TypeScript 入门指南：必要性、优势与实战配置

| 对比项目 | TypeScript | JavaScript | 注意 | 
| --------| --- | --- | --- | 
| 基本类型 |  boolean number string Array Tuple Enum any void null undefined never object | string number boolean null undefined symbol | TypeScript 中 object 表示的是不是 JavaScript 基本类型的类型| 
| 变量声明 | let const var| let const var | 基本一致 | 
| 接口 | interface | 无 |  TypeScript 的核心是类型检查，因此接口充当了命名这些类型的角色 | 
| 类 | class abstract class  readonly … | class 无 abstract class | 基本一致，但不同的可能发生在未来，TypeScript 使用 private 来定义私有，而 JavaScript 未来极有可能将 #.xx 来定义私有写入标准。TypeScript  支持抽象类，只读等等。| 
| 函数 | N | N | 基本一致，参数赋默认值，剩余参数等等，唯一不同的是 TypeScript 支持?可选参数 | 
| 泛型 | Generics | 无 | 泛型是一个特别灵活的可重用指定不同类型来控制具体类型的类型，TypeScript 支持 | 
| 枚举 | Enums | 无 | TypeScript 支持的枚举不仅可以默认从 0 开始，也可以赋值具体的字符串，它的操作空间非常大 | 
| 类型推断 | 支持 | 无 |  let x = 3; TypeScript 可以通过 3 来推断 x 的类型是 number | 
| 高阶类型 | & typeof  instanceof … | 无 | TypeScript 独有 | 
| Symbols | N | N | Symbol 一样 | 
| 迭代器 | N | N | 如果实现了 Symbol.iterator ，那么就被视为可迭代的，术语上和 JavaScript 定义的一样 | 
| Generators | N | N | 一样 |
| 模块系统 | N | export import | 事实上 TypeScript 支持多种多样的模块系统，既有 ESModule 也有 Commonjs 规范，甚至还有 AMD UMD 等 |
| 其他 | N | N | 由于 TypeScript 是 JavaScript 的超集，因此 ES2016 之后以及 ESNext 定义的  api 都可以直接在 TypeScript 中使用 并不需要语言支持，至于其他一些比如 JSX Mixins 等等，由于这些不属于 JavaScript 标准因此这里不再复述 。|

$ npm install -g typescript
$ tsc --version

let helloworld = "";

console.log(helloworld);
$ tsc helloworld.ts

let helloworld = "";

helloworld = 1;

$ tsc helloworld.ts
helloworld.ts:5:1 - error TS2322: Type '1' is not assignable to type 'string'.

5 helloworld = 1;

Found 1 error.

{
  "compilerOptions": {
    /* Basic Options */
    "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */
    "module": "commonjs",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
    // "lib": [],                             /* Specify library files to be included in the compilation. */
    // "allowJs": true,                       /* Allow javascript files to be compiled. */
    // "checkJs": true,                       /* Report errors in .js files. */
    // "jsx": "preserve",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
    // "declaration": true,                   /* Generates corresponding '.d.ts' file. */
    // "declarationMap": true,                /* Generates a sourcemap for each corresponding '.d.ts' file. */
    // "sourceMap": true,                     /* Generates corresponding '.map' file. */
    // "outFile": "./",                       /* Concatenate and emit output to single file. */
    // "outDir": "./",                        /* Redirect output structure to the directory. */
    // "rootDir": "./",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
    // "composite": true,                     /* Enable project compilation */
    // "removeComments": true,                /* Do not emit comments to output. */
    // "noEmit": true,                        /* Do not emit outputs. */
    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
    // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
    // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */

    /* Strict Type-Checking Options */
    "strict": true,                           /* Enable all strict type-checking options. */
    // "noImplicitAny": true,                 /* Raise error on expressions and declarations with an implied 'any' type. */
    // "strictNullChecks": true,              /* Enable strict null checks. */
    // "strictFunctionTypes": true,           /* Enable strict checking of function types. */
    // "strictBindCallApply": true,           /* Enable strict 'bind', 'call', and 'apply' methods on functions. */
    // "strictPropertyInitialization": true,  /* Enable strict checking of property initialization in classes. */
    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */

    /* Additional Checks */
    // "noUnusedLocals": true,                /* Report errors on unused locals. */
    // "noUnusedParameters": true,            /* Report errors on unused parameters. */
    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */

    /* Module Resolution Options */
    // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
    // "paths": {},                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
    // "typeRoots": [],                       /* List of folders to include type definitions from. */
    // "types": [],                           /* Type declaration files to be included in compilation. */
    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
    "esModuleInterop": true                   /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */

    /* Source Map Options */
    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */

    /* Experimental Options */
    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
  }
}

$ yarn add typescript awesome-typescript-loader source-map-loader --dev

var fs = require('fs')
var path = require('path')
var webpack = require('webpack')
const { CheckerPlugin } = require('awesome-typescript-loader');
var ROOT = path.resolve(__dirname)

module.exports = {
  entry: './src/index.tsx',
  devtool: 'inline-source-map',
  output: {
    path: ROOT + '/dist',
    filename: '[name].bundle.js',
    sourceMapFilename: '[name].bundle.map.js',
    publicPath: '//localhost:8889/dist/',
  },
  devServer: {
    inline: true,
    quiet: true,
    contentBase: "./",
    port: 8889
  },
  module: {
    rules: [
      { test: /\.ts[x]?$/, loader: "awesome-typescript-loader" },
      { enforce: "pre", test: /\.ts[x]$/, loader: "source-map-loader" },
    ]
  },
  resolve: {
    extensions: [".ts", ".tsx"],
    alias: {
      '@': path.resolve(ROOT,'src')
    }
  },
  plugins: [
    new CheckerPlugin(),
  ]
}