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CommonJs
. If you haven’t read it yet, you can find the column where this article is located to learn. CommonJs
has many excellent features, let’s briefly review them below: Module code only runs after loading;
The module can only be loaded once;
The module can request to load other modules;
Supported Circular dependencies;
Modules can define public interfaces, and other modules can observe and interact based on this public interface;
Es Module
is unique in that it can be loaded natively through the browser or with third-party loaders and build tools. Es module
modules can load the entire dependency graph from the top-level module, and it is done asynchronously. The browser will parse the entry module, determine the dependencies, and send a request for the dependent module. After these files are returned over the network, the browser will resolve their dependencies, and if these secondary dependencies have not been loaded, more requests will be sent. Es Module
not only borrows many excellent features of CommonJs
and AMD
, but also adds some new behaviors: Es Module
is executed in strict mode by default;
Es Module
does not share the global namespace;
Es Module
The value of the top-level this
is undefined
(regular script is window
);
The var
declaration in the module will not be added to the window
object;
##Es Module is loaded and executed asynchronously;
and
import. The
command is used to specify the external interface of the module, and the
import command is used to import the functions provided by other modules.
export const nickname = "moment"; export const address = "广州"; export const age = 18;
const nickname = "moment"; const address = "广州"; const age = 18; export { nickname, address, age };
export function foo(x, y) { return x + y; } export const obj = { nickname: "moment", address: "广州", age: 18, }; // 也可以写成这样的方式 function foo(x, y) { return x + y; } const obj = { nickname: "moment", address: "广州", age: 18, }; export { foo, obj };
is the original name, but can be renamed using the
as keyword.
const address = "广州"; const age = 18; export { nickname as name, address as where, age as old };
export default "foo"; export default { name: 'moment' } export default function foo(x,y) { return x+y } export { bar, foo as default };Incorrect use of export
if(true){ export {...}; }
Must provide an external interface:
// 1只是一个值,不是一个接口export 1// moment只是一个值为1的变量const moment = 1export moment// function和class的输出,也必须遵守这样的写法function foo(x, y) { return x+y }export foo复制代码Basic use of import
command to define the external interface of the module, other js files can be loaded through the
import command The entire module
import {foo,age,nickname} from '模块标识符'
The command accepts a curly bracket, which specifies the variable name to be imported from other modules, and the variable name must be the same as the name of the external interface of the imported module.
Assignment to constant variable
的类型错误。import
语句中同时取得它们。可以依次列出特定的标识符来取得,也可以使用 *
来取得:// foo.js export default function foo(x, y) { return x + y; } export const bar = 777; export const baz = "moment"; // main.js import { default as foo, bar, baz } from "./foo.js"; import foo, { bar, baz } from "./foo.js"; import foo, * as FOO from "./foo.js";
import
导入的模块是静态的,会使所有被导入的模块,在加载时就被编译(无法做到按需编译,降低首页加载速度)。有些场景中,你可能希望根据条件导入模块或者按需导入模块,这时你可以使用动态导入代替静态导入。import
可以像调用函数一样来动态的导入模块。以这种方式调用,将返回一个 promise
。import("./foo.js").then((module) => { const { default: foo, bar, baz } = module; console.log(foo); // [Function: foo] console.log(bar); // 777 console.log(baz); // moment});复制代码
await
必须在带有 async
的异步函数中使用,否则会报错:import("./foo.js").then((module) => { const { default: foo, bar, baz } = module; console.log(foo); // [Function: foo] console.log(bar); // 777 console.log(baz); // moment });
Top-level await
:const p = new Promise((resolve, reject) => { resolve(777); });const result = await p;console.log(result); // 777正常输出
import
是静态执行,所以不能使用表达式和变量,这些只有在运行时才能得到结果的语法结构。// 错误 import { 'b' + 'ar' } from './foo.js'; // 错误 let module = './foo.js'; import { bar } from module; // 错误 if (x === 1) { import { bar } from './foo.js'; } else { import { foo } from './foo.js'; }
type
属性设置为 module
用来告知浏览器将 script
标签视为模块。<script></script><script></script>
defer
的方式延迟你的 nomodule
脚本:<script> console.log("模块情况下的"); </script> <script></script> <script> console.log("正常 script标签"); </script>
nomodule
脚本会被执行多次,而模块只会被执行一次:<script></script> <script></script> <script></script> <script></script> <script></script>
nomodule
脚本会阻塞 HTML
解析。你可以通过添加 defer
属性来解决此问题,该属性是等到 HTML
解析完成之后才执行。defer
and async
are optional attributes, they can only choose one of them, under the nomodule
script, defer
The current script will not be parsed until HTML
is parsed, and async
will be parsed in parallel with HTML
and will not block the parsing of HTML
, the module script can specify the async
attribute, but it is invalid for defer
, because the module is delayed by default. async
