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

• tsconfig.json-TypeScript compiler configuration.
• typings-TypesScript declaration 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.

The Setup guide uses the following tsconfig.json:

```
{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "moduleResolution": "node",
    "sourceMap": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "lib": [ "es2015", "dom" ],
    "noImplicitAny": true,
    "suppressImplicitAnyIndexErrors": true
  }
}
```

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.

The documentation setup sets the noImplicitAny flag to true. 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 ```

The documentation setup sets this flag to true as well.

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 files -d.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 need to do nothing 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.

Since the QuickStart is targeting es5, you can override the list of declaration files to be included:

"lib": ["es2015", "dom"]

Thanks to that, you have all the es6 typings even when targeting es5.

Installable typings files

Many libraries-jQuery, Jasmine, and Lodash among them does 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 recognize them.

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

QuickStart identifies two typings, or d.ts, files:

• jasmine typings for the Jasmine test framework.
• node for code that references objects in the Node.js® environment;

QuickStart doesn't require these typings but many of the samples do.

target

By default, the target is es5, you can configure the target to es6 if you only want to deploy the application to es6 compatible browser. But if you configure the target to es6 in some old browser such as IE, Syntax Error will be thrown.

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