costco.js

var costco = function() {
  
  function evalFunction(funcString) {
    try {
      eval("var editFunc = " + funcString);
    } catch(e) {
      return {errorMessage: e+""};
    }
    return editFunc;
  }
  
  function previewTransform(docs, editFunc, currentColumn) {
    var preview = [];
    var updated = mapDocs($.extend(true, {}, docs), editFunc);
    for (var i = 0; i < updated.docs.length; i++) {      
      var before = docs[i]
        , after = updated.docs[i]
        ;
      if (!after) after = {};
      if (currentColumn) {
        preview.push({before: before[currentColumn], after: after[currentColumn]});      
      } else {
        preview.push({before: before, after: after});      
      }
    }
    return preview;
  }

  function mapDocs(docs, editFunc) {
    var edited = []
      , deleted = []
      , failed = []
      ;
    
    var updatedDocs = _.map(docs, function(doc) {
      try {
        var updated = editFunc(_.clone(doc));
      } catch(e) {
        failed.push(doc);
        return;
      }
      if(updated === null) {
        updated = {_deleted: true};
        edited.push(updated);
        deleted.push(doc);
      }
      else if(updated && !_.isEqual(updated, doc)) {
        edited.push(updated);
      }
      return updated;      
    });
    
    return {
      updates: edited, 
      docs: updatedDocs, 
      deletes: deleted, 
      failed: failed
    };
  }
  
  return {
    evalFunction: evalFunction,
    previewTransform: previewTransform,
    mapDocs: mapDocs
  };
}();