Forth-like DSL for JavaScript

forj.js

// Copyright 2020 Dmitriy Kubyshkin
// MIT license

const forj = () => {
  const stack = [];
  const controlStack = [1];

  const last = (stack) => stack.length && stack[stack.length - 1];

  function interpreter(arg) {
    if (last(controlStack) == 1) {
      stack.push(arg);
      //console.log("S:", stack);
    }
    return interpreter;
  };

  function define(name, getter) {
    Object.defineProperty(interpreter, name, {
      get() {
        if (last(controlStack) == 1) {
          //console.log("S:", stack);
          getter.call(this);
        }
        return this;
      }
    });
  }

  function directive(name, getter) {
    Object.defineProperty(interpreter, name, {
      get() {
        getter.call(this);
        return this;
      }
    });
  }

  define("print", () => console.log(stack.pop()));
  define("plus", () => stack.push(stack.pop() + stack.pop()));
  define("mul", () => stack.push(stack.pop() * stack.pop()));
  define("lt", () => stack.push(stack.pop() > stack.pop()));
  define("eq", () => stack.push(stack.pop() === stack.pop()));

  define("dup", () => stack.push(last(stack)));
  define("drop", () => stack.pop());

  interpreter.def = function(name, body) {
    define(name, () => {
      body(interpreter);
    });
    return this;
  };

  const loopStack = [];

  interpreter.LOOP = function(body) {
    loopStack.push(body);
    while (last(loopStack) === body) {
      body(interpreter);
    }
    return this;
  };

  define("CBREAK", () => {
    if (stack.pop()) loopStack.pop();
  });

  directive("IF", () => {
    //console.log("C:", controlStack);
    //console.log("S:", stack);
    if (last(controlStack)) {
      controlStack.push(Number(stack.pop()));
    } else {
      controlStack.push(-1);
    }
  });
  directive("ELSE", () => {
    if(last(controlStack) == -1) return;
    controlStack.push(Number(!controlStack.pop()));
  });
  directive("THEN", () => {
    controlStack.pop();
  });

  return interpreter;
};

forj()
  (1) .LOOP(_ => _
    .dup.print
    (1).plus
    .dup
    (11).eq
    .CBREAK
  )

forj()("").print;

forj()
  .def("square", _ => _
    .dup
    .mul
  )
  (9).square.print

forj()("").print;

forj()
(2)(3)(4).plus.mul.print
;

forj()("").print;

forj()
(2)(10).lt .IF
  (5)(10).lt .IF
    ("Yes").print
  .THEN
.ELSE
  (5)(10).lt .IF
    ("No").print
  .THEN
.THEN
;