Detect Russian / cyrillic in Javascript string?

Use pattern /[\u0400-\u04FF]/ to cover more cyrillic characters:

// http://jrgraphix.net/r/Unicode/0400-04FF
const cyrillicPattern = /^[\u0400-\u04FF]+$/;

console.log('Привіт:', cyrillicPattern.test('Привіт'));
console.log('Hello:', cyrillicPattern.test('Hello'));

UPDATE:

In some new browsers, you can use Unicode property escapes.

The Cyrillic script uses the same range as described above: U+0400..U+04FF

const cyrillicPattern = /^\p{Script=Cyrillic}+$/u;

console.log('Привіт:', cyrillicPattern.test('Привіт'));
console.log('Hello:', cyrillicPattern.test('Hello'));

Perhaps you meant to use the RegExp test method instead?

/[а-яА-ЯЁё]/.test(term)

Note that JavaScript regexes are not really Unicode-aware, which means the i flag will have no effect on anything that's not ASCII. Hence the need for spelling out lower- and upper-case ranges separately.