Cannot call a method within a class it defined it in ES6 in Node.js [duplicate]

Your method is being rebound to the Layer class within express, losing its original context. The way that express handles routes is by wrapping each one in a Layer class, which assigns the route callback to itself:

this.handle = fn;

That is where your problems arise, this assignment automatically rebinds the function context to Layer. Here is a simple example demonstrating the problem:

function Example() {    this.message = "I have my own scope"; } Example.prototype.logThis = function() {    console.log(this); }function ReassignedScope(logThisFn) {    this.message = "This is my scope now";   // simulation of what is happening within Express's Layer   this.logThis = logThisFn; }let example = new Example()let scopeProblem = new ReassignedScope(example.logThis);scopeProblem.logThis(); // This is my scope now

Others have already pointed out the solution, which is to explicitly bind your method to the ProductController instance:

app.post('/product', productController.create.bind(productController));

When you pass create method as method it is probably called in different context (this) as you expect. You can bind it:

app.post('/product', productController.create.bind(productController));

There are many other ways how to ensure this refers to correct object.

E.g. wrap it with function (either arrow or classical):

app.post('/product', (...args) => productController.create(...args));

Or bind methods in constructor:

constructor() {    this.create = this.create.bind(this);}