debugger is a statement which is meant for stopping the javascript execution environment and start the debugger at the statement. Modern debuggers and tooling no longer need this statement and leaving it in can cause the execution of your code to stop in production.

Invalid:

function isLongString(x: string) {
  debugger;
  return x.length > 100;
}

Valid:

function isLongString(x: string) {
  return x.length > 100; // set breakpoint here instead
}