10. Basic reference types

objects are considered to be instances of a particular reference type.
New objects are created by using the new operator followed by a constructor. A constructor is simply a function whose purpose is to create a new object.

Date()

let someDate = new Date(Date.parse("May 23, 2019"));
let someDate = new Date("May 23, 2019");   // 等价
// January 1, 2000 at midnight GMT
let y2k = new Date(Date.UTC(2000, 0));
let y2k = new Date(2000, 0);
// May 5, 2005 at 5:55:55 PM GMT
let allFives = new Date(Date.UTC(2005, 4, 5, 17, 55, 55));
let allFives = new Date(2005, 4, 5, 17, 55, 55);

Date.now()

// get start time
let start = Date.now();  // 得到的是毫秒
// call a function
doSomething();
// get stop time
let stop = Date.now(),
result = stop – start;

Regular Expression 正则表达式

let expression = /pattern/flags;

flags

• g: global
• i: case-insensitive
• m: multi-line
• y: sticky
• u: unicode

  // Match all instances of "at" in a string.
  let pattern1 = /at/g;
  // Match the first instance of "bat" or "cat", regardless of case.
  let pattern2 = /[bc]at/i;
  // Match all three-character combinations ending with "at", regardless of case.
  let pattern3 = /.at/gi;

  下列metacharacters需要转义
  ( [ { \ ^ $ | ) ] } ? * + .

  // Match the first instance of "[bc]at", regardless of case.
  let pattern2 = /\[bc\]at/i;
  // Match all instances of ".at", regardless of case.
  let pattern4 = /\.at/gi;

  RegExp()构造函数

  let pattern2 = new RegExp("[bc]at", "i");

  PRIMITIVE WRAPPER TYPES

  String(), Boolean(), Number()三种
  当基本类型调用对象方法时，经历了三步

1. 创建对象
2. 调用方法
3. 销毁对象

用new操作符创建的是对象，不用new则是类型转换

let a = new String("hello")
let b = new Boolean(true)
let c = new Number(1)

new Boolean(flase)转换为布尔值为true

Number

toFixed()指定小数点位数，四舍五入

let num = 10;
console.log(num.toFixed(2)); // "10.00"
let num = 10.005;
console.log(num.toFixed(2)); // "10.01"

toExponential()转换成e表达式

let num = 10;
console.log(num.toExponential(1)); // "1.0e+1"

toPrecision是toFixed和toExponential的综合

let num = 99;
console.log(num.toPrecision(1)); // "1e+2"
console.log(num.toPrecision(2)); // "99"
console.log(num.toPrecision(3)); // "99.0"

isInteger()

console.log(Number.isInteger(1)); // true
console.log(Number.isInteger(1.00)); // true
console.log(Number.isInteger(1.01)); // false