Similar intention as Underscore's _.defaults() algorithm, but with the ability to go deep into objects. Function will error out if Objects have circular references. Dependencies: UnderscoreJS

deep-defaults.js

function deepDefault(obj, source, depth, blankCheck){
  if (depth===undefined){
    depth=0;
  }
  if (depth>1000){
    throw new Error('Too many iterations, possible circular reference');
    return;
  }
  _.each(source, function(sourceVal, i){
    var val = obj[i];
    if (_.isFunction(val)){
      // do nothing
    }
    else if (_.isObject(val)){
      if( _.isObject(sourceVal) && !_.isFunction(sourceVal) ){
        deepDefault(val, sourceVal, depth+1, blankCheck);
      }
    }
    else if (val===undefined){
      if( _.isObject(sourceVal) && !_.isFunction(sourceVal) ){
        obj[i]={};
        deepDefault(obj[i], sourceVal, depth+1, true);
      }
      else{
        obj[i] = sourceVal;
      }
    }
  })
}

var a = {dog:'woof', cat:{sleepy:'purr'} };
var b = {mouse:'squeak', cat:{awake:'meow'}};
deepExtend(a,b);

console.log(a);

// {dog:'woof', cat:{sleepy:'purr', awake:'meow'}, mouse:'squeak'}

Had to be careful with the functions that look like objects!