A progress bar fills a horizontal track to show task completion percentage. It turns abstract "waiting" into concrete "how much is left" — the most basic state feedback in file uploads, data processing, and installers.
Key implementation: Use CSS transition: width 0.5s for smooth growth (10x better performance than JS animation). Set overflow: hidden + border-radius on the track to clip the fill bar. Use linear-gradient for visual depth. For auto-increment, use requestAnimationFrame synced with browser refresh rate.
Common pitfalls: Transition won't fire if using width attribute instead of style.width. Always clamp percentage with Math.min(pct, 100). Update label text and bar width together. Initial width of 0% may flash — use 2% minimum.
Variants: Segmented bars for multi-step, indeterminate bars with sliding animation, circular progress with SVG stroke-dashoffset, flowing gradient, threshold-based color switching.
Full copy-paste code available above. Questions? Leave a comment.
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