Utility that returns decimal strings to a certain precision.

toPrecisionString.ts

function toPrecisionString(num: number, precision: number): string {
  if (typeof num !== 'number' || typeof precision !== 'number') {
    throw new TypeError('toPrecisionString(n: number, precision: number): string => was called with non-number arguments')
  }
  if (isInt(num)) {
    let extension = ''
    if (precision === 0) {
      return String(num)
    } else {
      extension += '.'
      for (let i = 0; i < precision; i++) {
        extension += '0'
      }
    }
    return String(num) + extension
  } else {
    if (precision === 0) {
      return String(parseInt(num as any))
    }
    const str = String(num)
    let [base, extension] = str.split('.')
    if (extension.length > precision) {
      extension = extension.substring(0, precision)
    } else {
      for (let i = 0; i < precision - extension.length + 1; i++) {
        extension += '0'
      }
    }

    return base + '.' + extension
  }
}

function isInt(subject: unknown): boolean {
  if (typeof subject !== 'number') {
    return false
  }
  const str = String(subject)
  return /^[0-9]+$/.test(str)
}

const tables = [
  table([3.3333, 3], '3.333'),
  table([3, 3], '3.000'),
  table([3, 0], '3'),
  table([3.55, 4], '3.5500'),
  table([3.5, 0], '3', 'can be lossy'),
]

describe('toPrecisionString(num: number, precision: number): string', () => {
  tables.forEach(t => {
    it(t.message || `returns ${JSON.stringify(t.expectation)} when given ${JSON.stringify(t.input)}`, () => {
      const result = toPrecisionString(t.input[0], t.input[1])
      expect(result).toEqual(t.expectation)
    })
  })
})

function table(input, expectation, message?) { return { input, expectation, message } }