greensock/GSAP · error

Not tracking ${property} velocity.

Error message

Not tracking ${property} velocity.

What it means

VelocityTracker.get(property) looks up this._props[property]; if no tracker was added for that property it warns "Not tracking <property> velocity." and then dereferences pt, which typically throws a TypeError immediately after. It means velocity is only available for properties explicitly tracked via VelocityTracker.track() or gsap's trackVelocity option.

Source

Thrown at src/utils/VelocityTracker.js:82

}

export class VelocityTracker {

	constructor(target, property) {
		_coreInitted || _initCore();
		this.target = _toArray(target)[0];
		_lookup[_getID(this.target)] = this;
		this._props = {};
		property && this.add(property);
	}

	static register(core) {
		gsap = core;
		_initCore();
	}

	get(property, skipRecentTick) {
		let pt = this._props[property] || console.warn("Not tracking " + property + " velocity."),
			val, dif, rotationCap;
		val = parseFloat(skipRecentTick ? pt.v1 : pt.g(pt.t, pt.p));
		dif = (val - parseFloat(pt.v2));
		rotationCap = pt.rCap;
		if (rotationCap) { //rotational values need special interpretation so that if, for example, they go from 179 to -178 degrees it is interpreted as a change of 3 instead of -357.
			dif = dif % rotationCap;
			if (dif !== dif % (rotationCap / 2)) {
				dif = (dif < 0) ? dif + rotationCap : dif - rotationCap;
			}
		}
		return _round(dif / ((skipRecentTick ? pt.t1 : _ticker.time) - pt.t2));
	}

	getAll() {
		let result = {},
			props = this._props,
			p;
		for (p in props) {

View on GitHub (pinned to 13e2b79054)

Solutions

1. Track the property first: VelocityTracker.track(target, 'x', 'y') (or via gsap tween's trackVelocity / inertia)
2. Use VelocityTracker.getByTarget(target).get(property) only for names passed to track()
3. Query the correct property name (e.g., 'rotation' vs 'rotate')
4. Initialize tracking early so at least one tick has occurred before reading values

Example fix

// before
const tracker = new VelocityTracker(el);
tracker.get('x'); // never tracked
// after
VelocityTracker.track(el, 'x', 'y');
const tracker = VelocityTracker.getByTarget(el);
const vx = tracker.get('x');

import { VelocityTracker } from 'gsap/VelocityTracker';
function ensureTracked(target, props) {
  const tracker = VelocityTracker.getByTarget(target) || VelocityTracker.track(target, ...props);
  return tracker;
}
const tracker = ensureTracked(el, ['x', 'y']);
const vx = tracker.get('x');

function isTracked(tracker, prop) { return !!(tracker && tracker._props && tracker._props[prop]); }

try {
  const v = tracker.get('x');
} catch (e) {
  VelocityTracker.track(el, 'x');
  const v = 0; // no velocity yet on first tick
}

When it happens

Trigger: Calling tracker.get('rotation') or getVelocity for a property never tracked — creating VelocityTracker(target) without listing properties, not passing trackVelocity/--tracking in a tween, or querying before the first tween tick.

Common situations: Reading velocity in InertiaPlugin throwProps configs for props never tracked, assuming all tweened properties are tracked automatically, typos between tracked property name and queried name, or querying 'x' when only 'y' was tracked.

Related errors

• GSDevTools error: invalid animation.
• Please gsap.registerPlugin(CSSPlugin, CSSRulePlugin)
• Please gsap.registerPlugin(CustomEase, CustomBounce)
• Please gsap.registerPlugin(CustomEase)
• Please gsap.registerPlugin(CustomEase, CustomWiggle)

AI-assisted analysis of greensock/GSAP@13e2b79054 (2026-08-29). Data as JSON: /api/errors/21a8a9e9996d94d7. Report an issue: GitHub.