Before you can convert JavaScript files to TypeScript, you must add the TypeScript compiler to a project. If the project already uses a build step, the TypeScript compiler must be integrated into the build step. Otherwise, a new build step must be created. In the following sections, we will set up TypeScript and the build system.
In all cases, you should start with configuring TypeScript. This configuration will be used by code editors and the build tool. The most important setting is allowJs. This setting allows JavaScript files in the TypeScript project. Other important options are target and module. For target, you can choose between es3, es5, and es2015. The latest version of JavaScript, es2015, is not supported in all browsers at the time of writing. You can target es2015 when you write an application for NodeJS. You can target es5 for browsers. For very old environments, you must target es3.
If the project uses external modules, you should also...