Javascript“不是构造函数"创建对象时出现异常 [英] Javascript "Not a Constructor" Exception while creating objects

问题描述

我正在定义一个这样的对象:

I am defining an object like this:

function Project(Attributes, ProjectWidth, ProjectHeight) {
    this.ProjectHeight = ProjectHeight;
    this.ProjectWidth = ProjectWidth;
    this.ProjectScale = this.GetProjectScale();
    this.Attributes = Attributes;

    this.currentLayout = '';

    this.CreateLayoutArray = function()
    {....}
}

然后我尝试创建一个这样的实例:

I then try to create an instance like this:

var newProj = new Project(a,b,c);

但是抛出了这个异常:

Project is not a constructor

可能有什么问题?我在谷歌上搜索了很多，但我仍然无法弄清楚我做错了什么.

What could be wrong? I googled around a lot, but I still can't figure out what I am doing wrong.

推荐答案

问题中发布的代码无法生成该错误，因为 Project 不是用户定义的函数/有效的构造函数.

The code as posted in the question cannot generate that error, because Project is not a user-defined function / valid constructor.

function x(a,b,c){}
new x(1,2,3);               // produces no errors

你可能做过这样的事情:

You've probably done something like this:

function Project(a,b,c) {}
Project = {};               // or possibly   Project = new Project
new Project(1,2,3);         // -> TypeError: Project is not a constructor

使用 var 的变量声明是提升，因此总是在其余代码之前评估.因此，这也可能导致问题:

Variable declarations using var are hoisted and thus always evaluated before the rest of the code. So, this can also be causing issues:

function Project(){}
function localTest() {
    new Project(1,2,3); // `Project` points to the local variable,
                        // not the global constructor!

   //...some noise, causing you to forget that the `Project` constructor was used
    var Project = 1;    // Evaluated first
}

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