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HardPro challengeJavaScriptTypeScript

Transducer

Functional ProgrammingTransducersPerformance

A transducer is a composable, efficient transformation that decouples logic from data structure.
Instead of chaining .filter().map() (creates intermediate arrays), a transducer fuses the steps.

// Naive — creates two intermediate arrays
const result = [1, 2, 3, 4, 5]
  .filter(isEven)       // [2, 4]
  .map(double);         // [4, 8]

// Transducer — single pass, no intermediate arrays
const xf = compose(filterT(isEven), mapT(double));
transduce(xf, append, [], [1, 2, 3, 4, 5]); // [4, 8]

Implement:

  • mapT(fn) — a mapping transducer
  • filterT(pred) — a filtering transducer
  • transduce(xf, reducer, init, coll) — runs the transducer

Sample tests

Test #1filter(isEven) then map(double): [2,4] → [4,8]
Input: [[1,2,3,4,5],"filterMap"]
Output: [4,8]
Test #2map(addOne) then filter(isEven): [2,3,4,5,6] → [2,4,6]
Input: [[1,2,3,4,5],"mapFilter"]
Output: [2,4,6]
Test #3sum of evens: 2+4+6 = 12
Input: [[1,2,3,4,5,6],"sum"]
Output: 12
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