stan-js utils

This package is UNOFFICIAL set of utils for use with stan-js.

withMigrations

This is small util that helps to maintain backward compatibility of state when persisting it e.g. in localStorage. It accepts one parameter - configuration object with few params:

• version - this is current version of your state, you should increase it each time you change your state in non-backwards compatible way
• migrations - object with state migrations, each key is version number and value is function which will get old state and should return migrated state
• serialize/deserialize - custom (de)serializer, same as in stan-js

Small example

Let's say you stored user info in store and persisted it in localStorage:

type UserData = {
    username: string
}

const store = createStore({
  user: storage<UserData | null>(null, {
    ...withMigrations({
      version: 0,
      migrations: {},
    }),
  }),
})

Then, after some time you decide that you also want to have user's name in that state. You now have to do two steps:

1. increase state's version
2. add migration to new shape

const userDataV0Schema = z.object({
  username: z.string(),
})

type UserData = {
  username: string
  name: string
}

const store = createStore({
  user: storage<UserData | null>(null, {
    ...withMigrations({
      version: 1,
      migrations: {
        1: (prev: unknown) => {
          const { success, data } = userDataV0Schema.safeParse(prev)
		      return success ? { ...data, name: data.username } : null
		    },
	    },
	  }),
  }),
})

In above example we declare migration from version 0 to 1 which is using zod to parse data and add new property. In the future if you change state's shape again you will have to repeat those steps.

Let's add age property:

const userDataV0Schema = z.object({
  username: z.string(),
})

const userDataV1Schema = userDataV0Schema.extend({
  name: z.string(),
})

type UserData = {
  username: string
  name: string
}

const store = createStore({
  user: storage<UserData | null>(null, {
    ...withMigrations({
      version: 2,
      migrations: {
        1: (prev: unknown) => {
          const { success, data } = userDataV0Schema.safeParse(prev)
          return success ? { ...data, name: data.username } : null
        },
        2: (prev: unknown) => {
          const { success, data } = userDataV1Schema.safeParse(prev)
          return success ? { ...data, age: 0 } : null
        },
      },
    }),
  }),
})