Parsing Filter Expressions in NestJS with a Context-Free Grammar

The article addresses the limitation of flat query parameter filtering in NestJS, which cannot express recursive boolean logic like nested AND/OR conditions. It introduces Context-Free Grammars (CFGs) as a principled solution, where operator precedence emerges naturally from rule nesting rather than being hardcoded. The implementation uses Chevrotain, a TypeScript parser toolkit, to define a lexer that tokenizes filter expressions with carefully ordered token definitions.