useRef not allowing to access .current.clientHeight-CodePudding

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?

CodePudding user response：

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 witch 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 need to be rendered before a ref gets its value in current field.

A solution is to use a state for the top :

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

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