stolen from https://mcluck90.github.io/abusing-proxies-for-dsls/

dsl_fiz_buz.js

let tokens = [];
let dsl = new Proxy({}, {
  get(state, token) {
    // Before anything, we're going to try and access a variable
    if (tokens.length === 0 && state[token] !== undefined) {
      return state[token];
    }

    tokens.push(token);
    const rule = grammar[tokens[0]];
    if (rule) {
      if (tokens.length !== rule.length) {
        return dsl;
      }
      return rule({ dsl, state, tokens }, ...tokens.slice(1));
    }

    // No matching rule, just keep on chugging along
    return dsl;
  }
});

const grammar = {
  fizzz({ dsl, tokens }) {
    tokens.splice(0, tokens.length);
    dsl.log('Fizz');
    return dsl;
  },

  buzzz({ dsl, tokens }) {
    tokens.splice(0, tokens.length);
    dsl.log('Buzz');
    return dsl;
  },

  loop({ dsl, tokens }) {
    tokens.splice(0, tokens.length);

    return (predicate, body) => {
      while (predicate()) {
        body();
      }
      return dsl;
    };
  },

  iff({ dsl, tokens }) {
    tokens.splice(0, tokens.length);
    return (...args) => {
      for (let i = 0; i < args.length; i += 2) {
        let predicate = args[i];
        let body = args[i + 1];

        // If there's no body, this must be the `else` clause
        if (!body) {
          predicate();
          return dsl;
        }

        if (predicate()) {
          body();
          return dsl;
        }
      }
      return dsl;
    };
  },

  set({ dsl, state, tokens }, identifier) {
    tokens.splice(0, tokens.length);

    return (value) => {
      state[identifier] = value;
      return dsl;
    };
  },

  fizz({ dsl, tokens }, buzz) {
    if (buzz !== 'buzz') {
      throw new SyntaxError(`Expected "buzz", got "${buzz}"`);
    }
    tokens.splice(0, tokens.length);

    dsl.log('FizzBuzz');
    return dsl;
  },

  log({ dsl, tokens }) {
    tokens.splice(0, tokens.length);
    return (...args) => {
      console.log(...args);
      return dsl;
    };
  }
};

dsl.
  set.i(1).
  set.isFizz(x => x % 3 === 0).
  set.isBuzz(x => x % 5 === 0).
  loop(() => dsl.i <= 100, () =>
    dsl.
      iff(
        () => dsl.isFizz(dsl.i) && dsl.isBuzz(dsl.i), () =>
          dsl.fizz.buzz,
        () => dsl.isFizz(dsl.i), () =>
          dsl.fizzz,
        () => dsl.isBuzz(dsl.i), () =>
          dsl.buzzz,
        () => dsl.log(dsl.i)
      ).
      set.i(dsl.i + 1)
  );