Build a Privacy-First Image Compressor That Runs Entirely in Your Browser

The Problem Every online image compressor uploads your files to a server. That means: Your images pass through someone else's infrastructure Compression takes time due to upload/download Privacy-sensitive images (screenshots, documents) leave your device The Solution: Canvas API The HTML5 Canvas API can compress images entirely in the browser. No server. No upload. Your images never leave your device. Here's the core technique: function compressImage(file, quality = 0.7, maxWidth = 1920) { return new Promise((resolve) => { const img = new Image(); img.onload = () => { const canvas = document.createElement("canvas"); let { width, height } = img; // Resize if needed if (width > maxWidth) { height = (height * maxWidth) / width; width = maxWidth; } canvas.width = width; canvas.height = height; const ctx = canvas.getContext("2d"); ctx.drawImage(img, 0, 0, width, height); canvas.toBlob(resolve, "image/jpeg", quality); }; img.src = URL.createObjectURL(file); }); } Key Insights Qualit