4. data types

ES6里有6种简单数据类型：undefined, null, Number, Boolean, String, Symbol
1种复杂数据类型：Object

typeof 操作符

undefined

未初始化的变量为undefined，永远不要显示的指定一个变量值为undefined
undefined值用于比较

let xxx
console.log(xxx == undefined)  // true

typeof 一个未声明过的变量返回的是undefined

let xxx
typeof xxx
"undefined"
typeof ssssss
"undefined"

null

Logically, a null value is an empty object pointer
如果要声明一个对象，但不打算赋值，就应该指定为null

let car = null;
console.log(typeof car); // "object"

console.log(null == undefined); // true
console.log(null === undefined); // false

Boolean

Boolean()函数可以将任何值转为true或false
5个falsy值:
0，NaN, “”, null, undefined

Number

decimal number 十进制

octal (base 8) 八进制以0开头

let octalNum1 = 070; // octal for 56
let octalNum2 = 079; // invalid octal - interpreted as 79
let octalNum3 = 08; // invalid octal - interpreted as 8

hexadecimal (base 16) 十六进制以0x开头

let hexNum1 = 0xA; // hexadecimal for 10
let hexNum2 = 0x1f; // hexadecimal for 31

positive zero (+0) and negative zero (–0)

浮点数比整数占用双倍内存，JS总是想办法把数值转为整数

浮点数计算不精确

if (0.1 + 0.2 == 0.3) { // avoid!
console.log("You got 0.3.");
}

数值范围

Number.MIN_VALUE
5e-324
Number.MAX_VALUE
1.7976931348623157e+308
–Infinity // (negative infinity)
Infinity  // (positive infinity)
isFinite(-Infinity)  // 判断是否介于[MIN_VALUE, MAX_VALUE]
// false

NaN

// 任何不能转换为number的值，isNaN都返回true
console.log(isNaN(NaN)); // true
console.log(isNaN(10)); // false - 10 is a number
console.log(isNaN("10")); // false - can be converted to number 10
console.log(isNaN("blue")); // true - cannot be converted to a number
console.log(isNaN(true)); // false - can be converted to number 1

转换为number

Number("hello") // NaN
Number("123") // 123
Number(true)  // 1
Number(null) // 0
Number(undefined) // NaN
// 将字符串转换为数值
parseInt("0xf") // 15
parseInt("AF", 16); // 175  指定基数radix
parseInt("AF"); // NaN  默认10进制
// parseFloat只能转换10进制，不能指定基数
parseFloat("123") // 123
parseFloat("0xAF") // 0
parseFloat("1.22.33") // 1.22

String

toString()
可以将除null、undefined之外的任何值转为字符串
将数字转为字符串时可指定基数

0xAF.toString()
"175"
0xAF.toString(16)
"af"

String()可以将任何值转为字符串，有toString方法的调用toString方法

String(0xAF)
String(null)
String(undefined)

+””可以转换字符串

1 + ""
null + ""

文字模板template literals
用反引号backticks

let myMultiLineTemplateLiteral = `first line
second line`;

Interpolation插值替换

在``中用${}

let name = "jack"
let age = 19
console.log(`Hi,${name}, you're ${age} years old`)

tag function

第一个参数strings是去除了${}后分隔的片段组成的数组
剩下的参数是${}里的值
用途：可以把${ a } + ${ b } = ${ a + b }里的${}值作为参数分解出来

let a = 6;
let b = 9;
function simpleTag(strings, aValExpression, bValExpression, sumExpression) {
  console.log(strings);   // 第一个参数传入的肯定是数组
  console.log(aValExpression);
  console.log(bValExpression);
  console.log(sumExpression);
  return 'foobar';
}
// 调用标签函数
simpleTag`${ a } + ${ b } = ${ a + b }`    
// 结果
["", " + ", " = ", "", raw: Array(4)]
6
9
15
"foobar"

用spread operator改写

let a = 6;
let b = 9;
function simpleTag(strings, ...expressions) {
  console.log(strings);   // 第一个参数传入的肯定是数组
  for(const expression of expressions){
    console.log(expression)
  }
  return 'foobar';
}
// 调用标签函数
simpleTag`${ a } + ${ b } = ${ a + b }`

String.raw

console.log(`\u00A9`); // ©
console.log(String.raw`\u00A9`); // \u00A9

Symbol

symbol是唯一且不可变的，用于对象属性
symbols are intended to be used as unique tokens

let genericSymbol = Symbol();
console.log(genericSymbol); // Symbol()
let fooSymbol = Symbol('foo');
console.log(fooSymbol); // Symbol(foo);

Symbol()不能用new操作符

let myBoolean = new Boolean();
console.log(typeof myBoolean); // "object"
let myString = new String();
console.log(typeof myString); // "object"
let myNumber = new Number();
console.log(typeof myNumber); // "object"
let mySymbol = new Symbol(); // TypeError: Symbol is not a constructor

Object

let o = new Object();
o.__proto__  === Object.prototype
// 构造函数都有prototype

o.proto里的属性

• constructor —The function that was used to create the object. In the previous example, the

constructor is the Object() function.

• hasOwnProperty(propertyName) —Indicates if the given property exists on the object

instance (not on the prototype). The property name must be specified as a string (for
example, o.hasOwnProperty(“name”) ).