Logging

It is possible to set up logs for all queries using the log property when instantiating Kysely.

There are 2 ways to configure logging:

1. Provide an array with log level/s

You can provide an array of log levels to the log property when instantiating Kysely.

When 'query' is included in the array, Kysely will log all executed queries, not including parameter values. When 'error' is included in the array, Kysely will log all errors.

const db = new Kysely({
  ...
  log: ['query', 'error']
});

2. Provide a custom logging function

You can provide a custom logging function to the log property when instantiating Kysely. The custom logging function receives a log event as an argument.

The LogEvent interface is defined as follows:

interface LogEvent {
    level: 'query' | 'error';
    query: CompiledQuery; // this object contains the raw SQL string, parameters, and Kysely's SQL syntax tree that helped output the raw SQL string.
    queryDurationMillis: number; // the time in milliseconds it took for the query to execute and get a response from the database.
    error: unknown; // only present if `level` is `'error'`.
}

you can also use the function signature to have more flexibility on how you want to log the queries.

const db = new Kysely({
  dialect: new PostgresDialect(postgresConfig),
  log(event) {
    // you may want to log only the errored query :
    if (event.level === "error") {
        console.error("Query failed : ", {
          durationMs: event.queryDurationMillis,
          error: event.error,
          sql: event.query.sql,
          params: event.query.parameters,
        });
      }

    // or log all queries :
    console.log("Query executed : ", {
      durationMs: event.queryDurationMillis,
      sql: event.query.sql,
      params: event.query.parameters,
    });
  }
})

For more information check the docs for details on the interfaces KyselyConfig.