JIT Mappers

JIT mappers are disabled by default. Enable them with the jit option:

import { drizzle } from 'drizzle-orm/...';

const db = drizzle({ client, jit: true });

On initialization, Drizzle runs a compatibility check that tests whether the Function constructor works in the current runtime. If it doesn’t (e.g. some edge runtimes or CSP-restricted environments), it falls back to regular mappers with a console warning.

The jit option is available in every driver’s config

What are JIT mappers?

JIT (Just-In-Time) mappers are dynamically generated JavaScript functions that transform database rows into JS objects. Instead of interpreting column metadata at runtime for every row, JIT mappers compile a specialized function once and reuse it for all rows in the result set.

Why are they needed?

The regular Drizzle ORM mapper loops through column metadata for every single row:

For each row:
  For each column:
    → look up decoder
    → check if codec exists
    → check if value is null
    → apply codec if needed
    → apply decoder if needed
    → navigate nested path
    → assign to result object

With thousands of rows, this interpretive overhead adds up. JIT mappers eliminate it by generating a function where all the metadata lookups, null checks, and nested object construction are hardcoded into the function body at generation time