Why do strings given to the RegExp constructor need to be double escaped?

You are constructing the regular expression by passing a string to the RegExp constructor.

\ is an escape character in string literals.

The \ is consumed by the string literal parsing…

const foo = "foo";
const string = '(\s|^)' + foo;
console.log(string);

… so the data you pass to the RegEx compiler is a plain s and not \s.

You need to escape the \ to express the \ as data instead of being an escape character itself.