老师，看了你的教程，跟着做，然后就弄了个react练手，但是在写的过程中用的extend这个方法时出了个问题，就是for(const key in from)里无法拿出from里面的方法名，请问老师这个是什么问题，我用的create-react-app脚手架，下面我把代码贴出来，请老师指导一下
package.json

{
    "name": "thumbtack-web",
    "version": "0.1.0",
    "private": true,
    "dependencies": {
        "@types/jest": "24.0.19",
        "@types/node": "12.11.1",
        "@types/qs": "^6.5.3",
        "@types/react": "16.9.7",
        "@types/react-dom": "16.9.2",
        "@types/react-router-dom": "^5.1.0",
        "antd-mobile": "^2.3.1",
        "axios": "^0.19.0",
        "babel-plugin-import": "^1.12.2",
        "customize-cra": "^0.8.0",
        "http-proxy-middleware": "^0.20.0",
        "less": "^3.10.3",
        "less-loader": "^5.0.0",
        "node-sass": "^4.12.0",
        "qs": "^6.9.0",
        "react": "^16.10.2",
        "react-app-rewired": "^2.1.3",
        "react-dom": "^16.10.2",
        "react-router": "^5.1.2",
        "react-router-dom": "^5.1.2",
        "react-scripts": "3.2.0",
        "sass-loader": "^8.0.0",
        "tslint": "^5.20.0",
        "tslint-config-prettier": "^1.18.0",
        "tslint-config-standard": "^8.0.1",
        "typedoc": "^0.15.0",
        "typescript": "3.6.4"
    },
    "scripts": {
        "start": "react-app-rewired start",
        "build": "react-app-rewired build",
        "test": "react-app-rewired test",
        "eject": "react-app-rewired 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"
        ]
    }
}

tsconfig.json(说起这个，还有个小问题，create-react-app的这个compilerOptions在一运行yarn start的时候，不管你怎么改的，都会被重写，然后你改的就失效了，这是怎么做到的，有没有什么方法可以改呢？）

{
  "compilerOptions": {
    "target": "es5",
    "lib": [
      "dom",
      "dom.iterable",
      "esnext"
    ],
    "allowJs": true,
    "skipLibCheck": true,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "strict": true,
    "forceConsistentCasingInFileNames": true,
    "module": "esnext",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "isolatedModules": true,
    "noEmit": true,
    "jsx": "react"
  },
  "include": [
    "src"
  ],
  "exclude": [
    "src/setupProxy.js"
  ]
}

index.tsx

declare global {
    interface Window {
        customReturnClickFunc?: () => void
    }
}

interface ClickFunc {
    (): void;
}

class ReturnClickFunc {

    test: string = '123'

    a() {
        console.log(this);
    }

    addCustomReturnClickFunc(clickFunc: () => void) {
        if (clickFunc) {
            window.customReturnClickFunc = clickFunc;
        }
    }

    doClick() {
        if (window.customReturnClickFunc) {
            window.customReturnClickFunc();
            window.customReturnClickFunc = undefined;
        } else {
            window.history.back();
        }
    }
}

class Return {
    func?: ClickFunc & ReturnClickFunc;
}
/**
 * 这个方法里面key无论如何也拿不出来，导致在外部像调用addCustomReturnClickFunc无法被调用
 */
function extend<T, U>(to: T, from: U): T & U {
    for (const key in from) {
        console.log(key);
        (to as T & U)[key] = from[key] as any;
    }
    return to as T & U;
}

let context = new ReturnClickFunc();
let click = ReturnClickFunc.prototype.doClick.bind(context);
Return.prototype.func = extend(click, context);

export { Return }