module

Author: wang-jun-coder

模块/module.md

@@ -0,0 +1,101 @@
+## 模块机制
+
+### 循环引用问题
+
+```js
+// ---------- circular/a.js -----------
+
+console.log(`a begin`);
+exports.done = false;
+
+const b = require('./b');
+console.log(`a load b.js, b.done = ${b.done}`);
+
+exports.done = true;
+console.log(`b end`);
+
+// ---------- circular/b.js -----------
+console.log(`b.js begin`);
+exports.done = false;
+
+const a = require('./a');
+console.log(`b load a.js, a.done = ${a.done}`);
+
+exports.done = true;
+console.log(`b end`);
+
+
+// ---------- index.js -----------
+console.log(`circular load a.js and b.js begin`);
+const a = require('./circular/a');
+const b = require('./circular/b');
+console.log(`index.js a.done = ${a.done}, b.done = ${b.done}`);
+console.log(`circular load a.js and b.js end`);
+
+// ---------- console -----------
+/*
+circular load a.js and b.js begin
+a begin
+b.js begin
+b load a.js, a.done = false
+b end
+a load b.js, b.done = true
+b end
+index.js a.done = true, b.done = true
+circular load a.js and b.js end
+* */
+```
+加载流程如下  
+
+* index.js 加载 a.js, 
+* a 尝试加载 b.js
+* b.js 这时候尝试加载 a.js
+* 为防止死循环, 此时, b.js 只能拿到一个 a.js 的未完成的副本进行使用
+* b.js 完成加载后 a.js 拿到 b.js 的 exports, 
+* a.js 再完成剩余的加载
+* 最终, 在 index.js 中 a.js 和 b.js 均完成加载
+
+需注意：循环过程中，b.js 中拿到的 a.js 是不完整的，部分属性方法可能无法使用。虽然循环引用不会导致死循环，但也应该避免。
+
+
+### 全局变量泄露问题
+
+```js
+// ---------- global/leakGlobalVar.js -----------
+leak = 'this is a leak global variable'; // 可启用严格模式避免
+module.exports = {
+    name: 'leakGlobalVar.js'
+};
+
+// ----------index.js -----------
+const leakGlobalVar = require('./global/leakGlobalVar');
+console.log(`module: ${JSON.stringify(leakGlobalVar)}, leak: ${leak}`); // module: {"name":"leakGlobalVar.js"}, leak: this is a leak global variable
+```
+
+模块中泄露的全局变量可在其他模块中拿到， 无意识的变量泄露可通过启用严格模式避免
+
+```js
+// ---------- global/vm.js -----------
+const vm = require('vm');
+let code = `(
+    function() {
+        leakInVm = 'this is a leak object in vm';
+        return {
+            name: 'global/vm.js'
+        }
+    }
+)`;
+const vmFn = vm.runInThisContext(code);
+module.exports = vmFn();
+
+// ---------- index.js -----------
+
+const vm1 = require('./global/vm');
+console.log(vm1);       // { name: 'global/vm.js' }
+console.log(leakInVm);  // this is a leak object in vm
+
+```
+[vm.runInThisContext() compiles code, runs it within the context of the current global and returns the result. Running code does not have access to local scope, but does have access to the current global object.](https://nodejs.org/docs/latest-v8.x/api/vm.html#vm_vm_runinthiscontext_code_options)
+
+
+

模块/module/circular/a.js

@@ -0,0 +1,8 @@
+console.log(`a begin`);
+exports.done = false;
+
+const b = require('./b');
+console.log(`a load b.js, b.done = ${b.done}`);
+
+exports.done = true;
+console.log(`b end`);

模块/module/circular/b.js

@@ -0,0 +1,8 @@
+console.log(`b.js begin`);
+exports.done = false;
+
+const a = require('./a');
+console.log(`b load a.js, a.done = ${a.done}`);
+
+exports.done = true;
+console.log(`b end`);

模块/module/dirModule/dirModule1/index.js

@@ -0,0 +1,3 @@
+module.exports = {
+    name: "dirModule1/index.js"
+};

模块/module/dirModule/dirModule1/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "dirModule1",
+  "version": "1.0.0",
+  "main": "index.js",
+  "dependencies": {
+
+  }
+}

模块/module/dirModule/dirModule2/lib/index.js

@@ -0,0 +1,3 @@
+module.exports = {
+    name: "dirModule2/lib/index.js"
+};

