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TypeScript tips

Coding guidelines

Decorators

  • Series of articles on decorators and metadata.

Compiler options related to path aliases

The content under this section is copied from TSConfig reference page.

rootDir

Defaults to the longest common path of all non-declaration input files. If composite is set, the default is instead the directory containing the tsconfig.json file.

When TypeScript compiles files, it keeps the same directory structure in the ouput directory as exists in the input directory.

baseUrl

Sets a base directory from which to resolve bare specifier module names. For example, in the directory structure:

project
├── ex.ts
├── hello
│   └── world.ts
└── tsconfig.json

With "baseUrl": "./", TypeScript will look for files starting at the same folder as the tsconfig.json:

import { helloWorld } from 'hello/world';

console.log(helloWorld);

This resolution has higher priority than lookups from node_modules.

Important

This feature was designed for use in conjunction with AMD module loaders in the browser, and is not recommended in any other context. As of TypeScript 4.1, baseUrl is no longer required to be set when using paths.

NestJS make use of baseUrl because it uses Webpack for hot reloading in dev mode.

paths

A series of entries which re-map imports to lookup locations relative to the baseUrl if set, or to the tsconfig file itself otherwise.

paths lets you declare how TypeScript should resolve an import in your require/imports.

{
  "compilerOptions": {
    "paths": {
      "@app/*": ["./src/app/*"],
      "@config/*": ["./src/app/_config/*"],
      "@environment/*": ["./src/environments/*"],
      "@shared/*": ["./src/app/_shared/*"],
      "@helpers/*": ["./src/helpers/*"],
      "@tests/*": ["./src/tests/*"]
    }
  }
}

In this case, you can tell the TypeScript file resolver to support a number of custom prefixes to find code.

You can read more on path here.

include

Specifies an array of filenames or patterns to include in the program. These filenames are resolved relative to the directory containing the tsconfig.json file.

exclude

Specifies an array of filenames or patterns that should be skipped when resolving include.

Important

exclude only changes which files are included as a result of the include setting. A file specified by exclude can still become part of your codebase due to an import statement in your code, a types inclusion, a /// <reference ... /> directive, or being specified in the files list.

It is not a mechanism that prevents a file from being included in the codebase - it simply changes what the include setting finds.