Edit in JSFiddle

// eval part
var evaluate = function(x, env) {
  if (Array.isArray(x)) {
    if (x[0] === 'if') { // if というキーワードだった場合
      return String(evaluate(x[1], env)) !== 'false'
        ? evaluate(x[2], env)
        : evaluate(x[3], env);
    } else { // if ではない場合、今までどおりの処理を行う
      var exps = x.map(function(exp) {return evaluate(exp, env);});
      var proc = exps.shift();
      return proc.apply(null, exps);
    }
  } else {
    var y = env[x];
    return y !== undefined ? y : x;
  }
};
// library
var globalEnv = {
  '+': function(a, b) {return a + b;},
  '-': function(a, b) {return a - b;},
  '*': function(a, b) {return a * b;},
  '/': function(a, b) {return a / b;},
  'mod': function(a, b) {return a % b;},
  'not': function(x) {return !x;},
  '=': function(a, b) {return a === b;},
  '>': function(a, b) {return a > b;},
  '<': function(a, b) {return a < b;},
  '<=': function(a, b) {return a <= b;},
  '>=': function(a, b) {return a >= b;},
  'alert': function(a) {alert(a);}
};
// usage
d3.select('#out').text(
  evaluate(['if', ['<', 5, 6], 1, 2], globalEnv)
);

Result: <span id='out' />