A function domain as it some times might be called is the full range of arguments that are possible for a given function. So say I have a function that accepts a single argument that represents a singe side of a six sided die, in that case the range for the one argument would be the whole numbers 1 threw 6. However many functions will have a very wide range for an argument, and on top of that float numbers can be used. ALso often a function will have more than one argument, on top of having a wide range for one or more arguments. This can result in w very wide range of possibles for the domain of a function, making it hard to create a way to graph all possibles, or run threw all possible combinations of calls to make sure the function will always work as expected for all possible input values.
Lets say I just have a very simple function that will just take the numbers 0 there 10 as the first and only argument. On top of that the numbers that are expected are whole numbers so that means that the full function domain can just be a small array of numbers. So for this example it is possible to have a create domain function that will create an array of all possible values for the function. I can then use the array map method and call the function for each value in the domain array to get all possible return values for the domain also while I am at it.
The domain of a function is something that I want to run threw when it comes to testing a function that I wrote to make sure that I get experted results for all possible calls. However some times it might not be possible to do so, or it would require a whole lot of processing power and time to do so at least. So some times I test out a function by just going threw part of a range, or by a certain stepping value.
Often it might be a good idea to graph output, or create some kind of visual aid as a way to make sure that a function is working the way that it should for at least some kind of domain. Typically there is creating a simple graph using a canvas element for example.