Skip to main content

Primitives

TC39​

TC39

JavaScript = ECMAScript + DOM + BOM:

  • ECMAScript: ECMA-262.
  • DOM: DOM Core + DOM HTML (document).
  • BOM: Browser Object Model API (HTML5) (window/navigator/location/screen/performance etc).

Primitive Values​

Primitive data types:

  • Undefined.
  • Null.
  • Boolean.
  • Number.
  • String.
  • Symbol.
  • BigInt.

Undefined​

  • 对象属性未定义时, 该属性值为 undefined.
  • 未初始化变量的初值为 undefined (表示等待被赋值).

Null​

当引用为空或引用对象不存在时, 值为 null. null 值表示一个空对象指针.

Null

typeof null -> object.

Boolean​

Zero Value Expression​

零值表达式:

  • undefined.
  • null.
  • false.
  • NaN.
  • 0
  • 0n.
  • ''.

Boolean Conversion​

xBoolean(x)
undefinedfalse
nullfalse
booleanx
number0 → false, NaN → false
Other numbers → true
bigint0n → false
Other numbers → true
string'' → false
Other strings → true
symboltrue
objecttrue

Wrapper Objects​

Using the wrapper function without the new keyword is a useful way of coercing a value into a primitive type.

// Not recommended (primitive object wrapper):
const objectType = typeof new String(37) // object

// Safe (type coercion with wrapper function):
const stringType = typeof String(37) // string

// Object-wrapped string:
const falsy = new String(37) === '37' // false

// Type-coerced string:
const truthy = String(37) === '37' // true

// BAD!
const falseObject = new Boolean(false)
const result = falseObject && true
console.log(result) // true
console.log(typeof falseObject) // object
console.log(falseObject instanceof Boolean) // true

const prim = true
assert.equal(typeof prim, 'boolean')
assert.equal(prim instanceof Boolean, false)

const wrapped = Object(prim)
assert.equal(typeof wrapped, 'object')
assert.equal(wrapped instanceof Boolean, true)
assert.equal(wrapped.valueOf(), prim) // unwrap

Box and Unbox for primitive values:

  • 自动创建的原始值包装对象可以让原始值拥有对象的行为.
  • 自动创建的原始值包装对象只存在于访问它的那行代码执行期间.
  • 常数值加括号可转化为对象.
  • 可以对 primitive values 进行 ES6 解构语法.
const s1 = 'some text'
const s2 = s1.substring(2) // Call method on primitive string.
// let _s1 = new String(s1);
// const s2 = _s1.substring(2);
// _s1 = null;

const s3 = 'some text'
s3.color = 'red'
console.log(s3.color) // undefined
// primitive string
const greet = 'Hello there'
// primitive is converted to an object
// in order to use the split() method
const hello = greet.split(' ')[0] // "Hello"
// attempting to augment a primitive is not an error
greet.smile = true
// but it doesn't actually work
const undef = typeof greet.smile // "undefined"

不使用 new 关键字,包装类构造函数返回值为基本类型

const numberType = typeof Number(1) // "number"
const numberType = typeof Number('1') // "number"
const numberType = typeof Number(new Number()) // "number"
const stringType = typeof String(1) // "string"
const booleanType = typeof Boolean(1) // "boolean"