nodejs中模块函数/类的实例

Question

我有一个项目以 ES5.1 开始，后来我将其更改为支持 ES6，所以我使用的是 async-await，让...

现在我需要有类似实例的模块/对象。

我发现 this 帖子解释了这个过程，但是......

当我这样做时：

'use strict';

let _ = require('lodash');
var mappings;

function SafeRequestHandler(reqMappings) {

    if (!(this instanceof SafeRequestHandler)) {
        return new SafeRequestHandler((reqMappings));
    }

    mappings = reqMappings;
}

function safeHandleReq(req, res) {
    try {
        let pair = _.find(mappings, {'url': req.url});
        return pair.handler(req, res);
    } catch (err) {
        console.log(err);
    }
}

SafeRequestHandler.prototype.safe = safeHandleReq;

module.exports = SafeRequestHandler;

但现在，每次我这样做：

var handler = require('../util/safeRequestHandler');

let handlerMappings = [
    {
        url: def.party_create,
        handler: partyCreate
    },
    {
        url: def.party_modify,
        handler: partyModify
    },
    {
        url: def.party_get,
        handler: partyGet
    }
];

var handle = new handler(handlerMappings).safe;

我得到this.mappings 替换为SafeRequestHandler 例如。这不是一个实例。

所以我尝试了定义处理程序的 ES6 方法：

'use strict';

let _ = require('lodash');

class SafeRequestHandler {

    constructor(reqMappings) {
        this.mappings = reqMappings;
    }

    safeHandleReq(req, res) {
        try {
            let pair = _.find(this.mappings, {'url': req.url});
            return pair.handler(req, res);
        } catch (err) {
            console.log(err);
        }
    }

}

并像这样实例化它：

let handlerMappings = [
    {
        url: def.party_create,
        handler: partyCreate
    },
    {
        url: def.party_modify,
        handler: partyModify
    },
    {
        url: def.party_get,
        handler: partyGet
    }
];

let handle = new SafeRequestHandler(handlerMappings).safeHandleReq;

..但是这样我什至无法在safeHandleReq(...) 中联系mappings。 this 未定义，mappings 未定义

我想我只是没有完全理解基本原理，所以请您纠正并解释这两种方法有什么问题？

谢谢！

Answer 1

如果将特定类实例的method 存储到变量中，则会丢失上下文。

// handle doesn't have a pointer to its context (aka, this)
let handle = new SafeRequestHandler(handlerMappings).safeHandleReq;
// so handle() will output cannot read property mappings of undefined

改成：

const instance = new SafeRequestHandler(handlerMappings);

const handle = (...args) => instance.safeHandleReq(...args);

更多信息在这里：Losing "this" context in JavaScript when passing around members

Answer 2

您的第一个代码创建了一个全局变量，因此只有 一个 值。您也可以在那里使用 this：

 let _ = require('lodash');
function SafeRequestHandler(reqMappings) {
  if (!(this instanceof SafeRequestHandler)) {
    return new SafeRequestHandler((reqMappings));
  }
   this.mappings = reqMappings;
}

SafeRequestHandler.prototype.safeHandleReq =  function(req, res) {
    try {
        let pair = _.find(this.mappings, {'url': req.url});
        return pair.handler(req, res);
    } catch (err) {
        console.log(err);
    }
}
module.exports = SafeRequestHandler;