Difference between _.forEach and _.forOwn in lodash

What is the difference between these two methods when iterating over an object?

The difference lies in that if the collection over which you are iterating is an object which has a length property, then the _.forEach() will iterate over it as if it were an array, whereas the _.forOwn() will iterate over it like an object.

Assume you have the object:

a = {
  x: 100, 
  y: 200, 
  length: 2
}

If you iterate over it as:

_.forEach(a, function(val, key) {
  console.log('a[' + key + '] = ' + val); 
});

you'll get output:

a[0] = undefined
a[1] = undefined 

whereas iterating over it with _.forOwn() you'll get the more reasonable:

a[x] = 100
a[y] = 200
a[length] = 2