js types & primitive & object

js types & primitive & object

js 数据类型

typeof null
// "object"

typeof undefined
// "undefined"

typeof Symbol('symbol desc')
// "symbol"
typeof Symbol
// "function"

typeof `strings`
// "string"

typeof 123
// "number"
typeof NaN;
// "number"

typeof BigInt(1n);
// "bigint"

typeof true
// "boolean"

typeof function() {}
// "function"

typeof (() => {})
// "function"
typeof () => {}
// Uncaught SyntaxError: Malformed arrow function parameter list

typeof {}
// "object"

typeof []
// "object"

typeof NaN
// "object"

NaN

Number.isNaN(NaN)
// true
isNaN(NaN)
// true

Number.isNaN('hello world');
// false
isNaN('hello world');
// true

Number.isNaN(``)
// false
Number.isNaN(0)
// false

isNaN(``)
// false
isNaN(0)
// false

NaN === NaN;        // false
Number.NaN === NaN; // false
isNaN(NaN);         // true
isNaN(Number.NaN);  // true
Number.isNaN(NaN);  // true

function valueIsNaN(v) { return v !== v; }
valueIsNaN(1);          // false
valueIsNaN(NaN);        // true
valueIsNaN(Number.NaN); // true

isNaN('hello world');        // true

Number.isNaN('hello world'); // false

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN

Array

Array.isArray([])
// true
Array.isArray({})
// false

null

const nl = null;
// null
nl === null
// true

typeof null          // "object" (not "null" for legacy reasons)
typeof undefined     // "undefined"
null === undefined   // false

null  == undefined   // true

null === null        // true

null == null         // true

!null                // true
isNaN(1 + null)      // false
isNaN(1 + undefined) // true

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null

Symbol

typeof Symbol('symbol desc')
// "symbol"

typeof Symbol
// "function"

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol

function

const obj = {};
const func = () => {};

obj instanceof Object;
// true
func instanceof Object;
// true

func instanceof Function;
// true
obj instanceof Function;
// false

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof

Object

Object.prototype.toString

Object.prototype.toString.apply(obj)
// "[object Object]"

Object.prototype.toString.apply(func)
// "[object Function]"

Object.prototype.toString.apply

Object.prototype.toString.apply(Symbol(`s desc`))
// "[object Symbol]"

Object.prototype.toString.apply(NaN)
// "[object Number]"

Object.prototype.toString.apply(123)
// "[object Number]"

Object.prototype.toString.apply(BigInt(1n))
// "[object BigInt]"

Object.prototype.toString.apply(null)
// "[object Null]"

Object.prototype.toString.apply(``)
// "[object String]"

Object.prototype.toString.apply(true)
// "[object Boolean]"

Object.prototype.toString.apply(undefined)
// "[object Undefined]"

Object.prototype.toString.call

Object.prototype.toString.call(NaN)
"[object Number]"
Object.prototype.toString.call(undefined)
"[object Undefined]"
Object.prototype.toString.call(true)
"[object Boolean]"
Object.prototype.toString.call({})
"[object Object]"
Object.prototype.toString.call(func)
"[object Function]"
Object.prototype.toString.call(obj)
"[object Object]"
Object.prototype.toString.call(Symbol())
"[object Symbol]"
Object.prototype.toString.call(null)
"[object Null]"
Object.prototype.toString.call(123)
"[object Number]"
Object.prototype.toString.call(BigInt(1n))
"[object BigInt]"

The latest ECMAScript standard defines 8 data types:

7 data types that are primitives:

Boolean

Null

Undefined

Number

BigInt (ES10 / 2019 新增)

String

Symbol (ES6 / ES2015 新增)

1 data type that is reference:

Object

function() {}
() => {}

typeof

'number'

'string'

'boolean'

'undefined'

'bigint'

'symbol'

'object'

'function'

refs

https://flaviocopes.com/difference-primitive-types-objects/

https://flaviocopes.com/javascript-value-type/

TypeScript types

types & primitive & object

http://javascript.xgqfrms.xyz/pdfs/TypeScript Language Specification.pdf

  // const a: Object = [- 7 , 1, 5, 2, -5, 1];
  // const b: Object = [2, 3, 4, 2, 4];
  // const c: Object = [2, 3, 4, 3, 2];
  // const a: Number[] = [- 7 , 1, 5, 2, -5, 1];
  // const b: Number[] = [2, 3, 4, 2, 4];
  // const c: Number[] = [2, 3, 4, 3, 2];
  const a: number[] = [- 7 , 1, 5, 2, -5, 1];
  const b: number[] = [2, 3, 4, 2, 4];
  const c: number[] = [2, 3, 4, 3, 2];

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