module.factory-servise.js

module.factory('greeterFactory', function(a) {
    function Greeter(a) {
        this.greet = function() {
            return 'Hello ' + a;
        }
    }
    
    return new Greeter(a);
});

//or with service

module.service('greeterService', function(a) {
    this.greet = function() {
        return 'Hello ' + a;
    }
});

function Controller(greeterFactory, greeterService) {
    expect(greeter instanceof Greeter).toBe(true);
    expect(greeter.greet()).toEqual('Hello 123');
    
    const greeter2 = new greeter2;
    expect(greeter2 instanceof greeterFactory).toBe(true);
    expect(greeter2.greet()).toEqual('Hello 123');
}