What's the correct way to merge two arrays in Javascript?

I've got two arrays (for example):

var a1 = [{ id : 1, name : test}, { id : 2, name : test2}]

var a2 = [{ id : 1, count : 1}, {id : 2, count : 2}]

I want to be able to end up with something like:

var a3 = [{ id : 1, name : test, count : 1}, 

          { id : 2, name : test2, count : 2}]

Where the two arrays are being joined based on the 'id' field and extra data is simply being added.

I tried to use _.union to do this, but it simply overwrites the values from the second array into the first one