Javascript “Not a Constructor” Exception while creating objects

Question

I am defining an object like this:

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


        

Answer 1

I just want to add that if the constructor is called from a different file, then something as simple as forgetting to export the constructor with

module.exports = NAME_OF_CONSTRUCTOR

will also cause the "Not a constructor" exception.

Answer 2

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 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
}

Answer 3

For me it was the differences between import and require on ES6.

E.g.

// processor.js
class Processor {

}

export default Processor

//index.js
const Processor = require('./processor');
const processor = new Processor() //fails with the error

import Processor from './processor'
const processor = new Processor() // succeeds

Answer 4

To add to @wprl's answer, the ES6 object method shorthand, like the arrow functions, cannot be used as a constructor either.

Answer 5

An additional cause of this can be ES2015 arrow functions. They cannot be used as constructors.

const f = () => {};
new f(); // This throws "f is not a constructor"

Answer 6

I have a class in one file that I'm importing into a test file:

//Vec.js
class Vec {

}

module.exports.Vec = Vec;

Changing

//Vec.test.js
const Vec = require('./Vec');
const myVec = new Vec(); //TypeError: Vec is not a constructor

to

//Vec.test.js
const {Vec} = require('./Vec');
const myVec = new Vec(); //Succeeds!

resolved this error for me.