So, I am still a fresh man in JS. But I tried working around with my first bigger project. However, I just can't figure out why I can't access constructor members from functions that are outside of my main class. I assumed it had something to do with module.exports / require. So, that's how I did. Still, no luck.
Here is my code from the main.js file:
const funcs = require('./funcs.js');
class MainClass {
constructor(omegga, config, store) {
this.omegga = omegga;
this.config = config;
this.store = store;
}
async init() {
TestVehicle.on('cmd:test', async name => {
const dss = await this.store.get('cats'); // this works fine.
funcs.setSomething('29'); // this errors
console.log(dss);
});
}
}
module.exports = MainClass;
async stop() { }
My function funcs.setSomething('29') errors however. Here's the function from funcs.js:
var MainClass = require('./main.js');
const teampo = new MainClass();
async function setSomething(argument1) {
try {
teampo.store.set('cats', argument1); // this errors
} catch(e) { console.error(e); }
}
module.exports = { setSomething };
Now, JS tells me that something seems to be undefined. This is the error message I get:
"TypeError: Cannot read property 'set' of undefined"
I wonder why it is undefined? Shouldn't I be able to inherit store and all of its properties/methods? I've tried everything already, like changing require, the module.exports, etc. But it doesn't seem to work. Might the cause be more severe or is this some simple mistake I do and just have been overseeing the whole time?
When you are creating the instance of the class here: const teampo = new MainClass();, you're not passing it the 3 things it requires which is
constructor(omegga, config, store) {}.
Since you didn't pass it a store, this.store, and thereby teampo.store is undefined
When you instantiate your class with const teampo = new MainClass(); you’re not passing any arguments. The constructor sets store to one of the arguments (this.store = store). Since there’s no store argument, this.store is undefined.
So if you subsequently attempt to call set on it it errors.
I have a file called app.js:
let id = 0;
const Func = require('./func.js');
Func.myFunc();
console.log(id);
module.exports = {
id
};
Also I have another file called func.js:
const App = require('./app.js');
var myFunc = () => {
App.id = 100;
}
module.exports = {
myFunc
};
But console.log(id) returns: 0
What you are doing is termed circular dependencies. It is generally frowned upon and there is no allowed instance where this may be required. You're better off creating a third file that will use both of them...
Read this to have a better understanding:
https://nodejs.org/api/modules.html#modules_cycles
How to deal with cyclic dependencies in Node.js
Add another log like:
// app.js ...
console.log(module.exports);
module.exports = { id };
console.log(id);
to see that your code does work somehow, in the way that module.exports has an id property set, but you override that afterwards. Also the property of the export object has nothing todo with the id variable (well, the later gets copied into the first) so console.log will never log 100.
There are two problems with your piece of code:
the circular dependency
module.exports.id is not the same object as the variable id in app.js
In order to solve the first problem, you should create a third file where the variable will be declared and other modules will use that.
Now, to illustrate the second problem, look at the following.
// testModule.js
let variable = 0;
function get_variable() {
return variable;
}
module.exports = {variable, get_variable};
// app.js
const testModule = require('./testModule.js');
testModule.variable = 1234;
console.log(testModule.variable); // will print '1234'
console.log(testModule.get_variable()); // will print '0'
This little misunderstanding of modules, could lead to subtle nasty bugs. I consider that the best practice to solve this would be not to export the 'variable' property directly, but having getter/setter functions in the module, almost like transforming it into a class-like thing.
// testModule.js
let variable = 0;
function get_variable() {
return variable;
}
function set_variable(value) {
variable = value;
}
module.exports = {get_variable, set_variable};
I have a file called helpers.js in the 'helpers' folder. The contents are like below:
class Helpers {
constructor(config) {
if (this._singleton) {
throw new Error('A singleton has already been created.');
}
this._singleton = this;
}
/**
* Gets the singleton object.
* #returns {Helpers}
*/
static getSingleton() {
return this._singleton;
}
}
module.exports = Helpers;
Then in /helpers/user.js I want to get the helper's singleton instance.