模块/module/dirModule/dirModule2/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "dirModule2",
+  "version": "1.0.0",
+  "main": "lib",
+  "dependencies": {
+
+  }
+}

模块/module/dirModule/dirModule3/index.js

@@ -0,0 +1,3 @@
+module.exports = {
+    name: "dirModule3/index.js"
+};

模块/module/dirModule/dirModule3/index.json

@@ -0,0 +1,3 @@
+{
+  "name": "dirModule3/index.json"
+}

模块/module/dirModule/dirModule3/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "dirModule3",
+  "version": "1.0.0",
+  "dependencies": {
+    
+  }
+}

模块/module/dirModule/dirModule4/index.json

@@ -0,0 +1,3 @@
+{
+  "name": "dirModule4/index.json"
+}

模块/module/fileModule/file1

@@ -0,0 +1,3 @@
+module.exports = {
+    "name": "file1"
+};

模块/module/fileModule/file1.js

@@ -0,0 +1,3 @@
+module.exports = {
+    "name": "file1.js"
+};

模块/module/fileModule/file2.js

@@ -0,0 +1,3 @@
+module.exports = {
+    "name": "file2.js"
+};

模块/module/fileModule/file2.json

@@ -0,0 +1,3 @@
+{
+  "name": "file2.json"
+}

模块/module/fileModule/file3.json

@@ -0,0 +1,3 @@
+{
+  "name": "file3.json"
+}

模块/module/global/leakGlobalVar.js

@@ -0,0 +1,6 @@
+
+leak = 'this is a leak global variable'; // 可启用严格模式避免
+
+module.exports = {
+    name: 'leakGlobalVar.js'
+};

模块/module/global/vm.js

@@ -0,0 +1,15 @@
+const vm = require('vm');
+
+let code = `(
+    function() {
+        leakInVm = 'this is a leak object in vm';
+        return {
+            name: 'global/vm.js'
+        }
+    }
+)`;
+
+const vmFn = vm.runInThisContext(code);
+module.exports = vmFn();
+
+

