Question

When the myClass function returns a single string “hii”, testClass.getDetails() works fine:

function myClass(name, age) {
    this.name = name;
    this.age = age;

    return "hii";
}
myClass.prototype.getDetails = function() {
    return "mydetails";
}

var testClass = new myClass('aneesh', 27);
alert(testClass.getDetails());

But when I return an object in myClass:

function myClass(name, age) {
    this.name = name;
    this.age = age;
    return {};
}

I get an error:

testClass.getDetails is not a function

Why does that happen? In Javascript a string is also an object, right?

Answer 1

No, a string literal like the one you are returning ("hii") is a primitive value is not an object.

In JavaScript we have the following primitives: string, number, boolean, undefined and null.

If a constructor used with the new operator returns a primitive, the this value will be returned.

If an object is returned, like in your second example (which IMO is not really useful), the newly created object (this within the constructor) will be lost, and you get an error because it doesn’t contain a property named getDetails.

For example:

function Test () {
  // don't return anything (equivalent to returning undefined)
}

new Test() instanceof Test; // true

function Test2 () {
  return {};
}

new Test2 instanceof Test2; // false