Guide
Sequence

Generating Sequential Values

To generate sequential values, you can use seq. The first argument is the initial value, and the second argument is a function that specifies how to generate the values.

import { factory, seq } from "@factory-js/factory";
 
const userFactory = factory.define({
  props: {
    id: seq(1, (n) => n),
    name: seq(1, (n) => `user-${n}`), // Can generate non-numeric values as well
  },
  vars: {},
});
 
// 👇 [{ id: 1, name: "user-1" }, { id: 2, name: "user-2" }]
console.log(await userFactory.buildList(2));

You can also reference variables and other properties within seq.

import { factory, seq } from "@factory-js/factory";
 
const userFactory = factory
  .define({
    props: {
      key: later<string>(),
      name: () => "John",
    },
    vars: {
      role: () => "admin",
    },
  })
  .props({
    key: seq(
      1,
      async (n, { props, vars }) =>
        `${await props.name}:${await vars.role}:${n}`,
    ),
  });
 
// 👇 [{ id: 1, name: "John:admin:1" }, { id: 2, name: "John:admin:2" }]
console.log(await userFactory.buildList(2));

Note that test libraries like Vitest and Jest run tests in multiple threads by default, so the values output by seq may not be unique across threads. If you use seq to avoid unique constraints in the database, consider generating unique values by combining it with VITEST_WORKER_ID (opens in a new tab).

import { factory, seq } from "@factory-js/factory";
 
// `VITEST_WORKER_ID` has a different value for each thread
const workerId = process.env["VITEST_WORKER_ID"];
 
const userFactory = factory.define({
  props: {
    email: seq(1, (n) => `test-${n}-${workerId}@example.com`),
  },
  vars: {},
});
 
// 👇 [{ email: `test-1-1@example.com` }, { email: `test-2-1@example.com` }]
console.log(await userFactory.buildList(2));
VariableAfter Hook