Edit the lodash.js file to do your work.
Use live-server to see the unit tests as they run.
If you're feeling comfortable using some of the lodash methods, see if you can program your own javascript utility functions. https://github.com/DevMountain/utilities
This utilities repo has you implement some of the functionality that can be found in the underscorejs library. Underscore is a library that is similar to lodash. Its documentation can be found here