模块/module/index.js

@@ -0,0 +1,116 @@
+
+// 模块
+const circle = require('./shape/circle');
+console.log(`半径为 3 的圆的面积为: ${circle.area(3)}`);    // 半径为 3 的圆的面积为: 28.274333882308138
+
+const Square = require('./shape/square');
+const mySquare = new Square(4);
+console.log(`mySquare 的面积为: ${mySquare.area()}`);         // mySquare 的面积为: 16
+
+// 主模块
+const isMain = require.main === module;
+if (isMain) console.log(`main module: ${require.main.filename} `);// main module: /.../module/index.js
+
+// 获取模块加载名
+const cycleName = require.resolve('./shape/circle');
+console.log(`${cycleName}`); // /.../module/index.js
+
+// 模块加载缓存,
+const Square1 = require('./shape/square'); // square.js 中的 console.log 只输出一次
+const Square2 = require.cache[require.resolve('./shape/square')].exports;
+console.log(`Square === Square1: ${Square === Square1}\nSquare === Square2: ${Square === Square2}`); // Square === Square1: true Square === Square2: true
+
+// 循环导入问题
+console.log(`circular load a.js and b.js begin`);
+const a = require('./circular/a');
+const b = require('./circular/b');
+console.log(`index.js a.done = ${a.done}, b.done = ${b.done}`);
+console.log(`circular load a.js and b.js end`);
+/*
+circular load a.js and b.js begin
+a begin
+b.js begin
+b load a.js, a.done = false
+b end
+a load b.js, b.done = true
+b end
+index.js a.done = true, b.done = true
+circular load a.js and b.js end
+
+index.js 加载 a.js 时, a 尝试加载 b.js
+但是 b.js 这时候尝试加载 a.js, 为防止死循环, 此时, b.js 只能拿到一个 a.js 的未完成的副本进行使用
+b.js 完成加载后 a.js 拿到 b.js 的 exports, a.js 再完成剩余的加载
+最终, 在 index.js 中 a.js 和 b.js 均完成加载
+* */
+
+
+// require 加载规则 -> http://nodejs.cn/api/modules.html#modules_all_together
+// 文件加载 LOAD_AS_FILE(X)
+const file1 = require('./fileModule/file1');        // If X is a file, load X as JavaScript text.  STOP,
+console.log(`file1 is ${JSON.stringify(file1)}`);   //  file1 is {"name":"file1"}, 虽然 file1.js 存在, 但是查询到 file1 后停止
+
+const file2 = require('./fileModule/file2');        // If X.js is a file, load X.js as JavaScript text.  STOP
+console.log(`file2 is ${JSON.stringify(file2)}`);   // file2 is {"name":"file2.js"} 虽然 file2.json 存在, 但是查询到 file2.js 停止
+
+const file3 = require('./fileModule/file3');        // If X.json is a file, parse X.json to a JavaScript Object.  STOP
+console.log(`file3 is ${JSON.stringify(file3)}`);   // file3 is {"name":"file3.json"}, .node 文件未制作
+// .node 插件待制作, 也会尝试加载 后缀为 .node的文件
+
+
+// 目录加载, LOAD_AS_DIRECTORY(X)  ->  LOAD_INDEX(X)
+// LOAD_AS_DIRECTORY(X)
+// 1. If X/package.json is a file,
+//     a. Parse X/package.json, and look for "main" field.
+//     b. let M = X + (json main field)
+// c. LOAD_AS_FILE(M)
+// d. LOAD_INDEX(M)
+// 2. LOAD_INDEX(X)
+//
+// LOAD_INDEX(X)
+// 1. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
+// 2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
+// 3. If X/index.node is a file, load X/index.node as binary addon.  STOP
+
+
+const dirModule1 = require('./dirModule/dirModule1'); // package.json -> main -> load file
+console.log(`${JSON.stringify(dirModule1)}`);   // {"name":"dirModule1/index.js"}
+
+
+const dirModule2 = require('./dirModule/dirModule2'); // package.json -> main -> LOAD_INDEX(main)
+console.log(`${JSON.stringify(dirModule2)}`);   // {"name":"dirModule2/lib/index.js"}
+
+const dirModule3 = require('./dirModule/dirModule3'); // package.json -> main? -> LOAD_INDEX(dir)
+console.log(`${JSON.stringify(dirModule3)}`);   // {"name":"dirModule3/index.js"}  虽然也存在 index.json 但是优先级低
+
+const dirModule4 = require('./dirModule/dirModule4'); // package.json -> main? -> LOAD_INDEX(dir)
+console.log(`${JSON.stringify(dirModule4)}`);   // {"name":"dirModule4/index.json"}, addon 未准备
+
+// 从 node_modules 目录加载, 此处已 is 模块演示
+const is = require('is');
+console.log(`${is.string}`);   // function (value) {return toStr.call(value) === '[object String]';}
+
+
+// 从全局目录加载, 如安装的全局模块
+// NODE_PATH
+// $HOME/.node_modules
+// $HOME/.node_libraries
+// $PREFIX/lib/node
+
+
+// 模块封装器
+
+// (function(exports, require, module, __filename, __dirname) {
+// // 模块的代码实际上在这里
+// });
+// 保持顶层变量,作用在模块范围内, 而不是全局对象
+// 方便提供看似全局实际上是模块特有的变量
+
+// 子模块可以泄露全局变量吗?
+const leakGlobalVar = require('./global/leakGlobalVar');
+console.log(`module: ${JSON.stringify(leakGlobalVar)}, leak: ${leak}`); // module: {"name":"leakGlobalVar.js"}, leak: this is a leak global variable
+
+const vm1 = require('./global/vm');
+console.log(vm1);       // { name: 'global/vm.js' }
+console.log(leakInVm);  // this is a leak object in vm
+// https://nodejs.org/docs/latest-v8.x/api/vm.html#vm_vm_runinthiscontext_code_options
+//

模块/module/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "module-test",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "is": "^3.3.0"
+  }
+}

模块/module/shape/circle.js

@@ -0,0 +1,6 @@
+const { PI } = Math;
+
+exports.area = r => PI * r ** 2;
+exports.circumference = r => 2*PI*r;
+
+

模块/module/shape/square.js

@@ -0,0 +1,18 @@
+
+// exports 仅是 module.exports 的一个引用, 直接修改 exports 的指向是无效的
+module.exports = class Square{
+    constructor(width) {
+        this.width = width;
+    }
+
+    area() {
+        return this.width ** 2;
+    }
+};
+
+
+console.log(`${__filename} load`);  // /..../shape/Square.js load
+
+
+const circle = require('./circle');
+console.log(require.cache[require.resolve('./circle')].exports === circle);