Understanding Istanbul's code coverage

Question

I just started with an example file (example.js) which has the following statements:

x = 42;
if(false)
  x = -1;

I don't have any unit test file. but when I run

istanbul cover example.js 

I see some the below coverage data

=============================== Coverage summary ===============================
Statements   : 66.67% ( 2/3 )
Branches     : 50% ( 1/2 )
Functions    : 100% ( 0/0 )
Lines        : 66.67% ( 2/3 )
==============================================================================

Does that imply I have 2 statements out of 3 that are covered? If so, how can it report that this code is covered when I don't have any unit test file targeting that code?

From what I understand by definition of code coverage, it code that is covered by unit tests. I don't understand why istabul says that I have 2 statements covered when I have not written any unit tests.

Answer 1

Istanbul checks to see what lines of code were run. Simply running the file will execute two of the lines, and ignoring the third. If you were to create something like this:

module.exports = {
  myFunc: function() {
    x = 42;
    if(false) {
      x = -1;
    }
  }
};

You'd get a different result (x = 42 wouldn't be run at all). The one statement executed is module.exports

=============================== Coverage summary ===============================
Statements   : 25% ( 1/4 )
Branches     : 0% ( 0/2 )
Functions    : 0% ( 0/1 )
Lines        : 25% ( 1/4 )
================================================================================