技術標籤：js基礎前端

1. 資料型別
   基本資料型別：string、number、boolean、null、undefined、symbol
   引用資料型別：object、array、function
2. typeof
   typeof操作符返回一個字串，表示未經計算的運算元的型別

console.log(typeof 42);
// expected output: "number"

console.log(typeof 'blubber');
// expected output: "string"

console.log(typeof true);
// expected output: "boolean"
 


console.log(typeof undeclaredVariable);
// expected output: "undefined"

console.log(typeof null);
// expected output: "object"

var a = Symbol(3)
console.log(typeof a);
// expected output: "symbol"

var arr = [1,2,3]
console.log(typeof arr);
// expected output: "object"
 


var obj = {a:1}
console.log(typeof obj);
// expected output: "object"

var func = function () {
  console.log(1)
}
console.log(typeof func);
// expected output: "function"

typeof 可以正確檢測出number、string、boolean、undefined、symbol、function，而null、array、object檢測結果均為object，無法準確判斷
3. instanceof

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const auto = new Car('Honda', 'Accord', 1998);

console.log(auto instanceof Car);
// expected output: true

console.log(auto instanceof Object);
// expected output: true

var num = 2
console.log(num instanceof Number);
// expected output: false

var bool = false
console.log(bool instanceof Boolean);
// expected output: false

var str = 'string1'
console.log(str instanceof String);
// expected output: false

var temp = Symbol(2)
console.log(temp instanceof Symbol);
// expected output: false

console.log(null instanceof Object);
// expected output: false
console.log(undefined instanceof Object);
// expected output: false

var arr = [1,2,3]
console.log(arr instanceof Array);
// expected output: true

var func = function() {console.log('123')}
console.log(func instanceof Function);
// expected output: true

var obj = {a:1}
console.log(obj instanceof Object);
// expected output: true

instanceof不能區別undefined和null，基本型別不用new生命的也檢測不出來
4. constructor

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const auto = new Car('Honda', 'Accord', 1998);

console.log(auto.constructor ===  Car);
// expected output: true

var num = 2
console.log(num.constructor === Number);
// expected output: true

var bool = false
console.log(bool.constructor === Boolean);
// expected output: true

var str = 'string1'
console.log(str.constructor === String);
// expected output: true

var temp = Symbol(2)
console.log(temp.constructor === Symbol);
// expected output: true

//console.log(null.constructor === Object);
// Error: Cannot read property 'constructor' of null

//console.log(undefined.constructor === Object);
// Error: Cannot read property 'constructor' of undefined

var arr = [1,2,3]
console.log(arr.constructor === Array);
// expected output: true

var func = function() {console.log('123')}
console.log(func.constructor === Function);
// expected output: true

var obj = {a:1}
console.log(obj.constructor === Object);
// expected output: true

constructor無法判斷null和undefined
5. toString

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const auto = new Car('Honda', 'Accord', 1998);

console.log(Object.prototype.toString.call(auto));
// expected output: [object Object]

var num = 2
console.log(Object.prototype.toString.call(num));
// expected output: [object Number]

var bool = false
console.log(Object.prototype.toString.call(bool));
// expected output: [object Boolean]

var str = 'string1'
console.log(Object.prototype.toString.call(str));
// expected output: [object String]

var temp = Symbol(2)
console.log(Object.prototype.toString.call(temp));
// expected output: [object Symbol]

console.log(Object.prototype.toString.call(null));
// expected output: [object Null]
console.log(Object.prototype.toString.call(undefined));
// expected output: [object Undefined]

var arr = [1,2,3]
console.log(Object.prototype.toString.call(arr));
// expected output: [object Array]

var func = function() {console.log('123')}
console.log(Object.prototype.toString.call(func));
// expected output: [object Function]

var obj = {a:1}
console.log(Object.prototype.toString.call(obj));
// expected output: [object Object]

在任何值上呼叫object原生的toString()方法，都會返回一個[Object NativeConstructorName]格式的字串
6. 常用判斷