Line 0 parsing error cannot read properties of undefined reading map

Currently starting up the server on my client side, the error above is what I have been
getting. I am using TypeScript, ReactJS, ESLint. I can’t seem to go forward since this error
has been haunting me. The GitHub page for ESLint hasn’t been of much help either.

This error went up after I had created the useMutation component and exported it in index.ts.
Not sure how to get rid of this error.

Below is my package.json

    {
    "name": "tinyhouse_client",
    "version": "0.1.0",
    "private": true,
      "dependencies": {
      "@testing-library/jest-dom": "^4.2.4",
      "@testing-library/react": "^9.3.2",
      "@testing-library/user-event": "^7.1.2",
      "@types/jest": "^24.0.0",
      "@types/node": "^12.0.0",
      "@types/react": "^16.9.35",
      "@types/react-dom": "^16.9.0",
      "@typescript-eslint/parser": "^3.0.2",
      "react": "^16.13.1",
      "react-dom": "^16.13.1",
      "react-scripts": "3.4.1",
      "typescript": "~2.23.0"
      },
      "resolutions": {
     "@typescript-eslint/eslint-plugin": "^2.23.0",
     "@typescript-eslint/parser": "^2.23.0",
     "@typescript-eslint/typescript-estree": "^2.23.0"
     },
     "scripts": {
     "start": "react-scripts start",
     " build": "react-scripts build",
     "test": "react-scripts test",
     "eject": "react-scripts eject"
     },
     "eslintConfig": {
     "extends": "react-app"
     },
     "browserslist": {
     "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
      "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
     ]
     },
     **strong text** "proxy": "http://localhost:9000"
      }

Below is my index.ts

    export * from './server';
    export * from './useQuery';
    export * from './useMutation';

And my useMutation.ts

    import { useState } from 'react';
    import { server } from './server';

    interface State<TData> {
    data: TData | null;
    loading: boolean; 
    error: boolean;
    }

    type MutationTuple<TData, TVariables> = [
    (variables?: TVariables | undefined) => Promise<void>,
    State<TData>
    ];

    export const useMutation = <TData = any, TVariables = any>(
    query: string
    ): MutationTuple<TData, TVariables> => { 
    const [state, setState] = useState<State<TData>>({
    data: null,
    loading: false,
    error: false,
    })

    const fetch = async (variables?: TVariables) => {
    try {
      setState({ data: null, loading: true, error: false });

      const { data, errors } = await server.fetch<TData, TVariables>({ query, variables });
      if (errors && errors.length) {
        throw new Error(errors[0].message);
      }

      setState({ data, loading: false, error: false });
    } catch (err) {
      setState({ data: null, loading: false, error: true });
      throw console.error(err);
    }
   }

   return [fetch, state];
};

I considered giving a comment under the answer by taggon to this very question, but well, i felt it owed more explanation for those interested in details.

Uncaught TypeError: Cannot read property ‘value’ of undefined is strictly a JavaScript error.

(Note that value can be anything, but for this question value is ‘map’)

It’s critical to understand that point, just so you avoid endless debugging cycles.
This error is common especially if just starting out in JavaScript (and it’s libraries/frameworks).
For React, this has a lot to do with understanding the component lifecycle methods.

// Follow this example to get the context
// Ignore any complexity, focus on how 'props' are passed down to children

import React, { useEffect } from 'react'

// Main component
const ShowList = () => {
  // Similar to componentDidMount and componentDidUpdate
  useEffect(() => {// dispatch call to fetch items, populate the redux-store})

  return <div><MyItems items={movies} /></div>
}

// other component
const MyItems = props =>
  <ul>
    {props.items.map((item, i) => <li key={i}>item</li>)}
  </ul>


/**
  The above code should work fine, except for one problem.
  When compiling <ShowList/>,
  React-DOM renders <MyItems> before useEffect (or componentDid...) is called.
  And since `items={movies}`, 'props.items' is 'undefined' at that point.
  Thus the error message 'Cannot read property map of undefined'
 */

As a way to tackle this problem, @taggon gave a solution (see first anwser or link).

Solution: Set an initial/default value.
In our example, we can avoid items being ‘undefined’ by declaring a default value of an empty array.

Why? This allows React-DOM to render an empty list initially.
And when the useEffect or componentDid... method is executed, the component is re-rendered with a populated list of items.

// Let's update our 'other' component
// destructure the `items` and initialize it as an array

const MyItems = ({items = []}) =>
  <ul>
    {items.map((item, i) => <li key={i}>item</li>)}
  </ul>

Currently starting up the server on my client side, the error above is what I have been
getting. I am using Typescript, React, ESlint. I can’t seem to go forward since this error
has been haunting me. The github page for Eslint hasn’t been much help either.

This error went up after I had created the useMutation component and exported it in the
index.ts,
Not sure how to get rid of this error.

Below is my package.json

    {
    "name": "tinyhouse_client",
    "version": "0.1.0",
    "private": true,
      "dependencies": {
      "@testing-library/jest-dom": "^4.2.4",
      "@testing-library/react": "^9.3.2",
      "@testing-library/user-event": "^7.1.2",
      "@types/jest": "^24.0.0",
      "@types/node": "^12.0.0",
      "@types/react": "^16.9.35",
      "@types/react-dom": "^16.9.0",
      "@typescript-eslint/parser": "^3.0.2",
      "react": "^16.13.1",
      "react-dom": "^16.13.1",
      "react-scripts": "3.4.1",
      "typescript": "~2.23.0"
      },
      "resolutions": {
     "@typescript-eslint/eslint-plugin": "^2.23.0",
     "@typescript-eslint/parser": "^2.23.0",
     "@typescript-eslint/typescript-estree": "^2.23.0"
     },
     "scripts": {
     "start": "react-scripts start",
     " build": "react-scripts build",
     "test": "react-scripts test",
     "eject": "react-scripts eject"
     },
     "eslintConfig": {
     "extends": "react-app"
     },
     "browserslist": {
     "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
      "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
     ]
     },
     **strong text** "proxy": "http://localhost:9000"
      }

Below is my index.ts

    export * from './server';
    export * from './useQuery';
    export * from './useMutation';

And my useMutation.ts

    import { useState } from 'react';
    import { server } from './server';

    interface State<TData> {
    data: TData | null;
    loading: boolean; 
    error: boolean;
    }

    type MutationTuple<TData, TVariables> = [
    (variables?: TVariables | undefined) => Promise<void>,
    State<TData>
    ];

    export const useMutation = <TData = any, TVariables = any>(
    query: string
    ): MutationTuple<TData, TVariables> => { 
    const [state, setState] = useState<State<TData>>({
    data: null,
    loading: false,
    error: false,
    })

    const fetch = async (variables?: TVariables) => {
    try {
      setState({ data: null, loading: true, error: false });

      const { data, errors } = await server.fetch<TData, TVariables>({ query, variables });
      if (errors && errors.length) {
        throw new Error(errors[0].message);
      }

      setState({ data, loading: false, error: false });
    } catch (err) {
      setState({ data: null, loading: false, error: true });
      throw console.error(err);
    }
   }

   return [fetch, state];
};