js中判断数据类型

console.log(typeof 42); // "number"
console.log(typeof 'Hello'); // "string"
console.log(typeof true); // "boolean"
console.log(typeof undefined); // "undefined"
console.log(typeof null); // "object"（这是一个历史遗留问题）
console.log(typeof {}); // "object"
console.log(typeof []); // "object"
console.log(typeof function() {}); // "function"

console.log([] instanceof Array); // true
console.log({} instanceof Object); // true
console.log('Hello' instanceof String); // false（基本类型不是实例）
console.log(new String('Hello') instanceof String); // true（包装对象是实例）

function getType(value) {
  return Object.prototype.toString.call(value).slice(8, -1);
}

console.log(getType(42)); // "Number"
console.log(getType('Hello')); // "String"
console.log(getType(true)); // "Boolean"
console.log(getType(undefined)); // "Undefined"
console.log(getType(null)); // "Null"
console.log(getType({})); // "Object"
console.log(getType([])); // "Array"
console.log(getType(function() {})); // "Function"