If your browser does not support ECMAScript 6, you can use
>>> a.push.apply(a, b)
Or perhaps, if you think it's clearer:
Please note that all these solutions will fail with a stack overflow error if array
b is too long (trouble starts at about 100,000 elements, depending on the browser).
If you cannot guarantee that
b is short enough, you should use a standard loop-based technique described in the other answer.