@eternl/formats

Locale-aware display formatting and CSV-safe number helpers for browser-based dashboards and exports.

Installation

npm install @eternl/formats

Quick Start

import Big from 'big.js'
import {
  buildCsv,
  csvCurrencyCode,
  formatCsvDate,
  formatCsvNumber,
  formatCsvPercent,
  formatDisplayCurrency,
  formatDisplayPercent,
  formatDisplayCompact
}                             from '@eternl/formats'

const amount = new Big('1234567.890123')

const priceUS = formatDisplayCurrency(amount, {
  locale: 'en-US',
  currencyCode: 'USD',
  minDecimals: 2,
  maxDecimals: 2,
  useSymbol: true
}) // "$1,234,567.89"

const priceDE = formatDisplayCurrency(amount, {
  locale: 'de-DE',
  currencyCode: 'EUR',
  minDecimals: 2,
  maxDecimals: 2,
  useSymbol: true
}) // "1.234.567,89 €"

const adaDisplay = formatDisplayCurrency('123456789.123456', {
  locale: 'en-US',
  currencyCode: 'ADA',
  customSymbols: { ADA: '₳' },
  minDecimals: 2,
  maxDecimals: 6
}) // "₳123,456,789.123456"
const masked = formatDisplayCurrency('1234.5', {
  locale: 'en-US',
  currencyCode: 'USD',
  minDecimals: 2,
  maxDecimals: 2,
  isHidden: true
}) // "$*.**"

const maskedWithDash = formatDisplayCurrency('1234.5', {
  locale: 'en-US',
  currencyCode: 'USD',
  minDecimals: 2,
  maxDecimals: 2,
  isHidden: true,
  hiddenCharacter: '-'
}) // "$-.--"


const pctDisplay = formatDisplayPercent('0.125', {
  locale: 'en-US',
  minDecimals: 1,
  maxDecimals: 2
}) // "12.5%"

const compact = formatDisplayCompact('9876543', {
  locale: 'en-US',
  compact: true,
  maxDecimals: 1
}) // "9.9M"

const csvNumber = formatCsvNumber(amount, { maxDecimals: 6, trimTrailingZeros: true })
const csvPercent = formatCsvPercent('0.125')
const csvCurrency = csvCurrencyCode('ADA', { ADA: '₳' }, false)

const rows = [
  { ts: new Date('2025-10-06T12:00:00Z'), amount, currency: 'ADA', ratio: '0.125' }
]

const csv = buildCsv(rows, [
  { header: 'date', accessor: (row) => row.ts, formatter: (value) => formatCsvDate(value as Date) },
  { header: 'amount', accessor: (row) => row.amount, formatter: (value) => formatCsvNumber(value as Big, { maxDecimals: 6 }) },
  { header: 'currency', accessor: (row) => row.currency, formatter: (value) => csvCurrencyCode(String(value), { ADA: '₳' }, false) },
  { header: 'percent', accessor: (row) => row.ratio, formatter: (value) => formatCsvPercent(value as string, { maxDecimals: 4 }) }
])

The generated CSV is fully ASCII (. decimal separator, no grouping) and imports cleanly into Google Sheets, Koinly, and CoinTracking.

Display Formatting

• formatDisplayNumber handles localized decimals with grouping, optional sign display, and rounding controls.
• formatDisplayCurrency combines Intl.NumberFormat with optional custom symbols (₳, ₿, etc.). Spacing from the host locale is preserved unless a custom symbol removes it.
• formatDisplayPercent multiplies by 100 for display while respecting locale decimal and percent symbols.
• formatDisplayCompact applies configurable K/M/B/T suffixes using precise big.js arithmetic (no scientific notation).

Common options:

{
  minDecimals?: number
  maxDecimals?: number
  precision?: number        // significant digits
  roundingMode?: 'roundDown' | 'roundHalfUp' | 'roundHalfEven' | 'roundUp'
  trimTrailingZeros?: boolean (default: false)
  signDisplay?: 'auto' | 'always' | 'exceptZero' | 'never' | 'negative'
  useGrouping?: boolean
  isHidden?: boolean            // mask digits with maskCharacter (default '*')
  hiddenCharacter?: string
}

CSV Formatting

• formatCsvNumber outputs plain ASCII digits and a dot decimal separator, honoring min/max decimals, precision, and trimming.
• formatCsvPercent multiplies the ratio by 100 and emits an unadorned numeric string (no %).
• csvCurrencyCode keeps currency symbols or codes in a dedicated column so numeric fields stay clean.

formatCsvNumber('0.00000001', { maxDecimals: 8 }) // "0.00000001"
formatCsvPercent('0.125', { maxDecimals: 4 })      // "12.5"
csvCurrencyCode('btc', { BTC: '₿' }, true)        // "₿"

CSV Builder

buildCsv(rows, columns, delimiter = ',', lineEnding = '\n') applies per-column accessors and formatters, quoting fields with delimiters, quotes, or newlines (RFC 4180). Numeric results produced by formatCsvNumber never require quoting, so imports remain numeric.

Date Helpers

• formatDisplayDate(input, locale, options) caches Intl.DateTimeFormat instances.
• formatCsvDate(input) emits ISO-8601 strings: YYYY-MM-DD for midnight UTC, otherwise YYYY-MM-DDTHH:mm:ssZ.

Development

npm run lint   # type-check
npm test       # run Vitest suite
npm run build  # emit ESM + CJS bundles into dist/

The test suite covers locale differences, rounding edge cases, percent handling, large/small magnitudes, and CSV quoting rules.