TypeScript 中的 --strict 编译选项
Typescript 2.3 引入了一个新的 --strict 编译选项，它会开启一些相关的其他编译选项来使用更严格的类型检查。
严格类型检查选项
添加这个选项的初衷是让你可以方便地使用“默认严格模式”（strict-by-default mode）来得到更好的类型安全而不用一个个去开启相关的编译选项。你可以在命令行中添加 --strict 参数，或者在项目的 tsconfig.json 中设置 strict 选项来进入这个模式。
到 2021 年 8 月 Typescript 4.3 为止，--strict 选项会启用下面的八个编译选项：
- --alwaysStrict
- --strictBindCallApply
- --strictFunctionTypes
- --strictNullChecks
- --strictPropertyInitialization
- --noImplicitAny
- --noImplicitThis
- --useUnknownInCatchVariables
未来的 Typescript 版本可能会添加更多的类型检查选项到这个集合中。这意味着你不用跟踪每一次的 Typescript 发布使得你的项目能启用新的严格检查相关的选项。如果新的选项被添加到上面的集合中，只要升级了你项目的 Typescript 版本它们就会被自动激活。
选择性关闭某一些选型
每一个通过 --strict 开启的选项都可以被单独设置。也就是说，你依然可以在覆盖默认值的前提下默认使用严格模式。
举个例子，--alwaysStrict 选项会让编译器使用严格模式解析所有的源文件并且注入 “use strict” 指令，如果你想开启除此以外的所有严格类型检查选项，你可以像下面这样来设置你的 tsconfig.json 配置文件：
{
"strict": true,
"alwaysStrict": false
}
这个配置会如你期望的一样开启除 --alwaysStrict 以外的所有严格类型检查选项。你依然可以自动享受未来新增的严格检查选型的好处。
通过 tsc --init 自动生成配置文件模板
为了帮你初始化一个新的 Typescript 项目，编译器提供了自动生成配置文件的功能：
$ tsc --init
message TS6071: Successfully created a tsconfig.json file.
运行该命令后，你会在当前工作目录中发现一个 tsconfig.json 文件。生成的配置内容应该和下面的差不多：
{
"compilerOptions": {
/* Visit https://aka.ms/tsconfig.json to read more about this file */
/* Basic Options */
// "incremental": true, /* Enable incremental compilation */
"target": "es5" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019', 'ES2020', 'ES2021', or 'ESNEXT'. */,
"module": "commonjs" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', 'es2020', 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', 'react', 'react-jsx' or 'react-jsxdev'. */
// "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 */
// "tsBuildInfoFile": "./", /* Specify file to store incremental compilation information */
// "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. */
// "noUncheckedIndexedAccess": true, /* Include 'undefined' in index signature results */
// "noImplicitOverride": true, /* Ensure overriding members in derived classes are marked with an 'override' modifier. */
// "noPropertyAccessFromIndexSignature": true, /* Require undeclared properties from index signatures to use element accesses. */
/* 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. */
// "allowUmdGlobalAccess": true, /* Allow accessing UMD globals from modules. */
/* 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. */
/* Advanced Options */
"skipLibCheck": true /* Skip type checking of declaration files. */,
"forceConsistentCasingInFileNames": true /* Disallow inconsistently-cased references to the same file. */
}
}
注意 --strict 是默认开启的。这意味着初始化一个 Typescript 项目后，你会自动进入默认严格模式。