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 = ProjectWidth;
    this.ProjectScale = this.GetProjectScale();
    this.Attributes = Attributes;

    this.currentLayout = '';

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

I then try to create an instance like this:

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

but this exception is thrown:

Project is not a constructor

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

Answer 1

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 2

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 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() // succeed

Answer 4

I've googled around also and found this solution:

You have a variable Project somewhere that is not a function. Then the new operator will complain about it. Try console.log(Project) at the place where you would have used it as a construcotr, and you will find it.

Answer 5

For my project, the problem turned out to be a circular reference created by the require() calls:

y.js:
var x = require("./x.js");
var y = function() { console.log("result is " + x(); }
module.exports = y;

x.js:
var y = require("./y.js");
var my_y = new y(); // <- TypeError: y is not a constructor
var x = function() { console.log("result is " + my_y; }
module.exports = x;

The reason is that when it is attempting to initialize y, it creates a temporary "y" object (not class, object!) in the dependency system that is somehow not yet a constructor. Then, when x.js is finished being defined, it can continue making y a constructor. Only, x.js has an error in it where it tries to use the non-constructor y.

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.

Answer 7

In my case I was using the prototype name as the object name. For e.g.

function proto1()
{}

var proto1 = new proto1();

It was a silly mistake but might be of help to someone like me ;)

Answer 8

Sometimes it is just how you export and import it. For this error message it could be, that the default keyword is missing.

export default SampleClass {}

Where you instantiate it:

import SampleClass from 'path/to/class';
let sampleClass = new SampleClass();

Option 2, with curly braces:

export SampleClass {}
import { SampleClass } from 'path/to/class';
let sampleClass = new SampleClass();

Answer 9

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

const o = {
  a: () => {},
  b() {},
  c: function () {}
};

const { a, b, c } = o;

new a(); // throws "a is not a constructor"
new b(); // throws "b is not a constructor"
new c(); // works

Answer 10

Car.js

class Car {
 getName() {return 'car'};
}
export default Car;

TestFile.js

const object = require('./Car.js');
const instance = new object();

error: TypeError: instance is not a constructor

printing content of object

object = {default: Car}

append default to the require function and it will work as contructor

const object = require('object-fit-images').default;
const instance = new object();
instance.getName();

Answer 11

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 12

In my case this happened due to a circular reference between two classes. I imported class B in the class A file and imported class A in the class B file, so the program never reached to the point of actually define A as a class.

Answer 13

I just had a similar error when trying to use the BS5ModalJS in conjunction with webpack, meaning I was trying to import the js file.

Because the single .js file provided was designed to be used via the script tags, it took a while for me to realise that to avoid the "BSModal is not a constructor" error, I had to go into their code and add:

export {BSModal}

I was then able to use

import { BSModal } from './../thirdparty/BS5ModalJS/BS5Modal.js';

and do

let myModal enter code here= new BSModal(...)

without getting that error.

So if you're using webpack, perhaps make sure the classes and functions are exported (and therefore available) to the callers.

Answer 14

In my case I'd forgotten the open and close parantheses at the end of the definition of the function wrapping all of my code in the exported module. I.e. I had:

(function () {
  'use strict';

  module.exports.MyClass = class{
  ...
);

Instead of:

(function () {
  'use strict';

  module.exports.MyClass = class{
  ...
)();

The compiler doesn't complain, but the require statement in the importing module doesn't set the variable it's being assigned to, so it's undefined at the point you try to construct it and it will give the TypeError: MyClass is not a constructor error.

Answer 15

I had a similar error and my problem was that the name and case of the variable name and constructor name were identical, which doesn't work since javascript interprets the intended constructor as the newly created variable.

In other words:

function project(name){
    this.name = name;
}

//elsewhere...

//this is no good! name/case are identical so javascript barfs. 
let project = new project('My Project');

Simply changing case or variable name fixes the problem, though:

//with a capital 'P'
function Project(name){
    this.name = name;
}

//elsewhere...

//works! class name/case is dissimilar to variable name
let project = new Project('My Project');

Answer 16

It is happening because you must have used another variable named "project" in your code. Something like var project = {}

For you to make the code work, change as follows:

var project = {} into var project1 = {}

Answer 17

To add the solution I found to this problem when I had it, I was including a class from another file and the file I tried to instantiate it in gave the "not a constructor" error. Ultimately the issue was a couple unused requires in the other file before the class was defined. I'm not sure why they broke it, but removing them fixed it. Always be sure to check if something might be hiding in between the steps you're thinking about.

Answer 18

In browse (assuming your app is running), inspect, source, and make sure the javascript file for the constructor is loaded.

Answer 19

For me this happened because of a small typo.

I had a class export like ->

module.export = class TestClass {
   constructor() {}
}

And I was trying to use this class as ->

const testClass = new TestClass();

The problem and typo was at module.export, which should be module.exports so

module.exports = class TestClass { // module.exports here instead of export
   constructor() {}
}