Escaping backslashes JS

Trying to understand why the first eval expression doesn’t work, but the second does.

It is for a macro parser my values are “AM” and “ABC”.

eval('"AM\"!="ABC"');
> Error: Unexpected identifier
eval('"AM\\"!="ABC"');
> true

Can someone explain why you need four backslashes

console.log(eval('"AM\\"')); 
> "AM"