一、开宗明义

JS的每一个单独的文件都是一个独立的program。

二、数值与数据Value and Data

JS中的Value就是Data，有两种形式分别为primitive和object。其中primitive包括1）number（bigint）， 2）string（interpolate）3）boolean 4) undefined and null 5) Symbol

而剩下的所有的data都是object类型，比如array与function是特殊类型的object。

`typeof`检测数据的类型，但有个特殊的bug：

typeof null;                // "object" -- oops, bug!

声明变量的方式 Declare Variables

声明变量有var,let,const此三种方式，特殊形式为function的声明：

    function hello(myName) {
        console.log(`Hello, ${ myName }.`);
    }

三、函数 Function

在JS，function既是procedure又是value（data）。

    function awesomeFunction(coolThings) {
        // ..
        return amazingStuff;
    }

    coonstn awesomeFunction = function(coolThings) {
        // ..
        return amazingStuff;
    };

因为function是value，因此可将其作为object的properties。

    let saySomething = {
        greeting() {
            console.log("Hello!");
        },
        question() {
            console.log("What's your name?");
        },
        answer() {
            console.log("My name is Kyle.");
        }
    };

    saySomething.greeting();
    // Hello!

四、数值的比较 Comparison

历史的原因，JS的数值比较乃是个迷思，颇为复杂。

Triple-equals

最为常见的是triple-equals,但是有特殊案例：

    NaN === NaN;            // false
    0 === -0;               // truer

Quadruple-equals

解决方案:

> Number.isNaN(NaN)
true
> Object.is(0, -0)
false

其中object.is又被称之为quadruple-equals "====",不仅比较值，也比较类型。

更深层次的探索，JS的equality-comparision并不是content-aware的“structual equality”，而是identity-aware的reference-equality。

    var x = [ 1, 2, 3 ];

    // assignment is by reference-copy, so
    // y references the *same* array as x,
    // not another copy of it.
    var y = x;

    y === x;              // true
    y === [ 1, 2, 3 ];    // false
    x === [ 1, 2, 3 ];    // false

Coercive-comparision

Coercive-comparison的应用是inevitable，因为rational-operator会用到，比如：

    var arr = [ "1", "10", "100", "1000" ];
    for (let i = 0; i < arr.length && arr[i] < 500; i++) {
        // will run 3 times
    }

但是当比较的双方都是string的时候，就会出现错误：

> "11" > "9"
false

五、用Class如何组织我们的JS代码

Class

class组织代码的关键词是concrete，instantiated, new.

    class Page {
        constructor(text) {
            this.text = text;
        }

        print() {
            console.log(this.text);
        }
    }

    class Notebook {
        constructor() {
            this.pages = [];
        }

        addPage(text) {
            var page = new Page(text);
            this.pages.push(page);
        }

        print() {    class Publication {
        constructor(title,author,pubDate) {
            this.title = title;
            this.author = author;
            this.pubDate = pubDate;
        }

        print() {
            console.log(`
                Title: ${ this.title }
                By: ${ this.author }
                ${ this.pubDate }
            `);
        }
    }
            for (let page of this.pages) {
                page.print();
            }
        }
    }

    var mathNotes = new Notebook();
    mathNotes.addPage("Arithmetic: + - * / ...");
    mathNotes.addPage("Trigonometry: sin cos tan ...");

    mathNotes.print();
    // ..

Class Inheritance

inheritance可实现polymorphism.

    class Publication {
        constructor(title,author,pubDate) {
            this.title = title;
            this.author = author;
            this.pubDate = pubDate;
        }

        print() {
            console.log(`
                Title: ${ this.title }
                By: ${ this.author }
                ${ this.pubDate }
            `);
        }
    }

    class Book extends Publication {
        constructor(bookDetails) {
            super(
                bookDetails.title,
                bookDetails.author,
                bookDetails.publishedOn
            );
            this.publisher = bookDetails.publisher;
            this.ISBN = bookDetails.ISBN;
        }

        print() {
            super.print();
            console.log(`
                Publisher: ${ this.publisher }
                ISBN: ${ this.ISBN }
            `);
        }
    }

    class BlogPost extends Publication {
        constructor(title,author,pubDate,URL) {
            super(title,author,pubDate);
            this.URL = URL;
        }

        print() {
            super.print();
            console.log(this.URL);
        }
    }

六、用Module组织JS代码

以traditional方法实现module：

    function Publication(title,author,pubDate) {
        var publicAPI = {
            print() {
                console.log(`
                    Title: ${ title }
                    By: ${ author }
                    ${ pubDate }
                `);
            }
        };

        return publicAPI;
    }

    function Book(bookDetails) {
        var pub = Publication(
            bookDetails.title,
            bookDetails.author,
            bookDetails.publishedOn
        );

        var publicAPI = {
            print() {
                pub.print();
                console.log(`
                    Publisher: ${ bookDetails.publisher }
                    ISBN: ${ bookDetails.ISBN }
                `);
            }
        };

        return publicAPI;
    }

    function BlogPost(title,author,pubDate,URL) {
        var pub = Publication(title,author,pubDate);

        var publicAPI = {
            print() {
                pub.print();
                console.log(URL);
            }
        };

        return publicAPI;
    }

Traditional-module与class的区别之处就在于explicit的publicAPI。

ES Modules

ES modules (ESM)是基于file，一个file是一个module。ESM是single-instance即singletons.

    function printDetails(title,author,pubDate) {
        console.log(`
            Title: ${ title }
            By: ${ author }
            ${ pubDate }
        `);
    }

    export function create(title,author,pubDate) {
        var publicAPI = {
            print() {
                printDetails(title,author,pubDate);
            }
        };

        return publicAPI;
    }

在另外的文件中引用以上的函数。

    import { create as createPub } from "publication.js";

    function printDetails(pub,URL) {
        pub.print();
        console.log(URL);
    }

    export function create(title,author,pubDate,URL) {
        var pub = createPub(title,author,pubDate);

        var publicAPI = {
            print() {
                printDetails(pub,URL);
            }
        };

        return publicAPI;
    }

作为惯例，可以将所有的module都引入到main.js文件中。

建议不要将classic-module与class混用。