JSFiddle - React, Tailwind, and code Playground

JavaScript

// function that calls the callback
function call(cb) {
    cb();
}

class UnderTest {
    constructor() {
        this.number = 5;
    }

		// method to be called as a callback by someone else
    printNumber() {
        alert(this.number);
    }

		// current solution
    printNumberCB() {
        return function() {
            this.printNumber();
        }.bind(this); 
    }
}

var underTest = new UnderTest();
underTest.number = 7;
underTest.printNumber(); // print number directly
call(underTest.printNumberCB()); // print number by callback