postfix_js_thing.js

var functions = {
  "+": {
    argc: 2,
    func: function (a, b) {
      return a + b
    }
  },

  "-": {
    argc: 2,
    func: function (a, b) {
      return a - b
    }
  },

  "*": {
    argc: 2,
    func: function (a, b) {
      return a * b
    }
  },

  "/": {
    argc: 2,
    func: function (a, b) {
      return a / b
    }
  },

  "**": {
    argc: 2,
    func: function (a, b) {
      return Math.pow(a, b)
    }
  }
}

function execute (input, funcs) {
  return input.reduce(
    function (stack, curr) {
      if (curr in funcs) {
        return stack.concat(
          funcs[curr].func.apply(
            this, 
            stack.splice(-(funcs[curr].argc), funcs[curr].argc)
          )
        )
      } else {
        return stack.concat(curr)
      }
    },
    []
  )
}

function compile (stack, functions) {
  stack = stack.reduce(
    function (acc, curr) {
      if (curr === "defun") {
        var name = acc.pop(),
            argc = acc.pop(),
            funcCount = acc.pop(),
            funcs = acc.splice(-funcCount, funcCount);
        
        functions[name] = {
          argc: argc,
          func: function () {
            return run(Array.prototype.slice.call(arguments).concat(funcs))
          }
        }
        
        return acc;
      } else {
        return acc.concat(curr)
      }
    },
    []
  );
  return [stack, functions];
}

function run (str) {
  return execute.apply(
    this,
    compile(str, functions)
  )
}