Terrible Forth in JS

forth.js

var testScript = `
[ double dup + ]
[ quadruple double double ]
1 2 + print
double print
quadruple print
2 jmp
3 +
4 +
if 255 print 0 then
if 10 print then
`;

var Forth = function() {
  this.stack = [];
  this.dictionary = Object.create(Forth.BASE);
  this.memory = new Array(100);
  this.mode = Forth.modes.READING;
  this.pc = 0;
  this.code = [];
};

Forth.modes = {
  READING: 0,
  COMPILING: 1
};

Forth.BASE = {
  print: function(self) {
    console.log(self.top());
  },
  jmp: function(self) {
    var amount = self.stack.pop();
    self.pc += amount;
  },
  "+": function(self) {
    var [a, b] = self.pop(2);
    self.stack.push(a + b);
  },
  "-": function(self) {
    var [a, b] = self.pop(2);
    self.stack.push(a - b);
  },
  dup: function(self) {
    var val = self.stack.pop();
    self.stack.push(val, val);
  },
  noop: [],
  then: [],
  if: function(self) {
    var then = self.code.indexOf("then", self.pc);
    var condition = self.stack[self.stack.length - 1];
    if (condition === 0) {
      self.pc = then;
    }
  }
}

Forth.prototype = {
  top: function() {
    return this.stack[this.stack.length - 1];
  },
  pop: function(length) {
    var vals = [];
    for (var i = 0; i < length; i++) {
      vals.unshift(this.stack.pop());
    }
    return vals;
  },
  run: function(word) {
    if (typeof word == "function") {
      word(this);
    } else if (word instanceof Array) {
      word.forEach(w => this.word(w));
    } else {
      throw word
    }
  },
  word: function(word) {
    word = this.cast(word);
    var lookup = this.dictionary[word];
    if (lookup) {
      this.run(lookup);
    } else if (this.isLiteral(word)) {
      this.stack.push(word);
    } else {
      throw "Unrecognized word: " + word
    }
  },
  evaluate: function(code) {
    this.pc = 0;
    this.code = code.trim().split(/\s+/g);
    var buffer = [];
    for (; this.pc < this.code.length; this.pc++) {
      var w = this.code[this.pc];
      if (w == "[") {
        this.mode = Forth.modes.COMPILING;
      } else if (w == "]") {
        this.mode = Forth.modes.READING;
        if (buffer.length) {
          var name = buffer.shift();
          this.dictionary[name] = buffer;
          buffer = [];
        }
      } else {
        if (this.mode == Forth.modes.COMPILING) {
          buffer.push(w);
        } else {
          this.word(w);
        }
      }
    }
  },
  isLiteral: function(word) {
    return typeof word == "number";
  },
  cast: function(word) {
    if (typeof word == "string") {
      word = word.trim();
      if (word.match(/^\d+$/)) {
        return parseFloat(word);
      }
    }
    return word;
  }
};

var f = new Forth();
f.evaluate(testScript);