unni unni - 4 months ago 13
Javascript Question

Javascript "Not a Constructor" Exception while creating objects

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()
{....}
}


I then try to create and instance like this:

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


But this execption is thrown:

Project is not a constructor


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

Answer

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

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

Variable declarations are 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
}