToHundreds(1250) // is 1300
roundToHundreds(1249) // is 1200
const roundToTenth = roundToNearest('0.1')
roundToTenth(1.55) // is 1.6
roundToTenth(1.54) // is 1.5

const roundToHundreds = roundToNearest('100')
roundToHundreds(1250) // is 1300
roundToHundreds(1249) // is 1200

We often need to round numbers to specific precisions. Below, we have a few 100-meter track times. Official times need to be reported in hundredths, so let's round them.

Try It

const results = [
  { name: 'sarah', time: 15.926 },
  { name: 'matt', time: 16.143 },
  { name: 'jason', time: 15.907 },
]

const roundToHundredth = roundToNearest('0.01')
const roundTime = result => ({
  ...result,
  time: roundToHundredth(result.time),
})
const roundResults = mapValues(roundTime)
const roundedResults = roundResults(results)

console.log(roundedResults)
// is [
//   { name: sarah, time: 15.93 }
//   { name: matt, time: 16.14 }
//   { name: jason, time: 15.91 }
// ]
type Result = {
  name: string
  time: number
}
const results: Result[] = [
  { name: 'sarah', time: 15.926 },
  { name: 'matt', time: 16.143 },
  { name: 'jason', time: 15.907 },
]

const roundToHundredth = roundToNearest('0.01')
const roundTime = (result: Result) => ({
  ...result,
  time: roundToHundredth(result.time),
})
const roundResults = mapValues(roundTime)<Result[]>
const roundedResults = roundResults(results)

console.log(roundedResults)
// is [
//   { name: sarah, time: 15.93 }
//   { name: matt, time: 16.14 }
//   { name: jason, time: 15.91 }
// ]

Try It

const results = [
  { name: 'sarah', time: 15.926 },
  { name: 'matt', time: 16.143 },
  { name: 'jason', time: 15.907 },
]

const roundToHundredth = roundToNearest('0.01')
const roundTime = result => ({
  ...result,
  time: roundToHundredth(result.time),
})
const roundResults = mapValues(roundTime)
const roundedResults = roundResults(results)

console.log(roundedResults)
// is [
//   { name: sarah, time: 15.93 }
//   { name: matt, time: 16.14 }
//   { name: jason, time: 15.91 }
// ]
type Result = {
  name: string
  time: number
}
const results: Result[] = [
  { name: 'sarah', time: 15.926 },
  { name: 'matt', time: 16.143 },
  { name: 'jason', time: 15.907 },
]

const roundToHundredth = roundToNearest('0.01')
const roundTime = (result: Result) => ({
  ...result,
  time: roundToHundredth(result.time),
})
const roundResults = mapValues(roundTime)<
  Result[]
>
const roundedResults = roundResults(results)

console.log(roundedResults)
// is [
//   { name: sarah, time: 15.93 }
//   { name: matt, time: 16.14 }
//   { name: jason, time: 15.91 }
// ]

The precision is validated. Below shows the errors you'll receive with bad input.

Try It

try {
  roundToNearest('2')
} catch (err) {
  show(err)
  // prints
  // Without a decimal, roundToNearest requires a precision passing the regex
  // /^10{0,9}$/
}

try {
  roundToNearest('0.2')
} catch (err) {
  show('\n', err)
  // prints
  // With a decimal, roundToNearest requires a precision passing the regex
  // /^0\.0{0,9}1$/
}
// incorrect format presents a type error
roundToNearest('2')

Try It

try {
  roundToNearest('2')
} catch (err) {
  show(err)
  // prints
  // Without a decimal, roundToNearest requires a
  // precision passing the regex /^10{0,9}$/
}

try {
  roundToNearest('0.2')
} catch (err) {
  show('\n', err)
  // prints
  // With a decimal, roundToNearest requires a
  // precision passing the regex /^0\.0{0,9}1$/
}
// incorrect format presents a type error
roundToNearest('2')