normalize-it.js

// simple German normalize

function normalizeIt(str) {
  const normalized = str.normalize('NFC')
    .toLocaleUpperCase()
    .trim()
    .replace(/Ä/g, 'AE' )
    .replace(/Ö/g, 'OE' )
    .replace(/Ü/g, 'UE' )
    .replace(/ß/g, 'SS' )
    .replace(/ /g, '_' )
    .replace(/-/g, '_')
    .replace(/__/g, '_' );

  return str === normalized ? normalized : normalizeIt(normalized);
}

console.log(normalizeIt('test'));
console.log(normalizeIt('test '));
console.log(normalizeIt(' test '));
console.log(normalizeIt(' test'));
console.log(normalizeIt('test foo'));
console.log(normalizeIt('test-foo'));
console.log(normalizeIt('test -foo'));
console.log(normalizeIt('test - foo'));
console.log(normalizeIt('ä'));
console.log(normalizeIt('Ä'));
console.log(normalizeIt('ö'));
console.log(normalizeIt('Ö'));
console.log(normalizeIt('ü'));
console.log(normalizeIt('Ü'));
console.log(normalizeIt('ß'));
console.log(normalizeIt('a sÜß '));