FP #2 — transduce() — Map + Filter in One Pass

Normally, arr.map(f).filter(g) creates two intermediate arrays. A transducer fuses map and filter into a single reduction pass — composing transformers that run per-element without intermediate collections. This article builds transducers from scratch.

Prerequisites: FP #1 — compose/pipe

1. The Problem — Intermediate Arrays

For 10 items, no problem. For 10 million items, 3 intermediate allocations matter.

2. Transducer Building Blocks — Reducing Functions

A transducer is a function that transforms one reducer into another reducer:

3. Putting It All Together

4. Visual Comparison

5. Complete Minimal Transducer Library

Key Takeaways

• A transducer is (reducer) → reducer — it transforms how you accumulate.
• mapping(fn) → applies fn before passing to the next reducer.
• filtering(pred) → skips items that don't pass before passing to next reducer.
• Composition of transducers happens at the reducer level — items flow through all transforms in a single pass.
• Transducers are the foundation of Clojure's sequence operations and are available in JS via Ramda and other FP libraries.

Next: FP #3 — Maybe & Either Monads — build error-safe pipelines without null checks or try/catch.