Spring physics

React chapter 57 · react-spring · useSpring · source: pmndrs/react-spring/packages/core

The problem

CSS transition uses duration + easing. Interrupt halfway (click the other side) and the easing restarts from zero velocity. The motion feels mechanical. Gestures and follow-through need a model that keeps velocity.

The pattern that fixes it

A spring: acceleration from (target - x) * stiffness, damped by velocity. Clicking a new target keeps v. useSpring({ x: open ? 1 : 0 }) is that loop behind a declarative config. Duration is a consequence, not an input.

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