Question

Example 1:

function a(){
  return "ok";
}

function b(){
  this.c = a;
}

var d = new b();
alert(d.c()); // Result "ok".

Example 2:

function b(){
    this.c = a();
}

var d = new b();
alert(d.c); // Result "ok".

What do the parentheses mean in this case?

Answer 1

Putting () after a function calls it – causing the function to be executed. In your examples, the function is a.

Your two cases can be thought of as:

Example 1

d.c = a;
d.c(); // Call function a

Example 2

d.c = a(); // Call function a, put result in d.c
d.c;

In either case the function a is being called with () exactly once – hence they produce the same output.