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TypeScript Configuration

TypeScript is a primary language for Angular application development. It is a superset of JavaScript with design-time support for type safety and tooling.

Browsers can't execute TypeScript directly. Typescript must be "transpiled" into JavaScript using the tsc compiler, which requires some configuration.

This page covers some aspects of TypeScript configuration and the TypeScript environment that are important to Angular developers, including details about the following files:

tsconfig.json

Typically, you add a TypeScript configuration file called tsconfig.json to your project to guide the compiler as it generates JavaScript files.

For details about tsconfig.json, see the official TypeScript wiki.

The initial tsconfig.json for an Angular app typically looks like this example:

{
    "compileOnSave": false,
    "compilerOptions": {
      "baseUrl": "./",
      "outDir": "./dist/out-tsc",
      "sourceMap": true,
      "declaration": false,
      "module": "es2015",
      "moduleResolution": "node",
      "emitDecoratorMetadata": true,
      "experimentalDecorators": true,
      "importHelpers": true,
      "target": "es5",
      "typeRoots": [
        "node_modules/@types"
      ],
      "lib": [
        "es2018",
        "dom"
      ]
    }
   }

This file contains options and flags that are essential for Angular applications.

noImplicitAny and suppressImplicitAnyIndexErrors

TypeScript developers disagree about whether the noImplicitAny flag should be true or false. There is no correct answer and you can change the flag later. But your choice now can make a difference in larger projects, so it merits discussion.

When the noImplicitAny flag is false (the default), and if the compiler cannot infer the variable type based on how it's used, the compiler silently defaults the type to any. That's what is meant by implicit any.

When the noImplicitAny flag is true and the TypeScript compiler cannot infer the type, it still generates the JavaScript files, but it also reports an error. Many seasoned developers prefer this stricter setting because type checking catches more unintentional errors at compile time.

You can set a variable's type to any even when the noImplicitAny flag is true.

When the noImplicitAny flag is true, you may get implicit index errors as well. Most developers feel that this particular error is more annoying than helpful. You can suppress them with the following additional flag:

"suppressImplicitAnyIndexErrors": true

TypeScript Typings

Many JavaScript libraries, such as jQuery, the Jasmine testing library, and Angular, extend the JavaScript environment with features and syntax that the TypeScript compiler doesn't recognize natively. When the compiler doesn't recognize something, it throws an error.

Use TypeScript type definition filesd.ts files—to tell the compiler about the libraries you load.

TypeScript-aware editors leverage these same definition files to display type information about library features.

Many libraries include definition files in their npm packages where both the TypeScript compiler and editors can find them. Angular is one such library. The node_modules/@angular/core/ folder of any Angular application contains several d.ts files that describe parts of Angular.

You don't need to do anything to get typings files for library packages that include d.ts files. Angular packages include them already.

lib.d.ts

TypeScript includes a special declaration file called lib.d.ts. This file contains the ambient declarations for various common JavaScript constructs present in JavaScript runtimes and the DOM.

Based on the --target, TypeScript adds additional ambient declarations like Promise if the target is es6.

By default, the target is es2015. If you are targeting es5, you still have newer type declarations due to the list of declaration files included:

"lib": [
  "es2018",
  "dom"
]

Installable typings files

Many libraries—jQuery, Jasmine, and Lodash among them—do not include d.ts files in their npm packages. Fortunately, either their authors or community contributors have created separate d.ts files for these libraries and published them in well-known locations.

You can install these typings via npm using the @types/* scoped package and Typescript, starting at 2.0, automatically recognizes them.

For instance, to install typings for jasmine you run npm install @types/jasmine --save-dev.

target

By default, the target is es2015, which is supported only in modern browsers. You can configure the target to es5 to specifically support legacy browsers. Differential loading is also provided by the Angular CLI to support modern, and legacy browsers with separate bundles.

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https://angular.io/guide/typescript-configuration