reactjstypescriptvalidationmaterial-uitsx

Reputation: 3583

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

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

Upvotes: 304

Answers (13)

busterroni

Reputation: 683

I was missing brackets -- had

const example: [status in ExampleOne]: ExampleTwo = {

and changed it to

const example: { [status in ExampleOne]: ExampleTwo } = {

Upvotes: 0

Pramuditha Jayawardhana

Reputation: 161

First change the file type .tsx

then press F1 and type "Reload Window"

Upvotes: 14

alexander

Reputation: 1342

It can also happen if the TypeScript type is

const User = {
    email: string
}

instead of

interface User {
    email?: string | undefined
}

Upvotes: 0

Mehmet Efe Akça

Reputation: 1432

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

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

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.

Upvotes: 102

John Doe

Reputation: 165

Prepending the datatype with the keyword "typeof" did the trick for me.

let variable: typeof <data type> = ...

For example:

 let packs: typeof Card[] = await Card.find({ category: cat, rarity: rar });

Upvotes: 3

Tim Baum

Reputation: 61

For me, the error occurred as well and changing the import from:

import mytype from '../path/file'

import { mytype } from '../path/file'

resolved the issue.

Upvotes: 5

allan

Reputation: 131

For me, the error occurred due to the wrong filename extension - using .ts instead of .js

Upvotes: 4

IniasP

Reputation: 134

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 the import type approach is that you cannot use it as a value (e.g. constructing new MyClass(...)).

Upvotes: 11

Jakub Kurdziel

Reputation: 3762

change .ts file to .tsx

in my case it works

Upvotes: 298

jonyB

Reputation: 12065

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

Upvotes: 1143

user10643374

Reputation:

Another thing to check is if the import you're performing is surrounded by {} . One of the reasons this message appears is because anything that is not surrounded by {} is treated as the default export for a file.

Upvotes: 2

Ethan H

Reputation: 11

I had to change heroes: HEROES to heroes = HEROES.

I ran across this same x refers to a value, but is being used as a type here. Did you mean 'typeof x'? error building an Angular app. I spent two hours searching for the fix, reading this Stack Overflow post several times. Simple mistake on my part.

Upvotes: 0

Nicholas Obert

Reputation: 1628

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

Upvotes: 0

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

Answers (13)

in my case it works

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