How to define type for ref(binding on template) In Vue3, using TypeScript?

Well, that would depend on whether or not you need it typed and to provide member information (public properties, methods, etc). If you do, then yes you need to define a type for it; otherwise, you don't have to, and can simply access the unwrapped reference with .value which leaves it as type any (I bet you figured out this one).

But if you have to, you need to tell the compiler what it is or what it's going to be assigned on. To do that, you'll want to use the third overload of ref (with no argument) and explicitly set the generic type to the desired type—in your case, you want HTMLDivElement (or simply HTMLElement if you don't care about the specific members it has to offer).

export default defineComponent({  setup() {    const el = ref<HTMLDivElement>();    onMounted(() => {      el.value // DIV element    });    return {      el    }  }})

In JavaScript, you don't have type checking, so passing null on the ref function is as good as not passing anything (which is especially okay for template refs)*; it could even come across as being misleading in a sense that the unwrapped value actually resolves to something else but null.

* When using the Composition API, the concept of "reactive refs" and "template refs" are unified. And the reason we're accessing this particular type of ref on the mounted hook is because the DOM element will be assigned to it after initial render.

References:

• https://v3.vuejs.org/guide/composition-api-template-refs.html#template-refs

The line isn't correct:

    const el = ref<HTMLDivElement>();

The el is not an HTMLDivElement. Instead, it's a proxy to the element. The $el of that proxy is the actual HTMLDivElement. Though I'm not sure if I get it right. In a component, I managed to type it as follow:

import { ComponentPublicInstance, defineComponent, ref } from 'vue';export default defineComponent({  // snipped...  setup() {    const field = ref<ComponentPublicInstance<HTMLInputElement>>();    return {      field,      fun() { field.value.$el.value = 'some value'; }    };  }  // snipped...});