Have a look at the index.js, it enables Typescript checking, then declares a function’s types with JSDocs and commits various type errors.
The (mostly empty) jsconfig.json tells the Typescript compiler to use this configuration, and to generate an index.d.ts describing the JSDoc within the file. To generate that declaration file, as well as typecheck, you can run:
$ rm -f *.d.ts && tsc -p jsconfig.json
index.js:25:6 - error TS2345: Argument of type '"a"' is not assignable to parameter of type 'number'.
25 add2('a', 2);
~~~
index.js:26:6 - error TS2345: Argument of type '{}' is not assignable to parameter of type 'number'.
26 add3({}, 3);
~~
Found 2 errors.
$
Your IDE, if it uses the Typescript language server, will also detect these errors.