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:
A given Angular workspace contains several TypeScript configuration files. At the root tsconfig.json
file specifies the base TypeScript and Angular compiler options that all projects in the workspace inherit.
See the Angular compiler options guide for information about what Angular specific options are available.
The TypeScript and Angular have a wide range of options which can be used to configure type-checking features and generated output. For more information, see the Configuration inheritance with extends section of the TypeScript documentation.
For more information TypeScript configuration files, see the official TypeScript wiki For details about configuration inheritance, see the Configuration inheritance with extends section.
The initial tsconfig.json
for an Angular workspace typically looks like the following example.
{
"compileOnSave": false,
"compilerOptions": {
"baseUrl": "./",
"outDir": "./dist/out-tsc",
"sourceMap": true,
"declaration": false,
"downlevelIteration": true,
"experimentalDecorators": true,
"moduleResolution": "node",
"importHelpers": true,
"target": "es2015",
"module": "es2020",
"lib": [
"es2018",
"dom"
]
}
}
When you create new workspaces and projects, you have the option to use Angular's strict mode, which can help you write better, more maintainable code. For more information, see Strict mode.
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
For more information about how the TypeScript configuration affects compilation, see Angular Compiler Options and Template Type Checking.
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 don't need to do anything to get typings files for library packages that include
d.ts
files. Angular packages include them already.
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" ]
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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Licensed under the Creative Commons Attribution License 4.0.
https://v11.angular.io/guide/typescript-configuration