JIYIK CN >

Use type guards to resolve “Object is possibly null” errors with the useRef hook in React, for example if(inputRef.current){}. Once null is excluded from the type of ref, we can access properties on the ref that correspond to its type.

Below is sample code that causes this error to occur.

import {useEffect, useRef} from 'react';

export default function App() {
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    // ⛔️ Object is possibly 'null'.ts(2531)
    inputRef.current.focus();
  }, []);

  return (
    <div>
      <input ref={inputRef} type="text" id="message" />
      <button>Click</button>
    </div>
  );
}

The problem in the code snippet is that TypeScript doesn't guarantee that we'll assign the ref to an element or assign it a value, so its current property may be null.

To fix this error, we must use type guards to exclude null from ref's type before accessing its properties.

import {useEffect, useRef} from 'react';

export default function App() {
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    // 这里 ref 可以是 null
    if (inputRef.current != null) {
      // TypeScript 知道这里的 ref 不为 null
      inputRef.current.focus();
    }
  }, []);

  return (
    <div>
      <input ref={inputRef} type="text" id="message" />
      <button>Click</button>
    </div>
  );
}

We use a simple if statement as a type guard to ensure that the current property on the ref does not store a null value.

TypeScript knows that once we enter the if block, the current property of the ref object will not store a null value.

Make sure to use generics on the useRef hook to correctly type the current property of the ref

Notice that we pass a generic to the value of ref to the HTMLInputElement.

The types of DOM elements are uniformly named HTML***Element. Once you start typing HTML.., your IDE should be able to help you autocomplete.

Some commonly used types are: HTMLInputElement, HTMLButtonElement, HTMLAnchorElement, HTMLImageElement, HTMLTextAreaElement, HTMLDivElementetc.

If you store different values ​​in the ref, make sure to pass the specific type to the useRef hook's generics, ·const ref = useRef<{name: string}>(null);e.g.

(?.)We can also use the optional chaining operator to block if the current property on the ref stores a null value .

import {useEffect, useRef} from 'react';

export default function App() {
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    // 👇️ optional chaining (?.)
    inputRef.current?.focus();
  }, []);

  return (
    <div>
      <input ref={inputRef} type="text" id="message" />
      {/* Cannot find name 'button'.ts(2304) */}
      <button>Click</button>
    </div>
  );
}

If ref is empty (null or undefined), the optional chaining (?.)operator will block rather than raise an error.

In other words, if the current property on ref stores a null value, the operator will short-circuit and return undefined, rather than attempting to call the focus() method on the undefined value and causing a runtime error.

Another solution to the “object might be null” error with refs in React is to use the not null (!) assertion operator.

import {useEffect, useRef} from 'react';

export default function App() {
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    // 👇️ using non-null (!) assertion
    inputRef.current!.focus();
  }, []);

  return (
    <div>
      <input ref={inputRef} type="text" id="message" />
      {/* Cannot find name 'button'.ts(2304) */}
      <button>Click</button>
    </div>
  );
}

The exclamation mark is called the non-null assertion operator in TypeScript. It is used to remove null and undefined from a type without doing any explicit type checking.

When we use this, we basically tell TypeScript that the current property on the ref object does not store a value of null or undefined.

Note that this approach is not type-safe, as TypeScript does not perform any checks to ensure that the property is not null.

The “Object is possibly null” error is caused because the useRef() hook can be passed an initial value as a parameter, and we passed it null as the initial value.

The hook returns a mutable ref object whose .currentattributes are initialized to the passed arguments.

When we pass a ref prop to an element, for example <input ref={myRef} />, React .currentsets the properties of the ref object to the corresponding DOM node, but TypeScript can't determine whether we mean to set the ref to the DOM element or to set its value later in our code.