factory.ts

A library to ease creation of factories for test data for Typescript

Given an interface or type definition, create a factory for generating test data. Values for each key may be defaulted or be calculated each time based on a sequence number and the values for other keys.

Example

Interface

interface Person {
  id: number
  firstName: string
  lastName: string
  fullName: string
  age: number
}

Basic factory

import * as Factory from 'factory.ts'

const personFactory = Factory.makeFactory<Person>({
  id: Factory.each(i => i),
  firstName: 'Bob',
  lastName: 'Smith',
  fullName: 'Robert J. Smith, Jr.',
  age: Factory.each(i => 20 + (i % 10))
});

For each property of Person, you can specify a default value, or call Factory.each. Factory.each takes a lambda with a sequence number that is incremented automatically between generating instances of your type (Person in our example).

You can call personFactory.build with a subset of field data (Partial<Person>) to override defaults, and the output will be an object that conforms to Person using the definition specified in makeFactory.

const james = personFactory.build({firstName: 'James', fullName: 'James Smith'});
// { id: 1, firstName: 'James', lastName: 'Smith', fullName: 'James Smith', age: 21 };

const youngBob = personFactory.build({age: 5});
// { id: 2, firstName: 'Bob', lastName: 'Smith', fullName: 'Robert J. Smith, Jr.', age: 5 };

Extending factories

Occasionally you may want to extend an existing factory with some changes. For example, we might want to create a personFactory that emits a totally random age range:

const anyAgeFactory = personFactory.extend({
  age: Factory.each(() => randomAge(0,100)), // randomAge(min:number, max:number) => number
});

anyAgeFactory.build(); // { id: 1, ..., age: <random value> };

Extending a Factory creates a new, immutable Factory. Your initial factory remains unchanged.

Derived values

One specific way to extend an existing factory is to make a new factory where one of the keys/properties is determined by other properties. For example. we can use this to specify fullName from firstName and lastName:

const autoFullNameFactory = personFactory.withDerivation2(['firstName', 'lastName'], 'fullName', 
  (fName, lName) => `${lName}, ${fName} ${lName}`);

const jamesBond = autoFullNameFactory.build({ firstName: 'James', lastName: 'Bond' });
// { id: 1, firstName: 'James', lastName: 'Bond', fullName: 'Bond, James Bond', age: 21 };

The withDerivation*N* functions consume an array of dependent key names (of length N), then the name of the property to define, then a lambda that takes the appropriate types for arguments based on the values of the dependent keys, and expects a return type that matches the derived key value type. withDerivation1 through withDerivation5 are provided. Ideally these would be overrides, but I have yet to figure out a way to make the Typescript compiler accept this.

Note that any misspelling of dependent or derived key names in a call to withDerivation*N* will result in a compile error - aren't mapped types the best?

Alternatively, if you need to read more than 5 properties, or just don't want to specify dependencies explicitly, withDerivation expects a property key to derive and a lambda that goes from a value of the overall type being built to a value of the type of the dependent property. For our fullName case that would be:

const autoFullNameFactory = personFactory.withDerivation('fullName', (person) => 
  `${person.lName}, ${person.fName} ${person.lName}`);

Personally I prefer to be explicit about the dependent keys, but it doesn't really matter.

Derivations are processed in the order they are defined, and all withDerivation functions produce a new immutable Factory.