feat(transducers): add noop() xform, update readme

packages/transducers/README.md

@@ -519,11 +519,12 @@ is done dynamically via for loop.
 
 #### `compR(rfn: Reducer<any, any>, fn: (acc, x) => any): Reducer<any, any>`
 
-Helper function to build reducers.
+Helper function to compose reducers.
 
 #### `iterator<A, B>(tx: Transducer<A, B>, xs: Iterable<A>): IterableIterator<B>`
 
-Similar to `transduce()`, but emits results as ES6 iterator (and hence doesn't use a reduction function).
+Similar to `transduce()`, but emits results as ES6 iterator (and hence doesn't
+use a reduction function).
 
 #### `reduce<A, B>(rfn: Reducer<A, B>, acc: A, xs: Iterable<B>): A`
 
@@ -596,6 +597,8 @@ reducer and optional initial accumulator/result.
 
 #### `multiplexObj<A, B>(xforms: IObjectOf<Transducer<A, any>>, rfn?: Reducer<B, [PropertyKey, any]>): Transducer<A, B>`
 
+#### `noop<T>(): Transducer<T, T>`
+
 #### `padLast<T>(n: number, fill: T): Transducer<T, T>`
 
 #### `partition<T>(size: number, step?: number, all?: boolean): Transducer<T, T[]>`

packages/transducers/src/index.ts

@@ -57,6 +57,7 @@ export * from "./xform/mapcat";
 export * from "./xform/moving-average";
 export * from "./xform/multiplex";
 export * from "./xform/multiplex-obj";
+export * from "./xform/noop";
 export * from "./xform/pad-last";
 export * from "./xform/partition-by";
 export * from "./xform/partition-of";

packages/transducers/src/xform/noop.ts

@@ -0,0 +1,10 @@
+import { Transducer } from "../api";
+
+/**
+ * No-op / pass-through transducer, essentially the same as:
+ * `map(identity)`, but faster. Useful for testing and/or to
+ * keep existing values in a `multiplex()` tuple lane.
+ */
+export function noop<T>(): Transducer<T, T> {
+    return (rfn) => rfn;
+}