扩展 JavaScript Promise 时出错

Error when extending JavaScript Promise

本文关键字：出错 Promise JavaScript 扩展      更新时间：2023-09-26

我有一个接受回调的Worker类，但我想通过使其成为Promise来删除回调，所以我可以做new Worker(stuff, otherStuff).then(function() { ... });

为此，我使用了extends关键字。

起初，我在this.a = a;作业中遇到了ReferenceError: this is not defined错误，但我发现那是因为我不得不打电话给super();

现在，它在this.resolve({output: 'ok'});行处给出了TypeError: this.resolve is not a function，所以我试图强行添加this.resolve和this.reject函数，但错误不会消失。

这是(高度简化的(代码：

class Worker extends Promise {
    constructor(a, b) {
        super((resolve, reject) => {
            console.log('inside super');
            this.resolve = resolve;
            this.reject = reject;
        });
        console.log('start constructing');
        this.a = a;
        this.b = b;
        console.log('A: ' + JSON.stringify(a));
        console.log('B: ' + JSON.stringify(b));
        this.doStuff();
        if (!this.status) {
            this.resolve({output: 'ok'});
        } else {
            this.reject('There was an error');
        }
    }
    doStuff() {}
}
let a = {x: 1};
let b = {z: 2};
console.log('before creation');
let w = new Worker(a, b);
console.log('after creation');
w.then(function() {
    console.log('done');
});

我正在使用节点 6.1.0，它似乎支持Promise子类......我做错了什么？

最佳做法是避免扩展基元类，因为它们会发生变化。扩展它们会使您自己的类受到浏览器/引擎的怜悯。

当你尝试扩展 Promise 类时，this不会在构造函数中定义，因为现在大多数引擎都不支持子类化 Promise。

此外，工人不是承诺。工人应该工作并做出承诺。

解决方案是使用包装器或工厂模式，而不是继承。

例如，您可以在 Worker 类中定义一个 getter promise，该 getter 将创建一个新的 Promise 实例并返回它。

function serializeError(error({return{message：error.message||"，名称：错误.名称||"，堆栈：错误.堆栈||"}}class NativeResolver{constructor(({this.resolve;this.reject;this.promise=new Promise(((resolve，reject(=>{this.resolve=resolve;this.reject=reject}((}}function assertCast(condition({if(！condition({throw new Error("Failed assertion"(}return condition}const ReservedMessageTypes={RESPONSE_TYPE：">response"，ERROR_TYPE：">error"};function isGenerated Message(messageType({return messageType.substr(0,3(===">"}function throwIfReserved(messageType({if(isGeneratedMessage(messageType(({throw new Error( Unexpected reserved message type: '${messageType}' (}}class MessagePipe{constructor(targetOrigin，target，rethrowErrors=true({if(！target({const frame=document.querySelector( iframe[src^='${targetOrigin}'] (;if(！frame||！frame.contentWindow({throw new Error("T无法定位目标内容窗口"(}target=assertCast(frame.contentWindow(}this.target=target;this.targetOrigin=targetOrigin;this.rethrowErrors=rethrowErrors;this.logClientError=object=>console.error(JSON.stringify(object((;this.messageHandlers=new Map;this.pendingMessages_=new Map;this.nextMessageId_=0;this.messageListener_=m=>this.receiveMessage_(m(;console.assert(this.target_！==window，"target ！== window"(;window.addEventListener("message"，this.messageListener_(}registerHandler(messageType，handler({throwIfReserved(messageType(;if(this.messageHandlers_.has(messageType(({throw new Error( A handler already exists for ${messageType} (}this.messageHandlers_.set(messageType，handler(}async sendMessage(messageType，message={}({try{return await this.sendMessageImpl(messageType，message(}catch(errorResponse({const error=new Error( ${messageType}: ${errorResponse.message} (;error.name=errorResponse.name||"Unknown Error";error.stack+= 'nError from ${this.targetOrigin_}'n${errorResponse.stack} ;throw error}}async sendMessageImpl(messageType，message={}({throwIfReserved(messageType(;const messageId=this.nextMessageId_++;const resolver=new NativeResolver;this.pendingMessages_.set(messageId，resolver(;this.postToTarget_(messageType，message，messageId(;return resolver.promise}detach(({window.removeEventListener("message"，this.messageListener_(}handleMessageResponse_(messageType，message，messageId({const{RESPONSE_TYPE：RESPONSE_TYPE，ERROR_TYPE：ERROR_TYPE}=ReservedMessageTypes;const resolver=this.pendingMessages_.get(messageId(;if(messageType===RESPONSE_TYPE({resolver.resolve(message(}else if(messageType===ERROR_TYPE({this.logClientError(message(;resolver.reject(message(}else{console.error( Response for message ${messageId} received with invalid message type ${messageType} (}this.pendingMessages_.delete(messageId(}async callHandlerForMessageType_(messageType，message，messageId({const{RESPONSE_TYPE：RESPONSE_TYPE，ERROR_TYPE：ERROR_TYPE}=ReservedMessageTypes;let response;let error=null;let sawError=false;try{response=await this.messageHandlers_.get(messageType((message(}catch(err({sawError=true;error=err;response=serializeError(err(}this.postToTarget_(sawError？ERROR_TYPE：RESPONSE_TYPE，response，messageId(;if(sawError&&this.rethrowErrors({this.logClientError(error(;throw error}}receiveMessage_(event({const e=event;if(typeof e.data！=="object"||！数据||typeof e.data.type！=="string"({return}const{messageId：messageId，type：type，message：message}=e.data;const{ERROR_TYPE：ERROR_TYPE}=ReservedMessageTypes;if(e.origin！==this.targetOrigin_&&this.targetOrigin_！=="*"({return}if(isGenerated Message(type(&&this.pendingMessages_.has(messageId(({this.handleMessageResponse_(type，message，messageId(;return}if(isGeneratedMessage(type(({console.error( Response with type ${type} for unknown message received. (;return}if(！this.messageHandlers_.has(type(({const error=new Error( No handler registered for message type '${type}' (;const errorResponse=serializeError(error(;this.postToTarget_(ERROR_TYPE，errorResponse，messageId(;return}this.callHandlerForMessageType_(type，message，messageId(}postToTarget_(messageType，message，messageId({const messageWrapper={messageId：messageId，type：messageType，message：message||{}};this.target_.postMessage(messageWrapper，this.targetOrigin_(}}

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