Uncaught TypeError: Cannot read property 'apply' of undefined when calling function on parent LWC

The typeerror: cannot read property 'apply' of undefined error was I was trying to call a function on the parent directly from the child and that doesn't work. You need to create a custom event on the child that the parent is listening for like this:

Parent HTML

Parent JS
handleSlideClicked(event) { console.log('in parent handle click' + event.detail); }
Child HTML
Child JS
@api slideIndex; handleSlideClick(event) { this.slideIndex = event.currentTarget.dataset.index; // Creates the event with the data. const selectedEvent = new CustomEvent("slideclicked", { detail: this.slideIndex, bubbles:true }); // Dispatches the event. this.dispatchEvent(selectedEvent); }