Not everything that starts with a backslash is an escape sequence. Many characters are just not useful to escape sequences, and will simply cause a preceding backslash to be ignored.
"\H\e\l\l\o" === "Hello" // true
On the other hand, some characters like "u" and "x" will cause a syntax error when used improperly after a backslash.
The following is not a valid string literal because it contains the Unicode escape sequence prefix
\u followed by a character that is not a valid hexadecimal digit nor a curly brace:
"C:\Windows\System32\updatehandlers.dll" // SyntaxError
A backslash at the end of a line inside a string does not introduce an escape sequence, but indicates line continuation, i.e.
"contin\ uation" === "continuation" // true