實例

Question

我有一個開始作爲ES5.1的一個項目，後來我改成了支持ES6所以我是用異步的await，讓...

現在我需要具有類似於實例的模塊/對象。

我發現this文章，解釋的過程，但...

當我這樣做：

'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; 

但現在，每次我這樣做：

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 REPLACED在SafeRequestHandler例如。這不是一個實例。

所以我試圖ES6方法定義處理程序：

'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); 
     } 
    } 

} 

並創建實例，如：

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; 

..但這種方式在safeHandleReq(...)我甚至不能伸手mappings。 this是undefined，mappings是undefined

我想我只是不完全瞭解基本原理，所以能否請您正確&解釋這兩種方法有什麼問題？

謝謝！

Answer 1

你將失去​​的情況下：您可以使用此有過。

// 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 

將其更改爲：

const instance = new SafeRequestHandler(handlerMappings); 

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

更多的信息在這裏：Losing "this" context in JavaScript when passing around members

Answer 2

您的第一個代碼創建了一個全局變量，因此只有一個值。如果你存儲一個特定的類實例的method到一個變量

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;