Iterating arrays, objects, & strings
Manipulating & testing values
Creating composite functions
They are both functional libraries. Lo-Dash is a fork of Underscore, and still follows Underscore’s API enough to allow it to serve as a drop-in replacement. But under the hood, it’s been completely rewritten, and it’s also added a number of features and functions that Underscore does not provide.
Lo-Dash was created to provide more consistent cross-environment iteration support for arrays, strings, objects, and argument objects. It has since become a superset of Underscore providing extra features (like AMD support, deep clone, and deep merge), better overall performance and optimizations for large arrays/object iteration, and more flexibility with custom builds and template pre-compilation utilities.