Sneaky bugs: startsWith and indexOf with empty string

String.startsWith() returns a boolean after testing whether the string begins with the search string passed to it, and has a second argument to specify the starting location for the search.

All straightforward until you try to pass an empty search string, in which case it always returns true.

  const str = "Artyom";
  str.startsWith("Art"); // expected: true, actual: true
  str.startsWith("yom"); // expected: false, actual: false
  str.startsWith(""); // expected: false?, actual: true
  str.startsWith("", Infinity); // expected: false?, actual: true

I didn’t expect that to happen, and checking on MDN didn’t directly help since the page for startsWith doesn’t mention this behaviour.

However, the MDN page links to the specification which describes the steps performed while this method is called. It has a step that returns true if the search string’s length is 0.

  [...]
  9. Let searchLength be the length of searchStr.
  10. If searchLength = 0, return true.
  [...]

String.indexOf() has a lot of gotchas when it comes to empty strings but I expected this newer method to be better in this regard. At least the behaviour of indexOf is well defined in the MDN page.

Since I expected the call with empty search string to return false, I introduced a bug in the code which sneaked past me even after reading through the code as I wasn’t aware of this behaviour.