Your code has several issues that need to be addressed. Firstly, attempting to instantiate something by calling a constructor function with your testObject will result in a type error since it is not a function. To fix this, the testObject should be defined like this:
var TestObject = function () {
this.value = "this is my initial value";
};
TestObject.prototype.setup = function () {
this.value = "foo";
};
It's important to note the use of an uppercase T for the constructor function and how the setup method is defined on the prototype, which is more memory efficient compared to defining it as a property of the instance.
Now that TestObject is a valid function, you can create instances using the new operator:
var myFirstObject = new TestObject();
var mySecondObject = new TestObject();
By calling the setup method on an instance of TestObject, the changes will only apply to that specific instance because the value of this inside the method refers to the calling instance:
myFirstObject.setup();
console.log(myFirstObject.value); // 'foo'
console.log(mySecondObject.value); // 'this is my initial value'