Proposal: Replace internal fake patterns with a resolver function

[ST-DDT]

Continuation of/Requires: #2667, #2838

• Proposal: Design for standalone module functions #2667
• feat: introduce FakerCore #2838

Partially inspired by: #1832, #2664

• finance.iban does not create valid iban #1832
• Proposal: Use a single seed value per faker function invocation #2664

---

Clear and concise description of the problem

Glossary:

• fake patterns: String patterns used by faker.helpers.fake
• resolver functions: functions that take fakerCore as an argument and return a value based on a seed

Currently, some faker internal methods use fake patterns to generate some complex values based on certain parameters.
However parsing fake patterns is slow and error prone.

Suggested solution

Introduce a new helper method: faker.helpers.resolve(fns: ResolverFunction[])

type ResolverFunction<T> = (core: FakerCore) => T;

function resolve<T>(core: FakerCore, fns: ResolverFunction<T>[]): T{
    const fn = arrayElement(core, fns);
    return fn(core);
}

This could later be extended to support parameters as well.

Fake-Usage (OLD)

export const last_name_pattern = [
	'{{person.last_name}}',
	'{{person.last_name}}-{{person.last_name}}',
];

function lastName(core: FakerCore) {
	return fake(core, core.locale.person.last_name_pattern);
}

Resolve-Usage (NEW)

export const last_name_resolvers = [
	(core) => arrayElement(core, core.locale.person.last_name),
	(core) => arrayElement(core, core.locale.person.last_name) + '-' + arrayElement(core, core.locale.person.last_name),
];

function lastName(core: FakerCore) {
	return resolve(core, core.locale.person.last_name_resolvers);
}

Pros

• Compile time errors instead of runtime errors
• Supports auto completion when creating the resolver functions
• Significantly better performance
• No issues with json encoded parameters

Cons

• Breaking change for locale data
• Methods that use the new resolve patterns are harder to populate using a DB/file/string only store

Affected Methods

• company.name
• food.description
• food.dish
• location.zipCode
• location.city
• location.street
• location.streetAddress
• location.secondayAddress
• person.lastName
• person.bio
• person.jobTitle

And potentially:

• finance.iban
• ....

Alternatives

Alternatively, we could follow this pattern for each method individually.

export const last_name_resolvers = [
	(core) => arrayElement(core, core.locale.person.last_name),
	(core) => arrayElement(core, core.locale.person.last_name) + '-' + arrayElement(core, core.locale.person.last_name),
];

function lastName(core: FakerCore) {
    const lastNameResolver = arrayElement(core, core.locale.person.last_name_resolvers);
	return lastNameResolver(core);
}

[matthewmayer]

Hmm, i'm not really a big fan of this idea. I like the fact that the data sources for most methods are simple arrays. Also, I think looking at how Faker uses fake patterns internally is educational in figuring out how you might use it yourself.