Null 合体代入 (??=)

const a = { duration: 50 };

a.duration ??= 10;
console.log(a.duration);
// expected output: 50

a.speed ??= 25;
console.log(a.speed);
// expected output: 25

https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment

Null 合体演算子 (??)

const foo = null ?? 'default string';
console.log(foo);
// expected output: "default string"

const baz = 0 ?? 42;
console.log(baz);
// expected output: 0

https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator

オプショナルチェーン (?.)

const adventurer = {
  name: 'Alice',
  cat: {
    name: 'Dinah'
  }
};

const dogName = adventurer.dog?.name;
console.log(dogName);
// expected output: undefined

console.log(adventurer.someNonExistentMethod?.());
// expected output: undefined

https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Operators/Optional_chaining