I Built a QR Code Generator in Pure Vanilla JS — No Libraries, No Server, 202 Tests

QR codes look like magic — a grid of black and white squares that encodes anything from a URL to a business card. But how do they actually work? I decided to find out the hard way: implement the full QR Code Model 2 algorithm in vanilla JavaScript, zero external dependencies. The result: QR Code Generator — a free, client-side tool that generates QR codes from any text or URL. 👉 https://qr-code-generator-e83.pages.dev Why No Libraries? I maintain a collection of browser-only developer tools at devnestio . Every tool has the same rule: zero external dependencies. No npm installs, no CDN scripts, no servers. For most tools (JSON diff, Base64 encoder, UUID generator) that's easy. QR codes are different. The spec is a 126-page ISO document. Most developers just npm install qrcode and call it a day. But writing it from scratch taught me more about error-correcting codes, Galois field arithmetic, and matrix encoding than I ever expected. Worth every hour. What the Tool Does Real-time generation as you type (debounced at 80ms) Size selector — 128 × 128, 256 × 256, or 512 × 512 pixels Error correction level — L (7%), M (15%), Q (25%), H (30%) Color picker — any foreground and background color PNG download via canvas SVG download with crisp vector output at any scale How QR Codes Actually Work QR Code Model 2 (the standard you see everywhere) has six major steps. Here's the short version: 1. Data Encoding Text gets encoded into one of three modes based on content: Numeric ( 0-9 ): packs 3 digits into 10 bits — most compact Alphanumeric ( 0-9 A-Z $%*+-./:space ): 2 chars into 11 bits Byte (everything else): UTF-8, one byte per 8 bits The encoder picks the mode automatically and finds the minimum QR version (1–40) that fits the data. function detectMode ( text ) { if ( /^ \d +$/ . test ( text )) return NUMERIC_MODE ; if ( text . split ( '' ). every ( c => ALPHANUMS . includes ( c ))) return ALPHANUM_MODE ; return BYTE_MODE ; } 2. Reed-Solomon Error Correction This is the hard