JavaScript Hands On Notes

这篇文字随手用来记录 Js 和 Python 的区别

主要内容来自YouTube 视频 Learn JavaScript - Full Course for Beginners

[TOC]

Declare Variable{Declare Variable}:

Declaring

var can be declared for any types.

Without the var , the variable will be a global variable, which can be created in a function and accessed from outside of function.

Declare function:

function function(val)

Comparison:

===, ==

=== will looks at type difference, == will not

!== will checks types, != will not

Js has switch

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function sequentialSizes(val){
var answer = ""
switch(val){
case1:
case2:
case3:
answer = "low";
break;
case 4:
case 5:
case 6:
anwer = 'mid';
break;
}
}

null

null is called undefined

Dictionary(Object)

(the person in video called this “Object”) 这里是JSON Object!

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var Dogs = {
"name": "Camper", // called property: value
"legs": 4,
"tails":1,
"friends":["haha"]
};

var name = Dogs.name // one way of accessing
var name = Dogs[name]

//Delete
delete Dogs.name
delete Dogs[name]
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function checkObj(checkProp){  //check if a object has a certain property
if (myObj.hasOwnProperty(checkProp)){
return myObj[checkProp];
} else{
return "Not Found"
}
}

Object Exercise:

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var collection = {
"2548":{
"album": "slippery When Wet",
"artist": "Bon Jovi",
"tracks": [
"Let it Rock",
"You Give Love a Bad Name"
]
},
"2523":{
"album": "slippery When Wet",
"artist": "Bon Jovi",
"tracks": [
"Let it Rock",
"You Give Love a Bad Name"
]
}
}

var collectionCopy = JSON.parse(JSON.stringify(collection))
function updateRecords(id, prop, value){
if (value === ""){
delete collection[id][pro];
} else if(prop === "tracks"){
collection[id][prop] = collection[id][prop] || [];
collection[id][prop].push(value);
} else {
collection[id][prop] = value;
}
return collection
}

console.log(updateRecords(5439, "artist", "ABBA"));

Traverse the Array

和Java一样

Math

Js also has Math module

convert str to int

parseInt(str)

parseInt(str, 2) parse in base 2

Easy condition

condition? statement-if-true: statement-if-false

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function checkSign(num){
retun num > 0 ? "positive" : num <0 ? "negative" : "zero";
}

let, var, const

let cannot be declared twice

let is defined in scope. var is not

const is read-only, cannot write

const SENTENCE = "good!";

Array

array is immutable

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function freezeObj(){
"use strict";
const MATH_CONSTANTS = {
PI: 3.14
};

Object.freeze(MATH_CONSTANTS); //make object immutalbe
}

Anonymous function

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const magic = () => new Date(); // () means function, => means return 
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var myConcat = (arr1, arr2) => {arr1.concat(arr2)};

Array filter

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arr.filter(num => Number.isInteger(num) && num >0).map(x => x *x);

Default parameter

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const increment = (function(){
return funtion increment(number, value =1 ){
return number+value;
};
})();

Rest argument

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const num = (function(){
retuurn function sum(...args){
return args.reduce((a,b) => a+b, 0);
};
})();
console.log(sum(1,2,3,4));

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arr2 = [...arr1]; \\ copy values
arr2 = arr1; \\copy the same reference

const [, , ... arr] = list we can specify which index should be changed.

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const greeting = "Hello, my name is ${person.name}" \\inside curly brase is Js

Create an object

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const createPerson = (name, age, gender) => ({name, age, gender});
console.log(createPerson("Zodiac Hasbro", 56, "male"));
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var SpaceShuttle = function(targetPlanet){
this.targetPlanet = targetPlanet;
}
var zeus = new SpaceShuttle("Jupiter");

console.log(zeus.targetPlanet);
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class SpaceShuttle{
constructor(targetPlanet){
this.targetPlanet = targetPlanet;
}
}
var zeus = new SpaceShuttle("Jupiter");

console.log(zeus.targetPlanet);

Getter, Setter

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 class Book{
constructor(author){
this._author = author;
}
get writer(){
return this._author;
}
set writer(updatedAuthor){
this._author = updatedAuthor;
}
}

function makeClass(){
class Thermostat{
constructor(temp){
this._temp = 5/9 * (temp-32);
}
get temperature(){
return this._temp;
}
set temperature(updatedTemp){
this._temp = updatedTemp;
}
}
return Thermostat;
}

const Thermostat = makeClass();
const thermos = new Thermostat(76);

console.log(thermos._temp)

export, import

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import {capitalizeString} from "./string_function"
const cap = capitalizeString("hello!");

console.log(cap)

//在另一个文件里
export const capitalizeString = str => str.toUpperCase()
//capitalizeString 是一个方法,parameter 是str

export{capitalizeString};
export const foo = 'bar';

import * as capitalizing from "./where"

export default function subtract(x,y){return x-y};
import subtract from "./where"

差不多这就是一些Syntax区别,具体要用再查找吧! :smile:


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