simple visitor pattern implementation

visitor_pattern.js

var calculator = {
  add: function (node) {
    return visit(this, node.l) + visit(this, node.r);
  },
  sub: function (node) {
    return visit(this, node.l) - visit(this, node.r);
  },
  value: function (node) {
    return node.value;
  }
};
 
var dump = {
  add: function (node) {
    return visit(this, node.l) + ' + ' + visit(this, node.r);
  },
  sub: function (node) {
    return visit(this, node.l) + ' - '  +  visit(this, node.r);
  },
  value: function (node) {
    return String(node.value);
  }
};
 
var visit = function (visitor, node) {
  return visitor[node.operation].call(visitor, node);
}

var tree = {operation: 'add',
  l: {operation: 'value', value: 2},
  r: {operation: 'sub',
    l: {operation: 'value', value: 3},
    r: {operation: 'value', value: 1}
  }
};
 
console.log(visit(calculator, tree));
console.log(visit(dump, tree));