Method eventCallback

src/gsap-core.js:1405–1418

(type, callback, params)

	}

	eventCallback(type, callback, params) {
		let vars = this.vars;
		if (arguments.length > 1) {
			if (!callback) {
				delete vars[type];
			} else {
				vars[type] = callback;
				params && (vars[type + "Params"] = params);
				type === "onUpdate" && (this._onUpdate = callback);
			}
			return this;
		}
		return vars[type];
	}

	then(onFulfilled) {
		let self = this,