Data Types

Kysely deals with two distinct types of data types:

1. TypeScript types (compile-time)
2. JavaScript types (runtime)

TypeScript Types

• TypeScript types are compile-time only
• They don't affect runtime behavior
• You must manually align them with what your database driver returns
• Example: If you define a column as string but the database returns a number, you'll see:
  • TypeScript shows: string
  • Runtime receives: number

JavaScript Runtime Types

• Determined by your database driver (pg, mysql2, etc.)
• Kysely doesn't modify the data returned by drivers
• Exception: Plugins like CamelCasePlugin may modify column names

Configuring Runtime Types

PostgreSQL (pg driver)

Use pg-types to configure return types:

import { Kysely, PostgresDialect } from 'kysely'
import * as pg from 'pg'

// Configure bigint to return as number
const int8TypeId = 20
pg.types.setTypeParser(int8TypeId, (val) => parseInt(val, 10))

const db = new Kysely<Database>({
  dialect: new PostgresDialect({
    pool: new pg.Pool(config),
  }),
})

MySQL (mysql2 driver)

Use the typeCast pool property:

import { Kysely, MysqlDialect } from 'kysely'
import { createPool } from 'mysql2'

const db = new Kysely<Database>({
  dialect: new MysqlDialect({
    pool: createPool({
      ...config,
      // Convert tinyint(1) to boolean
      typeCast(field, next) {
        if (field.type === 'TINY' && field.length === 1) {
          return field.string() === '1'
        }
        return next()
      },
    }),
  }),
})

Automatic Type Generation

Third-party tools can generate TypeScript types from your database schema:

• kysely-codegen
• kanel-kysely

Note: These are external tools - refer to their documentation for issues with generated types.

Best Practices

1. Always check your database driver's documentation for default type mappings
2. Test runtime types in development to ensure TypeScript types match
3. Use type generators to maintain type consistency
4. Configure runtime type casting at the driver level, not in application code