Project 17: Unit Converter (Expert Mode)

📋 Project Brief

Build a comprehensive unit converter supporting scientific, financial, and everyday measurements. This includes live currency exchange rates via API, scientific notation for extreme values, and a searchable dropdown system for finding units quickly.

✅ Feature Requirements

Feature 1: Live Currency Exchange

Acceptance Criteria:

• Fetch real-time exchange rates from a financial API (e.g., exchangerate-api.com)
• Support at least 30 major currencies
• Cache rates for 1 hour to reduce API calls
• Show "Last updated: X minutes ago" timestamp
• Handle API failures by using cached/fallback rates with a warning badge

Feature 2: Scientific Notation

Acceptance Criteria:

• Support very large numbers (e.g., kilometers to nanometers: 1 km = 1,000,000,000,000 nm)
• Automatically switch to scientific notation (e.g., 1 × 10¹²) when numbers exceed 6 digits
• Support very small numbers (e.g., 0.000001 → 1 × 10⁻⁶)
• A toggle to switch between standard and scientific notation display

Feature 3: Searchable Dropdowns

Acceptance Criteria:

• Typing in the unit selector filters the list in real-time
• Search matches unit name, abbreviation, and category
• Recent/frequent units appear at the top of the list
• Keyboard navigation: arrow keys to move, Enter to select

Feature 4: Conversion History

Acceptance Criteria:

• A side panel shows the last 10 conversions
• Each entry shows: input value, input unit, output value, output unit, timestamp
• Clicking a history entry re-populates the converter
• History persists in localStorage

Feature 5: Reverse Toggle

Acceptance Criteria:

• A single button swaps the "From" and "To" units instantly
• The input value stays the same; the result recalculates
• The button has a rotate animation (↔) on click

📊 Unit Categories

Category	Example Units
Length	mm, cm, m, km, in, ft, yd, mi, nm, μm
Weight	mg, g, kg, oz, lb, ton, stone
Temperature	°C, °F, K
Volume	mL, L, gal, qt, pt, cup, tbsp, tsp, fl oz
Speed	m/s, km/h, mph, knots
Data	bit, byte, KB, MB, GB, TB, PB
Time	ms, s, min, hr, day, week, month, year
Currency	USD, EUR, GBP, JPY, INR, + 25 more

🧪 Testing Checklist

• Unit test all conversion formulas (especially temperature which isn't simple multiplication)
• Unit test scientific notation formatting
• Unit test search/filter algorithm
• Integration test: selecting units and entering a value produces correct output
• A11y: all dropdowns and inputs properly labeled

🚀 Stretch Goals

• Add a "Favorites" list for frequently used conversions
• Implement batch conversion (convert one value to multiple units simultaneously)
• Add a conversion formula display ("1 mile = 1.60934 kilometers")