React Native fetch：未定义不是对象答案

【问题标题】：React Native fetch: Undefined is not an objectReact Native fetch：未定义不是对象
【发布时间】：2020-03-25 21:29:10
【问题描述】：

尝试在 react native 中使用 fetch 并获取

Unable to symbolicate stack trace: Requiring module "node_modules\react-native\Libraries\Network\fetch.js", which threw an exception: TypeError: Attempted to assign to readonly property.

Unable to symbolicate stack trace: undefined is not an object (evaluating '_$$_REQUIRE(_dependencyMap[5], "../../Network/fetch").fetch')

这是我的代码

这是应用程序的第一个屏幕

import React, {Component} from 'react';
import {Text, View} from 'react-native';

export default class App extends Component {
    state = {
        loginURL:''
    };

    componentDidMount(): void {
        fetch('http://10.0.2.2:4607/imgur/login', {
            method: 'POST',
            headers: {
                Accept: 'application/json',
                'Content-Type': 'application/json',
            },
        })
            .then((response) => response.json())
            .then((json) => {
                if(json.type === 'redirect') this.setState({'loginURL': json.data});
            });
    };

    render(){
        return (<View><Text>Hello</Text></View>);
    };
}

我尝试删除 node_modules 目录并重新运行 npm install 但同样的问题。发生在模拟器上

我的 package.json 依赖项：

"dependencies": {
    "@fortawesome/fontawesome-pro": "^5.13.0",
    "@fortawesome/fontawesome-svg-core": "^1.2.28",
    "@fortawesome/pro-light-svg-icons": "^5.13.0",
    "@fortawesome/pro-regular-svg-icons": "^5.13.0",
    "@fortawesome/pro-solid-svg-icons": "^5.13.0",
    "@fortawesome/react-native-fontawesome": "^0.2.3",
    "@react-native-community/masked-view": "^0.1.7",
    "@react-navigation/drawer": "^5.3.4",
    "@react-navigation/native": "^5.1.3",
    "appcenter": "3.0.0",
    "appcenter-analytics": "3.0.0",
    "appcenter-crashes": "3.0.0",
    "react": "16.9.0",
    "react-native": "0.61.5",
    "react-native-gesture-handler": "^1.6.1",
    "react-native-reanimated": "^1.7.1",
    "react-native-render-html": "^4.2.0",
    "react-native-safe-area-context": "^0.7.3",
    "react-native-screens": "^2.4.0",
    "react-native-svg": "^11.0.1",
    "react-native-vector-icons": "^6.6.0",
    "react-native-webview": "^8.2.1",
    "react-navigation": "^4.3.3"
  },

【问题讨论】：

标签： react-native fetch

【解决方案1】：

这可能无关紧要，但是为什么您在 setState 上使用引号？不应该这样：

this.setState({'loginURL': json.data});

像这样：

this.setState({loginURL: json.data});

【讨论】：

谢谢，Javascript 两者都支持，所以没什么区别。

【解决方案2】：

所以当我启用调试器模式时，我收到了另一个错误

TypeError: Cannot assign to read only property 'forEach' of object '#<Headers>'
TypeError: Cannot read property 'fetch' of undefined

Error: Requiring module "node_modules\react-native\Libraries\Network\fetch.js", which threw an exception: TypeError: Cannot assign to read only property 'forEach' of object '#<Headers>'

在一个模块上，我包含了一个 forEach 填充物来循环和对象，就像你对一个数组一样。例如：

if (!Object.prototype.forEach) {
    Object.defineProperty(Object.prototype, 'forEach', {
        value: function (callback, thisArg) {
            if (this == null) throw new TypeError('Not an object');

            for (let key in this) {
                if (this.hasOwnProperty(key)) callback.call(thisArg, this[key], key, this);
            }
        }
    });

}

当我删除此代码时，一切都按预期工作！我找不到负责引发异常的代码，但至少我知道原因是我自己的代码:)

【讨论】：