Javascript str.replace() to replace final character in a string

Question

I'm trying to replace the apostrophes I have in strings to double apostrophes for a SQL query, I'm using str.replace(), but I'm messing something up, or I don't have the regex correct.

  var tester = "O'neal is walkin'";
  tester = tester.replace("'", "$&'");
  console.log(tester);

  // exp:    O''neal is walkin''
  // actual: O''neal is walkin'

I thought this might have to do with the apostrophe, but it seems to work that way for all end character edits:

  var tester = "O'neal is walkin";
  tester = tester.replace("n", "$&g");
  console.log(tester);

  // exp:    O'ngeal is walking
  // actual: O'ngeal is walkin

So what am I doing wrong here as to not catch that final character correctly?