Show number as fraction in javascript

toFraction.mjs

/**
 * Render a number as a fraction
 * @param {number} v the decimal value to convert to a fraction
 * @param {number[]} denom the list of denominators
 * @returns {string} the fraction closest to the decimal value
 */
const toFraction = (v, denom=[2,3,4,7,8,10,16,32,64,100]) => {
  denom = denom.sort((a,b)=>a-b)

  // whole and fractional parts
  var w = Math.trunc(v)
  const f = Math.abs(v - w)

  // start with the max denom
  var d = denom[denom.length-1]
  var n = Math.round(d*f)
  var delta = Math.abs(f - n/d)

  // check if other denom are closer
  for (var i=0; i < denom.length-1; i++) {
    const d1 = denom[i]
    const x = d1*f
    const n1 = Math.round(x)
    const delta1 = Math.abs(f - n1/d1)
    if (n1 && delta1 <= delta) {
      d = d1
      n = n1
      delta = delta1
      break
    }
  }
  if (n == d) {
    w++
    n = 0
  }
  const result = []
  if (w !== 0) result.push(w)
  if (n > 0)  result.push(`${n}/${d}`)
  return result.join(' ')
}
export default toFraction

toFraction.test.mjs

import toFraction from './toFraction.mjs'

const data = [
  [0.004, '' ], // close to 0
  [0.005, '1/100' ], // close to 0

  [0.994, '99/100' ], // close to 1
  [0.995, '1' ], // close to 1

  [0.01, '1/100'],
  [0.015625, '1/64'], // halfway between 1/100 and 1/64 - rounds up
  [0.016, '1/64'],
  [0.017, '1/64'],

  [(.015625 + .02)/2, '2/100'], // halfway between 1/64 and 2/100 - rounds up

  [0.018, '2/100'],
  [0.02, '2/100'],

  [0.03, '3/100'],

  [(.03+1/32)/2, '1/32'], // halfway between 3/100 and 1/32
  [0.03125, '1/32'],
  [0.035625, '1/32'],
  [(1/32 + .04)/2, '4/100'], // halfway between 1/32 and 4/100

  [0.04, '4/100'],
  [0.05, '5/100'],
  [0.06, '6/100'],

  [0.06125, '1/16'], // halfway between 6/100 and 1/16 - rounds up
  [0.0625, '1/16'],
  [0.06625, '1/16'], // halfway between 1/16 and 7/100 - rounds down

  [0.07, '7/100'],
  [0.08, '8/100'],
  [0.09, '9/100'],
  [0.1, '1/10'],

  [(.1 + 1/7)/2, '12/100'],
  [1/7, '1/7'],
  [(1/7 + 1/8)/2, '13/100'],

  [0.125, '1/8'],
  [0.2, '2/10'],
  [0.25, '1/4'],
  [0.3125, '5/16'],

  [0.33, '33/100'],
  [0.3316, '33/100'],

  [(.33 + (1/3))/2, '1/3'], // halfway between 33/100 and 1/3 - rounds up
  [(1/3), '1/3'],
  [((1/3) + .34)/2, '1/3'], // halfway between 1/3 and 34/100 - rounds down

  [.3367, '34/100'],
  [0.34, '34/100'],

  [5/8, '5/8'],
  [11/16, '11/16'],
  [23/32, '23/32'],
  [47/64, '47/64'],

  [0.375, '3/8'],

  [0.5, '1/2'],
  [2/3, '2/3'],
  [0.75, '3/4'],

  [Math.PI, '3 1/7'],
  [1.3333, '1 1/3'],
  [-1.3333, '-1 1/3'],

  [Math.PI, '3 16/113', [7,100,113,1000000]], // 355/113 is closer to PI than 3.141592, but not 3.1415926
]

const negative = x => `\x1b[0;31m${x}\x1b[0;m`
const positive = x => `\x1b[0;32m${x}\x1b[0;m`

var failures = 0
data.forEach(d => {
  const [v,e, denom] = d
  const f = toFraction(v, denom)

  console.log('toFraction(',v,'): expect',f,'to be',e, f === e ? positive('pass') : negative('fail'))
  failures += f === e ? 0 : 1
})

const vomit = '🤮'
const cheers = '🍻'
const passed = data.length - failures

console.log( // N passed, N failed emoji
  passed, passed > 0 ? `${positive('passed')},` : 'passed,',
  failures, failures > 0 ? negative('failed') : 'failed',
  failures > 0 ? vomit : cheers)