A simple expression evaluator, inspired by MongoDB.

expression.js

// a simple expression evaluator
/*
{
  "or": [
    {"ne": ['foo', 'bar']}
  ]
}
*/

function evaluate(expression) {

  var ALL_PREDICATES = {},
    SIMPLE_PREDICATES = {},
    subPredicates = null,
    key = null,
    value = null,
    i = 0;

  // define all predicates

  var lt = function (a, b) {
    return a < b ? true : false;
  };

  var gt = function (a, b) {
    return a > b ? true : false;
  };

  var lte = function (a, b) {
    return a <= b ? true : false;
  };

  var gte = function (a, b) {
    return a >= b ? true : false;
  };

  var ne = function (a, b) {
    return a != b ? true : false;
  };

  var eq = function (a, b) {
    return a === b ? true : false;
  };

  var regex = function (pattern, a) {
    return new RegExp(pattern).test(a);
  };

  SIMPLE_PREDICATES.lt = lt;
  SIMPLE_PREDICATES.gt = gt;
  SIMPLE_PREDICATES.lte = lte;
  SIMPLE_PREDICATES.gte = gte;
  SIMPLE_PREDICATES.ne = ne;
  SIMPLE_PREDICATES.eq = eq;
  SIMPLE_PREDICATES.regex = regex;

  var evaluateSimplePredicate = function (predicate) {
    var property = null;

    if (typeof(predicate) === 'object') {
      for (property in predicate) {
        if (predicate.hasOwnProperty(property) && SIMPLE_PREDICATES[property]) {
          return SIMPLE_PREDICATES[property].apply(null, predicate[property]);
        }
      }
    }
 };

  for (key in expression) {
    if (expression.hasOwnProperty(key)) {
      // simple predicate ?
      if (SIMPLE_PREDICATES[key]) {
        return evaluateSimplePredicate(expression);
      }
      if (key === 'or') {
        subPredicates = expression[key] || [];
        for (i = 0; i < subPredicates.length; i += 1) {
          if (evaluate(subPredicates[i])) {
            return true;
          }
        }
      }
      if (key === 'and') {
        subPredicates = expression[key] || [];
        for (i = 0; i < subPredicates.length; i += 1) {
          if (!evaluate(subPredicates[i])) {
            return false;
          }
        }
        return true;
      }
    }
  }

}