Does the Javascript Engine delete variables from scope if they are not used?

Question

I have a function called setupChannel that takes "socket" as a parameter. I am using redux-thunk so setupChannel returns an anonymous function with dispatch and getState as parameters. Inside that anonymous function, the debugger says socket is undefined unless I make specific reference of socket in the function.

Here socket is undefined when I use the debugger:

export const setupChannel = socket => {
  return (dispatch, getState) => {
    debugger;
  }
}

Here socket is defined when I console.log it

export const setupChannel = socket => {
  return (dispatch, getState) => {
    console.log(socket);
  }
}

Socket is also defined when I simply call a method on it:

export const setupChannel = socket => {
  return (dispatch, getState) => {
    socket.connect();
  }
}

So my question remains, why is it undefined when I pop the debugger in or possibly when I don't reference it?