What does 'is an instance of' mean in Javascript?

Question

The answer to this question: What is the initial value of a JavaScript function's prototype property?

has this sentence:

The initial value

Answer 1

JavaScript does not have classes, but it does have Types. You can define your own type and create a new instance using the new keyword:

function Foo(initialValue) {
    this.value = initialValue || 0;
}
var foo = new Foo(17);
var fooIsAFoo = foo instanceof Foo;  // true!
var fooIsAnObject = foo instanceof Object; // also true!  We have inheritance here :)