webpack-merge

webpack-merge provides a merge function that concatenates arrays and merges objects creating a new object. If functions are encountered, it will execute them, run the results through the algorithm, and then wrap the returned values within a function again.

This behavior is particularly useful in configuring webpack although it has uses beyond it. Whenever you need to merge configuration objects, webpack-merge can come in handy.

There's also a webpack specific merge variant known as merge.smart that's able to take webpack specifics into account (i.e., it can flatten loader definitions).