javascript中的内置对象和数据结构

var x; //create a variable but assign it no value

console.log("x's value is", x) //logs "x's value is undefined"

var bNoParam = new Boolean();
var bZero = new Boolean(0);
var bNull = new Boolean(null);
var bEmptyString = new Boolean('');
var bfalse = new Boolean(false);

var btrue = new Boolean(true);
var btrueString = new Boolean('true');
var bfalseString = new Boolean('false');
var bSuLin = new Boolean('Su Lin');
var bArrayProto = new Boolean([]);
var bObjProto = new Boolean({});

var x = new Boolean(false);
if (x) {
  // this code is executed
}

var x = false;
if (x) {
  // this code is not executed
}

var x = Boolean(expression);     // use this...
var x = !!(expression);          // ...or this
var x = new Boolean(expression); // don't use this!

function sanitise(x) {
  if (isNaN(x)) {
    return NaN;
  }
  return x;
}

console.log(sanitise('1'));
// expected output: "1"

console.log(sanitise('NotANumber'));
// expected output: NaN

> const x = 2n ** 53n;
9007199254740992n
> const y = x + 1n; 
9007199254740993n

Number('123')  // returns the number 123
Number('123') === 123  // true

Number("unicorn")  // NaN
Number(undefined)  // NaN

const theBiggestInt = 9007199254740991n

const alsoHuge = BigInt(9007199254740991)
// ↪ 9007199254740991n

const hugeString = BigInt("9007199254740991")
// ↪ 9007199254740991n

const hugeHex = BigInt("0x1fffffffffffff")
// ↪ 9007199254740991n

const hugeBin = BigInt("0b11111111111111111111111111111111111111111111111111111")
// ↪ 9007199254740991n

const string1 = "A string primitive";
const string2 = String('A string primitive');

let s_prim = 'foo'
let s_obj = new String(s_prim)

console.log(typeof s_prim) // Logs "string"
console.log(typeof s_obj)  // Logs "object"

let s1 = '2 + 2'              // creates a string primitive
let s2 = new String('2 + 2')  // creates a String object
console.log(eval(s1))         // returns the number 4
console.log(eval(s2))         // returns the string "2 + 2"

// Here are two symbols with the same description:
let Sym1 = Symbol("Sym")
let Sym2 = Symbol("Sym")

console.log(Sym1 === Sym2) // returns "false"

let sym = new Symbol()  // TypeError

let sym = Symbol('foo')
typeof sym      // "symbol" 
let symObj = Object(sym)
typeof symObj   // "object"

typeof null === 'object' // true

let o = {}
let o = {a: 'foo', b: 42, c: {}}

let a = 'foo', b = 42, c = {}
let o = {a: a, b: b, c: c}

let param = 'size'
let config = {
  [param]: 12,
  ['mobile' + param.charAt(0).toUpperCase() + param.slice(1)]: 4
}

console.log(config) // {size: 12, mobileSize: 4}

(function(){}).constructor === Function

var x = 10;

function createFunction1() {
    var x = 20;
    return new Function('return x;'); // this |x| refers global |x|
}

function createFunction2() {
    var x = 20;
    function f() {
        return x; // this |x| refers local |x| above
    }
    return f;
}

var f1 = createFunction1();
console.log(f1());          // 10
var f2 = createFunction2();
console.log(f2());          // 20

let today = new Date()
let birthday = new Date('December 17, 1995 03:24:00')
let birthday = new Date('1995-12-17T03:24:00')
let birthday = new Date(1995, 11, 17)            // the month is 0-indexed
let birthday = new Date(1995, 11, 17, 3, 24, 0)
let birthday = new Date(628021800000)            // passing epoch timestamp

let [month, date, year]    = ( new Date() ).toLocaleDateString().split("/")
let [hour, minute, second] = ( new Date() ).toLocaleTimeString().slice(0,7).split(":")

let fruits = ['Apple', 'Banana'];

console.log(fruits.length); // 2
console.log(fruits[0]);     // "Apple"

let fruits = new Array('Apple', 'Banana');

console.log(fruits.length); // 2
console.log(fruits[0]);     // "Apple"

let fruits = ['Apple', 'Banana']
fruits.forEach(function(item, index, array) {
  console.log(item, index)
})
// Apple 0
// Banana 1

let newLength = fruits.push('Orange')
// ["Apple", "Banana", "Orange"]

let last = fruits.pop() // remove Orange (from the end)
// ["Apple", "Banana"]

let first = fruits.shift() // remove Apple from the front
// ["Banana"]

let newLength = fruits.unshift('Strawberry') // add to the front
// ["Strawberry", "Banana"]

let removedItem = fruits.splice(pos, 1) // this is how to remove an item

// ["Strawberry", "Mango"]

let vegetables = ['Cabbage', 'Turnip', 'Radish', 'Carrot']
console.log(vegetables)
// ["Cabbage", "Turnip", "Radish", "Carrot"]

let pos = 1
let n = 2

let removedItems = vegetables.splice(pos, n)
// this is how to remove items, n defines the number of items to be removed,
// starting at the index position specified by pos and progressing toward the end of array.

console.log(vegetables)
// ["Cabbage", "Carrot"] (the original array is changed)

console.log(removedItems)
// ["Turnip", "Radish"]

let shallowCopy = fruits.slice() // this is how to make a copy
// ["Strawberry", "Mango"]

let contacts = new Map()
contacts.set('Jessie', {phone: "213-555-1234", address: "123 N 1st Ave"})
contacts.has('Jessie') // true
contacts.get('Hilary') // undefined
contacts.set('Hilary', {phone: "617-555-4321", address: "321 S 2nd St"})
contacts.get('Jessie') // {phone: "213-555-1234", address: "123 N 1st Ave"}
contacts.delete('Raymond') // false
contacts.delete('Jessie') // true
console.log(contacts.size) // 1

let myMap = new Map()
myMap.set(0, 'zero')
myMap.set(1, 'one')

for (let [key, value] of myMap) {
  console.log(key + ' = ' + value)
}
// 0 = zero
// 1 = one

for (let key of myMap.keys()) {
  console.log(key)
}
// 0
// 1

for (let value of myMap.values()) {
  console.log(value)
}
// zero
// one

for (let [key, value] of myMap.entries()) {
  console.log(key + ' = ' + value)
}
// 0 = zero
// 1 = one

myMap.forEach(function(value, key) {
  console.log(key + ' = ' + value)
})
// 0 = zero
// 1 = one

let mySet = new Set()

mySet.add(1)           // Set [ 1 ]
mySet.add(5)           // Set [ 1, 5 ]

mySet.has(1)              // true
mySet.delete(1)    // removes 1 from the set

// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2} 
for (let item of mySet) console.log(item)