JavaScript 中逻辑与 (&&) 和或 (||) 的灵活运用

if (23) { console.log(“truthy”); } // prints “truthy”
else { console.log(“falsy”); }

if (null) { console.log(“truthy”); } 
else { console.log(“falsy”); } // prints “falsy”

console.log(false && false); // false
console.log(true && false); // false
console.log(true && true); // true

console.log(0 && 1); // 0
console.log("a" && ""); // "" (an empty string)
console.log([] && [1, 2, 3]); // [1, 2, 3]

console.log(false || false); // false
console.log(true || false); // true
console.log(true || true); // true

console.log(1 || 0); // 1
console.log("" || "a"); // "a"
console.log(undefined || null); // null
console.log([] || [1, 2, 3]); // []

function person(name) {
    // if name is undefined, this.name will 
    // default to 'a person with no name'
    this.name = name || 'a person with no name';
    this.greet = function() {
        console.log('hello, i am ' + this.name + '.');
    };
}

// create person variables
var tyler = new person('tyler');
var mystery = new person(); 
// without an input, this.name defaults to the 
// second option since name is undefined.

// call greet() from each person object
tyler.greet(); // "hello, i am tyler."
mystery.greet(); // "hello, i am a person with no name."

function Person(firstName, lastName, age) {
  if (firstName && lastName && age) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.fullName = `${this.firstName} ${this.lastName}`;
    this.age = age;
    this.greet = function() {
      console.log(`Hello, my name is ${this.fullName} and I'm ${this.age} years old.`);
    };
  } 

  // If any argument is missing, the object will only have this property.
  else {
    this.greet = function() {
      console.log(`Hello, I am not a fully formed Person.`)
    };
  }
}

var tyler = new Person('Tyler', 'Meyer', 32);
var brad = new Person('Brad', '', 38);

tyler.greet(); // "Hello, my name is Tyler Meyer and I'm 32 years old."
brad.greet(); // "Hello, I am not a fully formed Person."