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Kysely

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Kysely is a type-safe and autocompletion-friendly TypeScript SQL query builder.

It lets developers build SQL queries with a fluent API. It also helps to detect errors in queries by leveraging TypeScript.

A Little Case

In the following example, we use the Kysely query builder to fetch a user from the database:

import { Kysely, PostgresDialect } from 'kysely';


interface User {
  id: number;
  name: string;
  email: string;
}


interface Post {
  id: number;
  title: string;
  content: string;
  authorId: number;
}


interface DB {
  users: User;
  posts: Post;
}


const db = new Kysely < DB > ({
  dialect: new PostgresDialect({
    pool: new Pool({
      host: process.env.DATABASE_HOST,
      database: process.env.DATABASE_DB,
      user: process.env.DATABASE_USER,
      password: process.env.DATABASE_PASSWORD,
      port: Number(process.env.DATABASE_PORT),
    }),
  }),
});


const user = db
  .selectFrom('users')
  .where('id', 1)
  .select(['id', 'name', 'email'])
  .executeTakeFirst();

Kysely uses the DB interface to type the result of the query. As a consequence, TypeScript infers the user variable as User | undefined (because the query may return an empty result).

Kysely helps to type the query result, and also the query itself. For example, if we try to select a column foo that does not exist in the user type, we will have a TypeScript error:

const user = db
  .selectFrom('users')
  .where('id', 1)
  // Type Error: Argument of type 'string[]' is not assignable to parameter of type 'SelectExpression<DB, "users">'.
  .select(['id', 'name', 'email', 'foo'])
  .executeTakeFirst();

The error message is not very clear, but it helps to detect the bug and prevent runtime errors.

More Features In The Video

Useful Resources