tags: mynote

• JS Conception
  • Basic
    • == and ===
  • Global Ececution Context
  • Function Execution Context
  • Hoisting
  • Variables
  • Scope
  • Conculsion
  • Closure(Scope chain)
  • CallStack
  • Functions
    • Constructor Function 物件原型的詳細介紹(有點多)
    • Prototype
    • Functions methods
    • Prototype Inheritance
    • Object.creat()
    • .proto .prototype差別
• JS
  • Arrow function
  • for loop
    • map
    • forEach
  • callback promise async
  • Destructing
  • AJAX and API
    • fetch API
    • CORS issue(同網域下才requast API)
• FAQ
  • import时如何正确使用花括号'{ }'

JS Conception

---

Basic

== and ===

==
左邊是int type右邊是string type，但是因為兩個等於會把 string 轉型為int，所以實際比對是
1 = 1
所以是true。

===
如果是三個等於（===），那麼就不會有轉換型別的問題，因此
1 === "1" //false
就是false。

---

Global Ececution Context

執行JS 後台會執行的事情

Function Execution Context

JS 後台會執行的事情

Hoisting

What is Hoisting??
function 或 var 在creation phase會先分配給memory
所以先執行再宣告也不會報錯

satHi() //Hi
function sayHi(){
    console.log("Hi")
}

function expression(arrow function), const, let都不支援Hoisting,
所以不能先執行再宣告

• initializer(const)
  是否需要先賦予變數值

Variables

const x // syntax error
let y // undefined
var z //undefined

• re-declaration(var)

const x = 1
const x = 2 // syntax error
let y = 1
let y = 2 // syntax error
var z = 1 
var z = 2 // ok!

• re-assignment(let, var)

const x = 1
x = 2 // syntax error
let y = 1
y = 2 // ok!
var z = 1 
z = 2 // ok!

Scope

• Global Scope(let, var, const)
• Function Scope(let, var, const)
• Block Scope(let, const)
  it can only be seen inside a loop or if statement

if(true){
    var x=1;
}
conole.log(x) //1

Conculsion

Closure(Scope chain)

function 的變數取決於function被定義的scope

CallStack

是一種資料結構, 遵循last in first out

Functions

All functions are objects

Constructor Function 物件原型的詳細介紹(有點多)

製作大量相似的obejects

function Person(name, age){
    this.name = name;
    this.age = age;
    this.hi = function(){
        console.log("Hi")
    }
}

let Mike = new Person("Mike", 33)
let Lex = new Person("Lex", 38)

Mike.hi() // Hi

如果有共有的function或是property, 可以使用prototype來指向同一個memory位置, 避免memory浪費

function Person(name, age){
    this.name = name;
    this.age = age;

}

Person.prototype.hi = function(){
        console.log("Hi")
    }

let Mike = new Person("Mike", 33)
let Lex = new Person("Lex", 38)

Mike.hi() // Hi

Prototype

Prototype is a simple reference to another object; this object contains common properties and methods across all instances.

string的兩種創建方式

myName = "Dennis"
myName = new String("Dennis")

用constructor創造的string就會繼承prototype
但是沒必要這樣做, myName = "Dennis"即可
要使用的時候內建function時候
會自動轉換myName = new String("Dennis")

Functions methods

funtion 內部的this是對應到window而不是function本身

• bind
  可以將function本身丟回去, 但是要賦予新的變數
  

• call
  可以將function本身丟回去, 不用賦予新的變數
  
  也可以傳參數
  

• apply
  和call一樣的用法, 只是參數要用list
  

Prototype Inheritance

Object.creat()

用來繼承prototype

.proto .prototype差別

• Function和Object都是构造函数
• Object.prototype是所有对象的顶层。
• Function的构造器指向自己

function Foo(name) {
  this.name = name;
}
var b = new Foo('b');
var a = new Foo('a');
b.say = function() {
  console.log('Hi from ' + this.whoAmI());
}
console.log(a.__proto__ === Foo.prototype); // true
console.log(a.__proto__ === b.__proto__); // true

