Javascript "Not a Constructor" Exception while creating objects
JavascriptObjectConstructorTypeerrorJavascript Problem Overview
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.
Javascript Solutions
Solution 1 - Javascript
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
}
Solution 2 - Javascript
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"
Solution 3 - Javascript
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
Solution 4 - Javascript
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.
Solution 5 - Javascript
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.
Solution 6 - Javascript
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.
Solution 7 - Javascript
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 ;)
Solution 8 - Javascript
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();
Solution 9 - Javascript
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
Solution 10 - Javascript
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();
Solution 11 - Javascript
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.
Solution 12 - Javascript
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.
Solution 13 - Javascript
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.
Solution 14 - Javascript
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');
Solution 15 - Javascript
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 = {}
Solution 16 - Javascript
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.
Solution 17 - Javascript
In browse (assuming your app is running), inspect, source, and make sure the javascript file for the constructor is loaded.