Number formatting

Number formatting covers digits grouping, decimal separators, currencies, percentages, and units per locale. `Intl.NumberFormat` is the browser/Node baseline; accounting rules (minor units, rounding) come from business requirements.

					new Intl.NumberFormat('de-DE', { style: 'currency', currency: 'EUR' }).format(1234.5);
// "1.234,50 €"

new Intl.NumberFormat('en-US', { style: 'percent', maximumFractionDigits: 1 }).format(0.847);
// "84.7%"
				

| Concern | Note | |---------|------| | Currency | Use ISO 4217 code; watch zero-decimal currencies (JPY) | | Rounding | Bankers vs away-from-zero—document policy | | Compact notation | `notation: 'compact'` for dashboards | | Parsing user input | Normalize using locale-aware parser, not `parseFloat` alone |

Never hard-code `,` or `.` as thousands/decimal separators in shared components.

On interviews: storing money as integer minor units; displaying crypto vs fiat; SSR formatting consistency; CLDR updates across environments.

Common pitfalls: floating-point money; mixing currency symbol position; formatting server numbers with wrong locale header; stripping separators before parse incorrectly.

The trade-off is simple string templates versus correct financial and analytics display worldwide.

Checklist:

• Format with explicit locale + currency/options.
• Store money in minor units or decimal type with policy.
• Parse input with locale-aware utilities.
• Test de-DE, en-US, and one RTL market.