Question

I want to copy all attributes from backbone.js Model object to another if they are not undefined. It’s a little like a child overriding the attributes of a parent. There will be some attributes of the child object which should be excluded from the entire process. This is what I have so far:

function inherit(parent, child) {
    var childAttributes = Object.keys(child.attributes);

    // Don't involve these attributes in any way, they're special.
    attributes = _.without(attributes, ['specialCase1', 'specialCase2', 'specialCase3']);

    // This array will store undefined child attributes
    var undefChildAttributes = new Array();

   for (i in attributes) {
          var attr = attributes[i];           // Get attribute name
          var value = child.get(attr);        // ... and value
          var type = RealTypeOf(value);       // ... and type

          if (type == 'undefined') {
                 undefChildAttributes.push(attr);
          }       
   }

   // Update the child attributes array to not include undefined any attributes
   attributes = _.without(attributes, undefChildAttributes);

   // Copy any attributes from child to parent

   // ------------------ PROBLEM AREA ------------------
   for (var m=0; m<attributes.length; m++) {
          var attr = attributes[m];                // Attribute to copy

          // I want to use _.extends in some way here, maybe like:
          _.extends(parent, child[attr]);
   }
}

I’m stuck at the point of how to copy the attribute into the parent. The only way I have atm is a hack:

if (type == 'string') {
    eval('var o = {' + attr + ':"' + child.get(attr) + '"};');
 } else {
    eval('var o = {' + attr + ':' + child.get(attr) + '};');
}

parent.set(o);

Answer 1

You can avoid eval by using the bracket notation:

var o = {};
o[attr] = child.get(attr);
parent.set(o);

And my take on your code, if I got it right

function inherit(parent, child) {
    var attributes = child.toJSON(),
        keepkeys = _.keys(attributes);

    // remove special cases
    keepkeys = _.without(keepkeys, 'specialCase1', 'specialCase2');
    // or keepkeys = _.difference(keepkeys, ['specialCase1', 'specialCase2']);

    // remove undefined values
    keepkeys = _.filter(keepkeys, function (key) {
        return typeof(attributes[key])!=="undefined";
    });

    // with underscore>=1.3.3
    var result = _.pick(attributes, keepkeys);

    // with underscore<1.3.3
    // var result = {};
    // _.each(keepkeys, function (key) {
    //    result[key] = attributes[key];
    // });

    parent.set(result);
}

var P = new Backbone.Model();
var C = new Backbone.Model({
    copy1: "c1", 
    specialCase1: "do not copy", 
    undefkey: undefined
});

inherit(P, C);
console.log(P.toJSON());

And a Fiddle http://jsfiddle.net/tdCEF/