This is my code:
const helpers = require('../helpers').getSingleton();
or
const Helpers = require('../helpers');
const helpers = Helpers.getSingleton();
The error I keep getting is:
TypeError: require(...).getSingleton is not a function
or
TypeError: Helpers.getSingleton is not a function
If I hover over Helpers in VSCode, I get this tooltip
And, whenever I hover over getSingleton() I get this tooltip:
So the path is correct, but it still gives me the errors.
The easiest way to implement the singleton pattern in JavaScript is to just not export the class at all, e.g.
class Helpers {}
let helper;
module.exports = function() {
if (!helper) helpers = new Helpers();
return helper;
};
// loaded with
var helpers = require('../helpers')(); // note the extra () to call it
or even better, since we aren't restricted to Java-like behavior, just skip the function entirely and do
class Helpers {}
module.exports = new Helpers();
// loaded with
var helpers = require('../helpers');
but then if all your module is exporting is a single instance of a class, there's very little reason to use a class in the first place. You might as well do
exports.helperMethodOne = function(){};
exports.helperMethodTwo = function(){};
exports.helperMethodThree = function(){};
// loaded with
var helpers = require('../helpers');
or
module.exports = {
helperMethodOne() {},
helperMethodTwo() {},
helperMethodThree() {},
};
// loaded with
var helpers = require('../helpers');
Your require statement is wrong, but its hard to tell you precisely the right syntax without knowing your environment.
const config = require('/path/to/file');
Is typical. So try:
const Helpers = require('../helpers');
You wrote '../helpers.js' in your screenshot, not '../helpers'
You get the error:
TypeError: require(...).getSingleton is not a function
Because require(...) resolves to something else, like null, and null.getSingleton is not a function.
Also, you cannot reference this meaninfully inside a static context. this ought to only be used for class instances, not static members.
You can do something like this to use it as Singleton.getInstance();
class Singleton {
static instance = new Singleton();
static getInstance = () => Singleton.instance;
constructor() {
throw new Error('Use Singleton.getInstance()');
}
}
module.exports = Singleton;
or even something more sneaky and use it as new Singleton()
class Singleton {
static instance;
constructor() {
if (!Singleton.instance) {
Singleton.instance = this;
}
return Singleton.instance;
}
}
module.exports = Singleton;
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.
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
}
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"
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
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.
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.
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.
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 ;)
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();
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.
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
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();
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.
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.
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.
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');
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 = {}
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.
In browse (assuming your app is running), inspect, source, and make sure the javascript file for the constructor is loaded.
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() {}
}
I'm using Electron to create a little desktop app and exporting with module.exports. On the 'server' side this works fine. However, when I'm using module.exports on the front end, as per Electron docs, I get this error.
Uncaught TypeError: this.showProgressbar is not a function"
var ViewController = {
getPageCount: function (res) {
this.total = res;
this.showProgressbar(res);
},
showProgressBar: function (num) {
$('.progress-container').addClass('show');
$('.progress-bar').style('width', '0%');
}
};
module.exports = ViewController;
On the client side, this is how I'm accessing this file.
var view = require(__dirname + '/client/ViewController.js');
ipc.on('page_count', view.getPageCount);
How should I be accessing internal methods in this instance?
ViewController is neither a 'Class' nor an instance, its a plain javascript object with two properties.
If you want it to behave like a class and be able to acccess to other properties from a method when creating an instance this is how you should do:
var ViewController = function(ipc){
this.ipc=ipc;
this.ipc.on('page_count', this.getPageCount);
};
ViewController.prototype.getPageCount: function (res) {
this.total = res;
this.showProgressbar(res);
},
ViewController.prototype.showProgressBar: function (num) {
$('.progress-container').addClass('show');
$('.progress-bar').style('width', '0%');
}
module.exports = ViewController;
You still need to instantiante ViewController :
var ViewController = require(__dirname + '/client/ViewController.js');
var controller = new ViewController(ipc);
That's because when the callback is being called, it's being called in the wrong context. To bind the context, use Function.bind.
ipc.on('page_count', view.getPageCount.bind(view));