Question

我已将方法的名称存储在列表中。

var list = ['fn1', 'fn2', 'fn3', 'fn4'];

我动态地使用一些标准选择方法。这些方法是使用'prototype

附加的较大类的一部分

MyObj.prototype.selectfn = function(criteria) {
    var fn = list[sel];
    this[fn].call(this, arg1);
}
MyObj.prototype.fn1 = function(args) { // do something }
MyObj.prototype.fn2 = function(args) { // do something}

等等。问题出在所选的“fn”函数中，this变量显示为全局对象，即使我使用call()我阅读了mozilla文档，但我无法理解为什么会这样，有人可以帮忙吗？

它有帮助，我的环境是node.js 0.10.12。

编辑：提供正确的示例代码有点困难，因为我的代码涉及很多地方的回调，但我会试着澄清。

假设有两个文件User.js和Helper.js。

User.js

var m, h;
var Helper = require('./Helper');

function init() {
  // to simplify, assume the `this` here refers to `User`
  h = new Helper(this);
}

function doSomething() {
  // pass some criteria string
  h.selectfn(criteria);
}

Helper.js

var Helper = module.exports = function(user) {
  this.user = user;
}

Helper.prototype.selectfn = function(criteria) {
  // based on some criteria string, choose the function name from "list" array
  // here, the string "sel" holds the selected function name
  var fn = list[sel];
  this[fn].call(this.user, arg1); 
  // if I print to console, `this.user` is correct over here, but inside the function it shows as undefined
}

Helper.prototype.fn1 = function(args) {
  // Here, I talk to databases, so I have callbacks. Say, on a callback, the user property is to be updated. This is why I want to use `call()` so that the `this` refers to `User` and can be updated.
  // For example, if we want to update the "last-seen" date.
  this.lastseen = new Date();
}

希望这个小例子让它更清晰。

Answer 1

call（）的第一个参数是你的函数上下文“this”

例如：

var someObject = {
  withAFunction: function(text) { alert('hello ' + text); }
};

var testFunction = function(text) {
   this.withAFunction(text);
};

testFunction.call(someObject, 'world');

Answer 2

我尝试检查您的代码：

var list = ['fn1', 'fn2', 'fn3', 'fn4'];

//MyObj is copy of Object?
var MyObj = Object;
//or a preudoclass:
//var MyObj =  function(){};

//inside this function you use sel and arg1 that seems undefined and use like arguments criteria that seems useless

MyObj.prototype.selectfn = function(sel,arg1) {
    var fn = list[sel];
    this[fn].call(this, arg1); // this is MyObj istance.


MyObj.prototype.fn1 = function(args) { console.log("fn1", args);/* do something */ }
MyObj.prototype.fn2 = function(args) { console.log("fn2",args); /* do something */ }

xxx = new MyObj();

xxx.selectfn(1,"ciao");
//call xxx.fn1("ciao");

请参阅控制台以获取回复。

使用call（）后，“this”是一个全局对象

2 个答案: