Tslint Rule

In a previous post I went over how to make a Flake8 lint. Creating a TSLint rule turned out to be pretty similar.

With Flake8 we use the ast module from Python’s standard library and inherit from ast.NodeVisitor. In TSLint we import and extend TSLint’s equivalent, Lint.RuleWalker.

Introspection of the TypeScript AST involves flipping between AST Explorer and your editor trying to get TypeScript to auto-complete the corresponding type guard, but then realizing that TrueKeyword doesn’t have a type guard and that you need to check the kind attribute against the SyntaxKind enum yourself.

Although AST Explorer is much nicer than ast.dump(ast.parse("foo()")), the Python AST has greater simplicity in refinement where the Python AST is just classes and isinstance() functions as the universal type guard.

Overall, TSLint’s docs for creating a custom rule are quite thorough and provide you with everything you need to create a lint.

A couple minor things I ran into:

1. make sure your rule files end in Rule, otherwise you will run into lint not found issues.

   Could not find implementations for the following rules specified in the configuration:
       jsx-no-true-attribute
   Try upgrading TSLint and/or ensuring that you have all necessary custom rules installed.
   If TSLint was recently upgraded, you may have old rules configured which need to be cleaned up.
   
   No valid rules have been specified for TypeScript files
   

2. create a tslint-$PKG_NAME.json file so that installing the lint is easier. This allows for extends: ['$PKG_NAME'] in tslint.json instead of having to configure the rules directories rulesDirectories: ['node_modules/$PKG_NAME/dist/rules']