better validation with higher-order functions

update I found this very nice library Superstruct which accomplishes everything & more what I describe in this blog post - it's pretty sweet :)

i've recently been faced with an issue of needing to validate a lot of different types of data-structures, including nested & arrays of things in express - which is a totally not nice - not in terms of any kind of techincal complexity but simply sheer amount of duplication.

as an ardent proponent of the DRY principle i wielded my FP power and spent a couple of days creating a new way of concisely expressing validation in a way that no longer makes me want to die :)

/** let d equal to:
* {
* iso_country: "GBR",
* social_info: {
* linkedin_url: "",
* facebook_url: "",
* instagram_url: "",
* }
* }

return await object(d, {
iso_country: IsISO31661Alpha3,
social_info: (v) =>
linkedin_url: IsUrl,
facebook_url: IsUrl,
instagram_url: IsUrl,
.withMessage("Some part of your social info is wrong!"),

which will return all the relevant errors in a nested data-structure as-in in the object.

as well as being able to be run imperatively, one can also use it as middleware against request body, queries & params;

validators: [
step: (v) => v.exists().toInt().isIn([1,2,3,4,5]),
// self reference if body is an array and has no field accessor
__this: v => v.isArray().custom(array({
// validating arrays of objects
param: IsString,
message: IsString,

another feature is composing validators, say for example validating an address, one could make something like:

type ObjectValidator<T> = { [index in keyof T]: CustomValidator };

const IAddress = (): ObjectValidator<Idless<IAddress>> => {
return {
city: (v) => IsString(v, "Must provide a city"),
iso_country_code: ISOCountry,
postcode: Postcode,
street_name: (v) => IsString(v, "Must provide a street name"),
street_number: (v) => IsInt(v, "Must provide a street number"),

and then use them in other validators;

await object(d, {
address: (v) => v.custom(single(IAddress())),