The lodash _.partial method in lodash can be used to create a new function from another function and some starting arguments. In other words it can be used to create a simplified function that only accepts a few arguments that will be used with some set static values when using another method that accepts more arguments. Simply put it is a way to create an abstraction, or an alternative interface for a complex method that accepts many arguments.
There are a number of ways to go about creating simplified methods that call other methods that are a little more complex and take many arguments. One way of doing so would be to just write a new function that calls the other function with certain arguments set to whatever static values that are to be used, or parse whatever values need to be pares for from what was given. Yet another way to deal with abstractions is to just not use them at all to begin with actually, and just work with functions that take many arguments just design the argument in such a way so that they take an options object rather than a set of arguments and have it so the function will just go with certain hard coded default values.
For a basic example of _.partial I made this quick example that involves a method that accepts two arguments a, and b called foo. I then create a new method called bar with _.partial passing foo as the first argument to _.partial, and then a number literal of the value 40. The bar method will then me a method that works just like foo only the a argument is fixed at the value of 40, and the first argument of the resulting bar method will be the value for b in the foo method.
For a more complex example of how this comes in handy sometimes say I have a method that takes a lot of arguments. For example a 3d distance formula that will except six arguments for two points that exist in 3d space. So I want a method that uses this distance method only with one point fixed at the origin call fromOrgin, so that it only takes three arguments.
Place holders can be used to set what arguments will be accepted from the resulting new method, and what arguments will be fixed. So if I want a method where the first argument is to be what is accepted by the new method and the others are fixed that ca be done with place holders.