Converting a number to a formatted string

convert.js

// @flow

/**
  * Adds a separator between triples of numbers.
  * For examples have a look at the test suite
  */
export const addThousands = (numString: string, separator: string = '.'): string => {
  const re = /\d(?=(?:\d{3})+(?!\d))/g

  const convert = (match = numString.search(re), acc = numString) => {
    if (match === -1) return acc

    const head = acc.slice(0, match + 1)
    const tail = acc.slice(match + 1)
    const newStr = `${head}${separator}${tail}`

    return convert(newStr.search(re), newStr)
  }

  return convert()
}

 // TESTS
 describe('addThousands()', () => {
  it('should handle default', () => {
    expect(addThousands('1000')).toBe('1.000')
  })

  it('should handle different separator', () => {
    expect(addThousands('1000', ',')).toBe('1,000')
  })

  it('should add separator for each triple before decimals', () => {
    expect(addThousands('1000000')).toBe('1.000.000')
  })

  it('should handle decimal dot on num', () => {
    expect(addThousands('1000.50')).toBe('1.000.50')
  })
 })
   
type FormattedStrConfig = {|
  decimalSeparator?: string,
  thousandsSeparator?: string,
  unit?: string
|}

/**
  * Converts a number into a formatted string
  * For examples have a look at the test suite.
  */
export const numToFormattedString = (
  num: number,
  {
    decimalSeparator = ',',
    thousandsSeparator = '.',
    unit = '€'
  }: FormattedStrConfig
): string => {
  const numStrWithDecimals = num
    .toFixed(2)
    .toString()
    .replace('.', decimalSeparator)
  const numStr = addThousands(numStrWithDecimals, thousandsSeparator)
  const finalStr = `${numStr}${unit}`

  return finalStr
}

describe('numToFormattedString()', () => {
  it('should num with default formatting', () => {
    const result = numToFormattedString(1000.50, {})

    expect(result).toBe('1.000,50€')
  })

  it('should handle decimal overwrite', () => {
    const result = numToFormattedString(1000.50, {
      decimalSeparator: ':'
    })

    expect(result).toBe('1.000:50€')
  })

  it('should handle thousands overwrite', () => {
    const result = numToFormattedString(1000.50, {
      thousandsSeparator: ':'
    })

    expect(result).toBe('1:000,50€')
  })

  it('should handle unit overwrite', () => {
    const result = numToFormattedString(1000.50, {
      unit: '$'
    })

    expect(result).toBe('1.000,50$')
  })

  it('should always have two decimals', () => {
    const result = numToFormattedString(1000, {})

    expect(result).toBe('1.000,00€')

    const result2 = numToFormattedString(1000.555, {})

    expect(result2).toBe('1.000,55€')
  })
})