Kysely brings compile-time type safety to SQL query building without hiding SQL behind an abstraction layer. Test your understanding of Kysely's type model, plugins, and migration system.
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What is Kysely's main advantage over writing raw SQL strings in TypeScript?
Kysely's type safety means that if you reference a column that doesn't exist in your schema definition, TypeScript shows a type error before you run the code. Table names, column names, join conditions, and return types are all checked at compile time — catching SQL errors that raw template literals would only reveal at runtime.
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How do you provide your database schema types to Kysely?
Kysely requires you to provide a Database interface: interface Database { users: UsersTable; posts: PostsTable; } where each table maps to an interface defining its columns and types. You pass this to new Kysely<Database>({ dialect }) and all queries are then type-checked against it.
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What does the CamelCasePlugin in Kysely do?
The CamelCasePlugin handles the common mismatch between JavaScript naming (camelCase) and PostgreSQL column naming (snake_case). With it enabled, you write { firstName: 'John' } in TypeScript and Kysely maps it to the first_name column in SQL automatically — in both read and write directions.
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How does Kysely support database migrations?
Kysely's Migrator runs migration files that export up and down functions. Each function receives a Kysely instance (via db.schema) to create/alter/drop tables. A lock-protected migrations table tracks which migrations have run, ensuring they execute once and in order across environments.
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What is Kysely's plugin system designed for?
Kysely plugins implement the KyselyPlugin interface with transformQuery and transformResult hooks. They intercept the compiled query node tree or raw results, enabling cross-cutting concerns like the CamelCasePlugin (transforming column names), soft-delete plugins (adding WHERE deleted_at IS NULL automatically), or custom logging.