attribute is present, the module script and all its dependencies will be parsed and fetched in parallel, and the module script will be executed as soon as it is available. Es Module
module, you must first understand the Es Module
and Commonjs
are completely different, they have three completely different types: CommonJS
The module outputs a copy of the value, Es Module
The output is a reference to the value; CommonJS
module is loaded at runtime, and Es Module
is the compile-time output interface. CommonJS
The require()
of the module is to load the module synchronously, and the import
command of the ES6 module is to load asynchronously and has an independent module dependency analysis stage. CommonJS
loads an object (that is, the module.exports
property), which is only available when the script is running It will be generated after completion. And Es Module
is not an object. Its external interface is just a static definition, which will be generated during the static analysis phase of the code. Commonjs
What is output is a copy of the value. That is to say, once a value is output, changes within the module will not affect the value. For details, please see the previous article. The operating mechanism of Es Module
is different from CommonJS
. JS Engine
When statically analyzing a script, a read-only reference will be generated when the module loading command import
is encountered. When the script is actually executed, the value will be retrieved from the loaded module based on this read-only reference. In other words, import
is a connection pipe. If the original value changes, the value loaded by import
will also change accordingly. Therefore, Es Module
is a dynamic reference and does not cache values. The variables in the module are bound to the module in which they are located. Module Record
) encapsulates structural information about the import and export of a single module (the current module). This information is used to link the import and export of the connected module set. A module record consists of four fields, which are only used when executing the module. The four fields are: Realm
: Create the scope of the current module; Environment
: Module The top-level binding environment record of Runtime property-based access. Module namespace objects have no constructor; HostDefined
: field is reserved for use by
Module Environment Record
Immutable binding means that the current module introduces other modules, and the introduced variables cannot be modified. This is the unique immutable binding of the module. Construction
), find the js
file according to the address, download it through the network, and parse the module file to Module Record
; Instantiation
), instantiate the module, allocate memory space, parse the import and export statements of the module, and point the module to the corresponding memory address;Evaluation
), run the code, calculate the value, and fill the value into the memory address;loader
is responsible for addressing and downloading modules. First we modify an entry file, which in HTML
is usually a <script type="module"></script>
tag to represent a module file. import
statement. There is a module declaration identifier in the import
declaration statement. Character (ModuleSpecifier
), which tells loader
how to find the address of the next module. module record (Module Record)
, and each module record
Contains JavaScript code
, execution context
, ImportEntries
, LocalExportEntries
, IndirectExportEntries
, StarExportEntries
. The ImportEntries
value is a ImportEntry Records
type, and LocalExportEntries
, IndirectExportEntries
, StarExportEntries
is a ExportEntry Records
type. ImportEntry Records
contains three fields ModuleRequest
, ImportName
,LocalName
;ModuleSpecifier
);ModuleRequest
The name of the required binding for the module export of the module identifier. The value namespace-object
indicates that the import request is for the namespace object of the target module;ImportEntry Records
fields imported using import
Example: Import Statement From | Module identifier(ModuleRequest) | Import name(ImportName) | LocalName | |
---|---|---|---|---|
"react" | "default" | "React" | ||
"react" | namespace -obj | "Moment" | ||
"react" | "useEffect" | "useEffect" | ||
"react" | "useEffect" | "effect" |
Export declaration | Export name | Module identifier | Import name | Local name |
---|---|---|---|---|
export var v; | "v" | null | null | "v" |
export default function f() {} | "default" | null | null | "f" |
export default function () {} | "default" | null | null | "default" |
export default 42; | "default" | null | null | "default" |
"x" | null | null | "x" | |
"x" | null | null | "v" | ##export {x} from "mod"; |
"mod" | "x" | null | export {v as x} from "mod"; | |
"mod" | "v" | null | ##export * from "mod"; | |
"mod" | all-but-default | null | ##export * as ns from "mod"; | "ns |
all | null |
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