TypeScript
TypeScript is, in our opinion, the best language to write your Screeps codebase in. It combines the familiarity of JavaScript with the power of static typing.
Static type checkers like TypeScript and Flow help reduce the amount of bugs in your code by detecting type errors during compile time. In general, using static typing in your JavaScript code can help prevent about 15% of the bugs that end up in committed code. Not only static typing, TypeScript also provides various productivity enhancements like advanced statement completion, as well as smart code refactoring.
To read more about how TypeScript can help you in Screeps, read this Screeps World article by @bonzaiferroni.
This section provides TypeScript-specific tips & tricks for you to make the best out of the ecosystem.

Strict mode

The --strict compiler flag was introduced in TypeScript 2.3 which activates TypeScript's "strict mode". The strict mode sets all strict typechecking options to true by default.
As of TypeScript 2.7, the affected options are:
  • --noImplicitAny
  • --noImplicitThis
  • --alwaysStrict
  • --strictNullChecks
  • --strictFunctionTypes
  • --strictPropertyInitialization
Starting from version 2.0 of the starter kit, we've enabled the --strict flag in tsconfig.json. If this gives you compile time errors, you can try setting "strict" to false, or by overriding one or more of the options listed above.

ESLint

ESLint checks your TypeScript (and JavaScript) code for readability, maintainability, and functionality errors, and can also enforce coding style standards.
This project provides ESLint rules through a .eslintrc.js file, which extends the recommended rules from ESLint defined here.
We've made some changes to these rules, which we considered necessary and/or relevant to a proper Screeps project:
  • set the guard-for-in rule to off, it was forcing for ( ... in ...) loops to check if object members were not coming from the class prototype.
  • set the no-console rule to off, in order to allow using console.
  • set the no-underscore-dangle to warn.

Customising ESLint

You can also customise your .eslintrc.js file to match the preferences of your codebase. Click here, to find out how, and click here for a complete list of rules available.
If you believe that some rules should not apply to a part of your code (e.g. for one-off cases like having to use require() to include a module), you can use flags to let ESLint know about it: https://eslint.org/docs/user-guide/configuring/rules#disabling-rules
More info about ESLint: https://eslint.org/