String.toUpperCase() does not uppercase accents

If i upperCase string with accents, it does not work on accents!

let txt= "béèàîâ";
txt = txt.toUpperCase();
console.log(txt); // Béèàîâ
let txtUp = "bÉÈÀÎÂ";
txtUp = txtUp.toUpperCase();
console.log(txtUp); // BÉÈÀÎÂ
for( let c of txt) {
  console.log(c, c.charCodeAt(0));
}
for( let c of txtUp) {
  console.log(c, c.charCodeAt(0));
}

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