What causes "ERROR: No velocity was defined for ${target} property: ${p}" in greensock/GSAP?

Provide velocity explicitly: inertia:{x:{velocity:500, end:1000}}

ERROR: No velocity was defined for ${target} property: ${p}

Error message

ERROR: No velocity was defined for ${target} property: ${p}

What it means

When building an inertia tween, each property needs a starting velocity. It can come from an explicit velocity value in vars or from live tracking data. If neither exists for a property, InertiaPlugin warns that no velocity was defined for that target/property and continues with undefined behavior for that property.

Source

Thrown at src/InertiaPlugin.js:276

			p, curProp, curVal, unit, velocity, change1, end, change2, linkedProps;
		vars = _processingVars;
		_processingVars = 0;
		//when there are linkedProps (typically "x,y" where snapping has to factor in multiple properties, we must first populate an object with all of those end values, then feed it to the function that make any necessary alterations. So the point of this first loop is to simply build an object (like {x:100, y:204.5}) for feeding into that function which we'll do later in the "real" loop.
		linkedProps = _processLinkedProps(target, vars, getVal, resistance);

		for (p in vars) {
			if (!_reservedProps[p]) {
				curProp = vars[p];
				_isFunction(curProp) && (curProp = curProp(index, target, targets));
				if (_isNumber(curProp)) {
					velocity = curProp;
				} else if (_isObject(curProp) && !isNaN(curProp.velocity)) {
					velocity = +curProp.velocity;
				} else {
					if (tracker && tracker.isTracking(p)) {
						velocity = tracker.get(p);
					} else {
						console.warn("ERROR: No velocity was defined for " + target + " property: " + p);
					}
				}
				change1 = _calculateChange(velocity, duration);
				change2 = 0;
				curVal = getVal(target, p);
				unit = _getUnit(curVal);
				curVal = parseFloat(curVal);
				if (_isObject(curProp)) {
					end = curVal + change1;
					if ("end" in curProp) {
						curProp = _parseEnd(curProp, (linkedProps && p in linkedProps) ? linkedProps : end, curProp.max, curProp.min, p, vars.radius, velocity);
					}
					if (("max" in curProp) && +curProp.max < end) {
						if (preventOvershoot || curProp.preventOvershoot) {
							change1 = curProp.max - curVal;
						} else {
							change2 = (curProp.max - curVal) - change1;
						}

View on GitHub (pinned to 13e2b79054)

Solutions

Provide velocity explicitly: inertia:{x:{velocity:500, end:1000}}
Call InertiaPlugin.track(target) and ensure isTracking(p) for each inertia property
Use Draggable with inertia:true so tracking/throwing is wired automatically
Match property names between track() and the inertia vars

Example fix

// before
gsap.to(el, { inertia: { x: { end: 300 } } }); // no velocity source
// after
InertiaPlugin.track(el);
gsap.to(el, { inertia: { x: { velocity: "auto", end: 300 } } });
// or explicit:
gsap.to(el, { inertia: { x: { velocity: 400, end: 300 } } });

Defensive patterns

Strategy: validation

Validate before calling

Object.keys(inertiaVars).forEach(p => { const prop = inertiaVars[p]; if (!(typeof prop === 'object' && !isNaN(prop.velocity)) && !InertiaPlugin.getByTarget?.(target)?.isTracking?.(p)) { console.warn('No velocity for ' + p); } });

Type guard

const hasVelocitySource = (target, p, curProp) => (curProp && typeof curProp === 'object' && !isNaN(curProp.velocity)) || InertiaPlugin.track && !!tracker;

Prevention

Always supply velocity explicitly when not relying on tracking
Keep tracked property names identical to tweened property names
Track the target right before the throw/inertia tween executes

When it happens

Trigger: gsap.to(el, {inertia:{x:{end:500}}}) where neither a velocity nor InertiaPlugin tracking provides x's velocity; using property names that differ from the tracked ones (e.g. tracking x but tweening left).

Common situations: Hand-written inertia configs omitting velocity; transformed properties tracked under different names than the tween props; Draggable's tracking thrown away before the throw tween runs.

Related errors

AI-assisted analysis of greensock/GSAP@13e2b79054 (2026-08-29). Data as JSON: /api/errors/3e944c608864fc24. Report an issue: GitHub.