What is "stranger"? Whom can I call "stranger"?
Native speaker’s answer
Rebecca
A "stranger" is someone that is unknown or familiar. Here, she is referring to the fact that she is not unfamiliar with the dark. You can use this expression in the following ways: to not be a stranger to [x], to be no stranger to [x]. Ex. I am not a stranger to loneliness, I grew up far away from my family and friends. Ex. She is no stranger to adversity as she has overcome many challenges despite being so young.