let a;
    console.log(a?.name); //undefined
    console.log(a.name); //報(bào)錯

    let obj = {};
    console.log(obj.name); //undefined
    console.log(obj?.name?.a); //undefined
    console.log(obj.name.a); //報(bào)錯

// ??
    console.log(undefined ?? 2);  // 2
    console.log(null ?? 2);   // 2
    console.log(0 ?? 2);  // 0
    console.log("" ?? 2);   // ''
    console.log(true ?? 2);  // true
    console.log(false ?? 2);   // false
 
// ||
    console.log(undefined || 2);   // 2
    console.log(null || 2);     // 2
    console.log(0 || 2);        // 2
    console.log("" || 2);     // 2
    console.log(true || 2);     // true
    console.log(false || 2);      // 2

    const obj = { a: { b: [{ name: "obj" }] } };
 
    // 原本的寫法
    console.log(obj && obj.a && obj.a.b.length && obj.a.b[0].name); //obj
 
    // 可選鏈寫法
    console.log(obj?.a?.b?.[0]?.name); // obj
    console.log(obj?.b?.c?.d); // undefined

    const obj = { a: { name: "obj" } };
 
    console.log(obj?.a?.b ?? "hello world"); // hello world

    let a = "你好";
 
    let b = null;
    let c = undefined;
    let d = 0;
    let e = "";
    let f = true;
    let g = false;
 
    console.log((b ??= a)); // 你好
    console.log((c ??= a)); // 你好
    console.log((d ??= a)); // 0
    console.log((e ??= a)); // ''
    console.log((f ??= a)); // true
    console.log((g ??= a)); // false
 
    console.log(b); // 你好
    console.log(c); // 你好
    console.log(d); // 0
    console.log(e); // ''
    console.log(f); // true
    console.log(g); // false

let travelPlans = {
  destination: 'DC',
  monday: {
    location: 'National Mall',
    budget: 200,
    host: null
  }
}
let res = travelPlans?.tuesday?.location ?? "locahost"; // => locahost
let res2 = travelPlans?.host; // => undefined

let res = travelPlans?.tuesday?.location; 
<=等價=> 
let res = travelPlans&& travelPlans.tuesday&& travelPlans.tuesday.location