Module 39 // Core JavaScript

Intl API

Module Objective

Localization, number and date formatting

Mental Model Realtime Simulation

INTERACTIVE_CANVAS
Editor_Pane
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Console_Output
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Practical Code Examples

// Example 1
// 1. Currency Formatting
const price = 1234.56;

const usFormat = new Intl.NumberFormat('en-US', { 
  style: 'currency', currency: 'USD' 
}).format(price);

const inFormat = new Intl.NumberFormat('en-IN', { 
  style: 'currency', currency: 'INR' 
}).format(price);

console.log(usFormat); // "$1,234.56"
console.log(inFormat); // "₹1,234.56"
💡 `Intl.NumberFormat` handles decimal points, thousands separators, and currency symbols automatically based on the locale.
// Example 2
// 2. Full Date/Time
const now = new Date();
const formatter = new Intl.DateTimeFormat('fr-FR', {
  weekday: 'long',
  year: 'numeric',
  month: 'long',
  day: 'numeric'
});

console.log(formatter.format(now)); // "mercredi 17 décembre 2025"
💡 Instead of complex regex or string hacking, `Intl` provides native, performant translation of date components.
// Example 3
// 3. Relative Time
const rtf = new Intl.RelativeTimeFormat('en', { style: 'short' });

console.log(rtf.format(-1, 'day')); // "1 day ago"
console.log(rtf.format(5, 'minute')); // "in 5 min."
💡 Relative time formatting is perfect for social media apps or news feeds where 'just now' or '2 days ago' is more useful than a raw date.

Engine & Memory Architecture

The Localization Engine

1. Heavy Built-in:

  • The Intl object is a massive subsystem in the engine.
  • It relies on the ICU (International Components for Unicode) library, which is a large binary stored in your browser's installation directory.

2. Formatting Performance:

  • Creating an Intl formatter object is relatively expensive in terms of CPU cycles.
  • Performance Tip: If you're formatting thousands of numbers in a loop, create the formatter once and reuse it (const f = new Intl...) instead of creating a new one inside the loop.

3. Locale Data:

  • Locales are loaded from RAM only when requested. The engine optimizes memory by keeping only the most common locale data resident.
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