ReactJS and Typescript : refers to a value, but is being used as a type here (TS2749)

Question

I'm coding a ReactJS class with Typescript and Material-ui, in a .tsx file. In one of my custom components, I want to create a reference to one of the components that I use in my custom component.

export class MyTextField extends React.Component<MyProps, MyState> {
  private refTextField: React.RefObject<TextField>;
  constructor(props: MyProps) {
    super(props);
    this.refTextField = React.createRef();
  }

  render(): JSX.Element {
    const { id, label, value: defaultValue } = this.props;
    const { value } = this.state;
    const element = (
      <TextField ref={this.refTextField} id={id} label={label} defaultValue={defaultValue} value={value} />
    );

    return element;
  }
}

During compilation, I get an error on the declaration of my reference:

'TextField' refers to a value, but is being used as a type here. TS2749

I tried to put "typeof TextField" in my declaration, but I have another message, when valuing the ref property in my render :

Type 'RefObject<(props: TextFieldProps) => Element>' is not assignable to type '((instance: HTMLDivElement | null) => void) | RefObject | null | undefined'. Type 'RefObject<(props: TextFieldProps) => Element>' is not assignable to type 'RefObject'. Type '(props: TextFieldProps) => Element' is missing the following properties from type 'HTMLDivElement': align, addEventListener, removeEventListener, accessKey, and 238 more. TS2322

Any ideas ? thank you so much

Answer 1

So I ran into this problem multiple times in my code before but only figured out the reason this was happening today.

TL;DR at the end for the actual answer to the question.

When you create a class in TypeScript, the name of that class refers to both the instance type and the Javascript class value. If you reference that class as a type, TypeScript detects that automatically as the instance type of that class. And if you reference that class in the runtime, it just uses it as in the Javascript meaning. And it's all good and dandy till now.

class MyClass {}
let abc: MyClass; // ts recognizes as instance type
abc = new MyClass(); // completely fine, used here as the javascript value

However, the real problem is when you export the class from a module dynamically. When you export the class in some ways, TypeScript can only export the Javascript value of the class and does not export the type. So if you import it in another module and try to reference it as a type, you will get TS2749.

let intervariable = class MyClass{}
export const MyClass = intervariable; // TypeScript does not export type here.

import {MyClass} from './myclass';

let abc: MyClass; // TypeScript error TS2749

When this happens, especially if it is out of your control, my solution to get the instance type was simply to use InstanceType and typeof:

import {MyClass} from './myclass';

let abc: InstanceType<typeof MyClass>; // no error
// the rest...

The typeof operator gets you the class constructor type for a class value, and the InstanceType generic gets you the instance type that you want.

TL;DR: In your case you just have to write InstanceType<typeof TextField> instead of TextField.

Answer 2

Make sure you're on a .tsx file and not a .ts file

Answer 3

change *.ts file to *.tsx

Answer 4

try to add a type to your createRef function like this:

 private refTextField: React.RefObject<TextField>;
  constructor(props: MyProps) {
    super(props);
    this.refTextField = React.createRef<React.RefObject<TextField>>(null);
  }

Answer 5

For future viewers, the error might also be caused by you not importing the class in the first place.

Answer 6

This should have been a comment, but I don't have enough reputation. I wanted to share that a clean way to use the solution recommended by the accepted answer is to declare a type alias with the same name (it does not cause a conflict). Then you can use your type in the same way you would normally use an instance type.

Adding to the example:

import {MyClass} from './myclass';

type MyClass = InstanceType<typeof MyClass>;

let abc: MyClass; // still no error!

Edit: A type import seems to work as well and it looks cleaner:

import type {MyClass} from './myclass';

The caveat with this approach is that you cannot use it as a value (e.g. constructing new MyClass(...)).