React，useRef 不允许访问当前属性，得到 Uncaught TypeError: Cannot read properties of undefined (reading 'clientHeight')

Question

I'm storing a reference to an image item using: const renderedImageRef = useRef() . The ref is then assigned in render() function using:

<img ref={renderedImageRef} src=... />

In another JSX item below, I try to access renderedImageRef.current.clientHeight using:

<div style={{top:`${renderedImageRef.current.clientHeight}px`}}>
   Hello world
</div>

But this produces an error in the console:

Uncaught TypeError: Cannot read properties of undefined (reading 'clientHeight')

Strangely enough, if I try to access renderedImageRef.current.clientHeight from inside a useEffect hook, it displays the height correctly:

useEffect(() => {
    if(renderedImageRef !== null) {
        console.log(renderedImageRef)
    }
}, [renderedImageRef])

Why am I getting the console error?

Answer 1

A possible answer is that this line:

<div style={{top:`${renderedImageRef.current.clientHeight}px`}}>Hello world</div>

is coming in your code before this one:

<img ref={renderedImageRef} src=... />

In which case, it is normal that you are getting the error. It is to know that when you call const renderedImageRef = useRef() , the value of renderedImageRef is {current:undefined} . The JSX must be rendered before a ref gets its value in the current field.

A solution is to use a state for the top :

const [top, setTop] = useState(0);
useEffect(() => {
  // At this point, the current is defined. This if is useful in case you use TypeScript
  if (renderedImageRef.current) {
    setTop(renderedImageRef.current.clientHeight);
  }
}, []);

<div style={{top:`${top}px`}}>Hello world</div>

Answer 2

If you are using Next.JS, I think you have to change the dependency to renderedImageRef.current. So something like this:

const [top, setTop]=useState(0)
useEffect(() => {
    if(renderedImageRef.current) {
        setTop(renderedImageRef.current.clientHeight);
    }
}, [renderedImageRef.current])