support dynamic defaults via registered callback functions

[eldridge]

Thank you for joi-json. This package is a missing piece of functionality to joi, in my humble opinion. The ability to define validation in data structures instead of raw code is a key draw to joi/joi-json.

Unfortunately, there are a few things that joi can do that joi-json cannot. This PR adds support for dynamic defaults by registering callback functions with the parser that can be invoked if a string is supplied as a default that contains enclosing parentheses.

For example, if we had a list of Express routes with separate validation profiles:

[
    {
        method: "SEARCH",
        url: "/author",
        validation: {
            page_size: "number:integer,min=1,max=1000,default=50"
        }
    },
    {
        method: "SEARCH",
        url: "/publication",
        validation: {
            page_size: "number:integer,min=1,max=1000,default=50"
        }
    }
]

What if we wanted "50" to be a global default defined in a central configuration using the config package, we could define a callback that would pull it from a global configuration:

const lodash = require("lodash");
const config = require("config");
const joijson = require("joi-json").builder();

joijson.registerFunction("config", path => lodash.get(config, path));

const schema = joijson.build({
    page_size: "number:integer,min=1,max=1000,default=config(app.default_page_size)"
});

Pretty powerful, yeah?

[richardhyatt]

I like it - could we extend the scope larger than just default values? Could we use a similar syntax to specify a set of properties that could be returned from the function to allow a more dynamic way to define more of the mundane validation properties? Should we also look at adding functionality like lodash's get() to make it easy to define path based values?