createContext 不接受 defaultValue

Question

I'm trying to build a context by using createContext from the react API:

import React, { useState, createContext } from 'react';

export const MovieContext =  createContext();

export const MovieProvider = (props) => {
  const [movies, setMovies] = useState(
    [
      {
        original_title: 'name of movie',
        poster_path: 'path_to_poster',
        id: 1
      }
    ]
  );
  return (
    <MovieContext.Provider value={[movies, setMovies]}>
      {props.children}
    </MovieContext.Provider>
  );
}

export default MovieProvider;

The createContext() function shows this error:

Expected 1 arguments, but got 0.ts(2554) index.d.ts(385, 9): An argument for 'defaultValue' was not provided.

If I make it a string, and pass a string as value the app builds:

export const MovieContext =  createContext('');

<MovieContext.Provider value={''}>
  {props.children}
</MovieContext.Provider>

What kind of value do I add as a createContext() parameter to make it work with the useState hook? I'm following this guys tutorial and he does it in JS but I'm building my app in TS so it's a bit more strict.

Answer 1

These are the Typescript types, they are described here . There is a comment in the code why this argument should be required, although the documentation doesn't say it is required.

function createContext<T>(
    // If you thought this should be optional, see
    // https://github.com/DefinitelyTyped/DefinitelyTyped/pull/24509#issuecomment-382213106
    defaultValue: T,
): Context<T>;

In your case will correctly set the default parameter like

type IMovie = {
    original_title: string;
    poster_path: string;
    id: number;
};

type IMovieContext = [IMovie[], React.Dispatch<React.SetStateAction<IMovie[]>>];

export const MovieContext = createContext<IMovieContext>([[], () => null]);

export const MovieProvider = props => {
    const [movies, setMovies] = useState<IMovie[]>([
        {
            original_title: 'name of movie',
            poster_path: 'path_to_poster',
            id: 1,
        },
    ]);
    return <MovieContext.Provider value={[movies, setMovies]}>{props.children}</MovieContext.Provider>;
};