Typescript

Project Structure With tsconfig.json

The rootDir config defines the root of all source files. By default this is the longest common path of all non-declaration input files.

The structure under rootDir will be preserved in the output directory. Relative imports are relative to this directory.

The include config will define more specifically which files should be included. You could include all *.ts files here for example.

If include pulls in files that are not in rootDir, you will get an error. But presumably you could stash files under rootDir that the include parameters does not pull in, and that would not be an error.

Targets and Modules

Typescript adopts the ES6 module syntax exclusively. But nothing runs Typescript directly; it must be transpiled to Javascript for anything to run, so the question arises: what module system will the output Javascript file use? And related: what version of Javascript will it use?

Specifying the target in tsconfig.json will define what version of Javascript the output will be, in terms of language features. Lots of choices here but es3, es5 and es6 are popular ones. Default is es5.

Next, you specify the module system the output will use, with the module field in tsconfig.json. Several options here but if you’re targeting node, then you’ll probably want commonjs or es6. Default is commonjs if the target is es5, otherwise es6.

Interesting combinations:

• target: es6, module: es6. Everything looks modern.
• target: es5. module: commonjs. Everything is old fashioned, with requires for modules.
• target: es6, module: commonjs. New language feature with old-fashioned require based module system
• target: es5, module: es6. This one is a bit odd. This means an old fashioned set of language feature with new import/export syntax. Not sure why you would do this, or in what context this would work.

You can also specify a moduleResolution field, but I don’t think I’ve ever used anything but node here.