      //Function的原型对象
      console.log(1,Function.prototype);
      //Object的原型对象
      console.log(2,Object.prototype);
      //Function的原型对象的原型对象
      console.log(3,Function.prototype.__proto__);
      console.log(4,Object.prototype === Function.prototype.__proto__);

JS

---

Arrow function

let sayHi = ()=>{}

feature

• no hoisting. 一定要先宣告再使用
• no this key word. (this key word refers to window object)

for loop

map

创建一个新的数组，其中每一个元素由调用数组中的每一个元素执行提供的函数得来。

let arr =[1,2,3,4,5,6]
let list = arr.map(value => {
 return value * value;
});

//list = [1,4,9,16,25,36]

forEach

针对每一个元素执行提供的函数。

let arr =[1,2,3,4,5,6]
arr.forEach((value, key) => {
 return arr[key] = value * value;
});

//[1,4,9,16,25,36]

callback promise async

一般同步的狀況
一般同步的狀況2的等待時間比較短會先打印

window.setTimeout(() => {console.log(1)},2000)
window.setTimeout(() => {console.log(2)},1000)
//2
//1

callback的狀況

function delay(message, time) {
  return new Promise(function (resolve, reject) {
    window.setTimeout(() => {resolve(`${message}`)}, time)});
}
p = delay;
p(1,2000).then((a) => {
  console.log(a);
  return delay(2, 1000);
}).then(b=>console.log(b))
//1
//2

Promise的狀況

function delay(message, time) {
  return new Promise(function (resolve, reject) {
    window.setTimeout(() => {resolve(`${message}`)}, time)});
}
p = delay;
p(1,2000).then((a) => {
  console.log(a);
  return delay(2, 1000);
}).then(b=>console.log(b))
//1
//2

Promise用then接正確的結果, 用catch接錯誤的結果

function delay(message, time) {
  return new Promise(function (resolve, reject) {
    window.setTimeout(() => {resolve(`${message}`)}, time)});
}
p = delay;
p(1,2000).then((a) => { //正確的結果顯示這裡
  console.log(a);
  return delay(2, 1000);
}).then(b=>console.log(b))
  .catch((err) => { //錯誤的結果顯示這裡
    console.log("err", err);
  });
//1
//2

ASYNC的狀況

function delay(message, time) {
  return new Promise(function (resolve, reject) {
    window.setTimeout(() => {resolve(`${message}`)}, time)});
}

async function test(){
  console.log(await delay(1,2000))
  console.log(await delay(2,1000))
}

test()

Destructing

AJAX and API

AJAX = Asynchronous JavaScript And XML

​​​​Q: Why AJAX?
​​​​A: connect with API

fetch API

promise

fetch(requestURL)
    .then((response) => {
        return response.json();
    })
    .then((data) => {
        console.log(`Title: ${data.title}`);
        if (data.completed !== true){
          console.log("Undo");
        } else {
          console.log("Done!!");
        }
    })
    .catch((error) => {
        console.log(`Error: ${error}`);
    })

async

  const Find = async () => {
    const dataFetch = await fetch(
      "https://dennis89linebot.herokuapp.com/myNote",
      {
        method: "GET",
        headers: {
          Accept: "application/json",
        },
      }
    );
    let parseData = await dataFetch.json();
    setDatas(parseData);
    console.log("first", parseData);
  };

fetch如果不用async or promise
下面的code會先執行，導致沒有讀到值

  const dataFetch1 =  fetch(
    "https://dennis89linebot.herokuapp.com/myNote",
    {
      method: "GET",
      headers: {
        Accept: "application/json",
      },
    }
  );
  let parseData1 =  dataFetch1.json();

CORS issue(同網域下才requast API)

https://ithelp.ithome.com.tw/articles/10268821

FAQ

import时如何正确使用花括号'{ }'