JavaScript 类型检查操作符typeof 和 instanceof 的区别对比

typeof 42;           // "number"
typeof "hello";      // "string"
typeof true;         // "boolean"
typeof undefined;    // "undefined"
typeof null;         // "object" (历史遗留问题)
typeof {};           // "object"
typeof [];           // "object"
typeof function(){}; // "function"

[] instanceof Array;      // true
[] instanceof Object;     // true
new Date() instanceof Date; // true
​
42 instanceof Number;     // false
"str" instanceof String;  // false

typeof 42;        // "number"
typeof "hello";   // "string"
typeof true;      // "boolean"
typeof undefined; // "undefined"
typeof null;      // "object" (这是唯一例外)

42 instanceof Number;        // false
"hello" instanceof String;   // false
true instanceof Boolean;     // false

typeof [];      // "object"
typeof {};      // "object"

[] instanceof Array;    // true
{} instanceof Array;    // false

typeof null;  // "object" (这是历史遗留问题)

null instanceof Object;  // false

class Animal {}
class Dog extends Animal {}
let dog = new Dog();
dog instanceof Dog;     // true
dog instanceof Animal;  // true

typeof dog;  // "object" (无法知道是Dog还是Animal)

if (typeof variable === 'undefined') {
  // 变量未定义
}

function add(a, b) {
  if (typeof a !== 'number' || typeof b !== 'number') {
    throw new Error('参数必须是数字');
  }
  return a + b;
}

if (typeof callback === 'function') {
  callback();
}

if (value instanceof Date) {
  console.log(value.getFullYear());
}

class User {}
let user = new User();
if (user instanceof User) {
  // 处理用户对象
}

if (dog instanceof Animal) {
  // 处理动物对象
}

// 假设array来自另一个iframe
let iframeArray = window.frames[0].Array;
let arr = new iframeArray(1, 2, 3);
console.log(arr instanceof Array); // false

console.log(Array.isArray(arr)); // true

function Foo() {}
let foo = new Foo();
console.log(foo instanceof Foo); // true
// 修改原型
Foo.prototype = {};
console.log(foo instanceof Foo); // false

function getType(obj) {
  if (obj === null) return "null";
  if (Array.isArray(obj)) return "array";
  if (obj instanceof Date) return "date";
  return typeof obj;
}
console.log(getType([]));      // "array"
console.log(getType(null));     // "null"
console.log(getType(new Date())); // "date"
console.log(getType(42));       // "number"

function isNumber(value) {
  return typeof value === 'number' || value instanceof Number;
}