Building a 403-Function Casio fx-991ES PLUS Scientific Calculator in Pure JavaScript & PWA

If you have ever taken high school or university physics, engineering, or calculus, you have almost certainly held a Casio fx-991ES PLUS scientific calculator. Its two-line Natural Textbook Display (V.P.A.M.), multi-mode engine, and tactile layout have made it a staple in classrooms and exam halls worldwide.

However, most online "scientific" calculators available today are deceptively basic. They give you standard arithmetic, basic trigonometry, and logarithms, but fall completely flat when you need:

Numerical definite integration and derivatives
$3 \times 3$ Matrix determinants and inversions
Complex number arithmetic in polar ($r\angle\theta$) and rectangular ($a+bi$) forms
Statistical regressions (linear, quadratic, exponential, logarithmic)
Base-N conversions with bitwise logic (BIN, OCT, DEC, HEX)
Polynomial & simultaneous linear equation solvers

To bridge this gap without paywalls or forced app installs, we built WideMath Scientific Calculator Online—a full-featured, zero-dependency browser recreation of the fx-991ES PLUS workflow that runs entirely on client-side web standards.

🛠️ Architecture & Engineering Highlights

1. Two-Dimensional Natural Textbook Display

A core challenge of recreating the fx-991ES PLUS is rendering mathematical expressions naturally: fractions stacked vertically, arbitrary $n$-th roots with indices, and definite integrals with upper and lower limit boxes.

Rather than pulling in heavy external math rendering dependencies that inflate load times, WideMath uses a modular HTML/CSS layout engine paired with math.js for safe tokenization and evaluation without eval().

2. Multi-Mode State Machine

The calculator operates across 10 discrete calculation modes:

COMP: General scientific calculations with automatic bracket completion
CMPLX: Rectangular ($a+bi$) and polar ($r\angle\theta$) calculations
STAT: 1-variable and 2-variable regression analysis
BASE-N: Integer conversions with AND, OR, XOR, XNOR, and NOT bitwise operators
EQN: 2-to-3 variable linear systems, quadratics ($ax^2+bx+c=0$), and cubics
MATRIX: Matrix storage ($A, B, C$) with determinants, transpositions, and matrix inversion
VECTOR: Vector storage and dot/cross product calculations
TABLE: Function value tables across customizable ranges and step intervals

3. Shift / Alpha Key Modifiers with Dynamic Illumination

To preserve the intuitive hardware feeling, pressing the SHIFT (yellow) or ALPHA (red) key dynamically dims unavailable buttons and illuminates only the keys that have corresponding functions in that mode.

4. Zero-Install Progressive Web App (PWA)

By implementing a standalone manifest.json and service worker caching strategy, the entire calculator can be added to any smartphone or desktop home screen and operated completely offline.