@thi.ng/parse/combinators/lookahead.js

Purely functional parser combinators & AST generation for generic inputs

const lookahead = (parser, ahead, capture = false, id = "lookahead") => (ctx) => {
  if (ctx.done) return false;
  ctx.start(id);
  let pass = false;
  while (true) {
    const state = capture ? null : ctx.state.copy();
    if (ahead(ctx)) {
      !capture && (ctx.state = state);
      return pass ? ctx.end() : ctx.discard();
    }
    if (!parser(ctx)) return ctx.discard();
    pass = true;
  }
};
export {
  lookahead
};