JavaScript 變數的資料型態

• primitive type ( 原始型態 )
  • null
  • undefined
  • string
  • number
  • boolean
  • symbol (ES6)
• others
  • object(array, function, date…)

• primitive type 與 object 資料型態的差別

  其中最大差別在於 primitive type 為 Immutable

  var str = 'hello'
  var newStr = str.toUpperCase()
  console.log(str, newStr) // hello HELLO
  
  var arr = [1]
  arr.push(2)
  console.log(arr) // [1, 2]

typeof

• 可使用 typeof 了解目前的資料型態

  console.log(typeof 42);
  // expected output: "number"
  
  console.log(typeof 'blubber');
  // expected output: "string"
  
  console.log(typeof true);
  // expected output: "boolean"
  
  console.log(typeof declaredButUndefinedVariable);
  // expected output: "undefined";

• answer
  

  "number"
  "string"
  "boolean"
  "undefined"

Additional information

• null

  In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the “object” typeof return value.

  // This stands since the beginning of JavaScript
  typeof null === 'object';

Description

The following table summarizes the possible return values of typeof

Type	Result
Undefined	“undefined”
Null	“object”
Boolean	“boolean”
Number	“number”
BigInt	“bigint”
String	“string”
Symbol (new in ECMAScript 2015)	“symbol”
Host object (provided by the JS environment)	Implementation-dependent
Function object (implements [[Call]] in ECMA-262 terms)	“function”
Any other object	“object”
—
但因為使用 typeof 在某些情況下可能不是很準確，例如無法檢測出是否為 array，因此可使用另外兩種方式

Array.isArray()

The Array.isArray() method determines whether the passed value is an Array.

Array.isArray([1, 2, 3]);  // true
Array.isArray({foo: 123}); // false
Array.isArray('foobar');   // false
Array.isArray(undefined);  // false

prototype

console.log(Object.prototype.toString.call('123'))
console.log(Object.prototype.toString.call(456))
console.log(Object.prototype.toString.call([]))
console.log(Object.prototype.toString.call(new Date()))
console.log(Object.prototype.toString.call(null))

• answer
  

  [object String]
  [object Number]
  [object Array]
  [object Date]
  [object Null]

Reference

• typeof
• Array.isArray()