Property | FactoryJS

Overwriting Property Values

To overwrite the property values of a factory, use .props. In the example below, the value of firstName is overwritten from John to Alice.

import { factory } from "@factory-js/factory";
 
const userFactory = factory.define({
  props: {
    firstName: () => "John",
    lastName: () => "Doe",
  },
  vars: {},
});
 
const user = await userFactory.props({ firstName: () => "Alice" }).build();
 
console.log(user); // 👉 { "firstName": "Alice", "lastName": "Doe" }

.props can be called multiple times in a method chain. If the same property value is overwritten multiple times, the last value takes precedence.

const user = await userFactory
  .props({ firstName: () => "Alice", lastName: () => "Smith" })
  .props({ firstName: () => "Bob" })
  .props({ firstName: () => "Tom" })
  .build();
 
console.log(user); // 👉 { firstName: "Tom", lastName: "Smith" }

⚠️

You cannot add new properties with .props. You can only overwrite the values of properties defined in .define. If you want to add properties, refer to Extending Factory.

Defining Dependent Properties

You can also define properties that reference the values of other properties. In the example below, the value of fullName is generated based on the values of firstName and lastName.

const userFactory = factory
  .define({
    props: {
      firstName: () => "John",
      lastName: () => "Doe",
      // Dependent properties cannot be defined within `.define`, so set an empty string initially
      fullName: () => "",
    },
    vars: {},
  })
  .props({
    fullName: async ({ props }) =>
      // Since there might be a Promise, you need to use `await` to get the property values
      `${await props.firstName} ${await props.lastName}`,
  });
 
// 👇 { fullName: "John Doe", firstName: "John", lastName: "Doe" }
console.log(await userFactory.build());

⚠️

Avoid creating properties that reference each other circularly, as this could result in an infinite loop.

Note that dependent properties cannot be defined within .define. Therefore, you need to set a dummy value such as an empty string initially and then overwrite the value later with .props. This constraint helps keep the object within .define simple, allowing TypeScript to correctly infer types.

FactoryJS provides a function called later to easily define placeholder. For more details, refer to Placeholder.

A convenient aspect of this feature is that it works correctly even if you change the value of a property later. For example, in the following example, the value of firstName is changed from John to Tom after setting the value of fullName, and the value of fullName is updated automatically.

const userFactory = factory
  .define({
    props: {
      firstName: () => "John",
      lastName: () => "Doe",
      fullName: "",
    },
    vars: {},
  })
  .props({
    fullName: async ({ props }) =>
      `${await props.firstName} ${await props.lastName}`,
  });
 
const user = await userFactory.props({ firstName: () => "Tom" }).build();
 
// 👇 { fullName: "Tom Doe", firstName: "Tom", lastName: "Doe" }
console.log(await userFactory.build());

Defining Factory Placeholder