feat: Add transformSafely.

src/util/StreamUtil.ts

@@ -1,5 +1,5 @@
-import type { Writable, ReadableOptions } from 'stream';
-import { Readable } from 'stream';
+import type { Writable, ReadableOptions, DuplexOptions } from 'stream';
+import { Readable, Transform } from 'stream';
 import arrayifyStream from 'arrayify-stream';
 import { getLoggerFor } from '../logging/LogUtil';
 import type { Guarded } from './GuardedStream';
@@ -43,6 +43,59 @@ export function pipeSafely<T extends Writable>(readable: NodeJS.ReadableStream,
   return guardStream(destination);
 }
 
+export interface AsyncTransformOptions<T = any> extends DuplexOptions {
+  /**
+   * Transforms data from the source by calling the `push` method
+   */
+  transform?: (this: Transform, data: T, encoding: string) => any | Promise<any>;
+
+  /**
+   * Performs any final actions after the source has ended
+   */
+  flush?: (this: Transform) => any | Promise<any>;
+}
+
+/**
+ * Transforms a stream, ensuring that all errors are forwarded.
+ * @param source - The stream to be transformed
+ * @param options - The transformation options
+ *
+ * @returns The transformed stream
+ */
+export function transformSafely<T = any>(
+  source: NodeJS.ReadableStream,
+  {
+    transform = function(data): void {
+      this.push(data);
+    },
+    flush = (): null => null,
+    ...options
+  }: AsyncTransformOptions<T> = {},
+):
+  Guarded<Transform> {
+  return pipeSafely(source, new Transform({
+    ...options,
+    async transform(data, encoding, callback): Promise<void> {
+      let error: Error | null = null;
+      try {
+        await transform.call(this, data, encoding);
+      } catch (err: unknown) {
+        error = err as Error;
+      }
+      callback(error);
+    },
+    async flush(callback): Promise<void> {
+      let error: Error | null = null;
+      try {
+        await flush.call(this);
+      } catch (err: unknown) {
+        error = err as Error;
+      }
+      callback(error);
+    },
+  }));
+}
+
 /**
  * Converts an iterable to a stream and applies an error guard so that it is {@link Guarded}.
  * @param iterable - Data to stream.

test/unit/util/StreamUtil.test.ts

@@ -1,6 +1,7 @@
 import { PassThrough } from 'stream';
+import arrayifyStream from 'arrayify-stream';
 import streamifyArray from 'streamify-array';
-import { guardedStreamFrom, pipeSafely, readableToString } from '../../../src/util/StreamUtil';
+import { guardedStreamFrom, pipeSafely, transformSafely, readableToString } from '../../../src/util/StreamUtil';
 
 describe('StreamUtil', (): void => {
   describe('#readableToString', (): void => {
@@ -41,6 +42,104 @@ describe('StreamUtil', (): void => {
     });
   });
 
+  describe('#transformSafely', (): void => {
+    it('can transform a stream without arguments.', async(): Promise<void> => {
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely(source);
+      transformed.setEncoding('utf8');
+      const result = await arrayifyStream(transformed);
+      expect(result).toEqual([ 'data' ]);
+    });
+
+    it('can transform a stream synchronously.', async(): Promise<void> => {
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely<string>(source, {
+        encoding: 'utf8',
+        transform(data: string): void {
+          this.push(`${data}1`);
+          this.push(`${data}2`);
+        },
+        flush(): void {
+          this.push(`data3`);
+        },
+      });
+      const result = await arrayifyStream(transformed);
+      expect(result).toEqual([ 'data1', 'data2', 'data3' ]);
+    });
+
+    it('can transform a stream asynchronously.', async(): Promise<void> => {
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely<string>(source, {
+        encoding: 'utf8',
+        async transform(data: string): Promise<void> {
+          await new Promise((resolve): any => setImmediate(resolve));
+          this.push(`${data}1`);
+          this.push(`${data}2`);
+        },
+        async flush(): Promise<void> {
+          await new Promise((resolve): any => setImmediate(resolve));
+          this.push(`data3`);
+        },
+      });
+      const result = await arrayifyStream(transformed);
+      expect(result).toEqual([ 'data1', 'data2', 'data3' ]);
+    });
+
+    it('catches source errors.', async(): Promise<void> => {
+      const error = new Error('stream error');
+      const source = new PassThrough();
+      const transformed = transformSafely<string>(source);
+      source.emit('error', error);
+      await expect(arrayifyStream(transformed)).rejects.toThrow(error);
+    });
+
+    it('catches synchronous errors on transform.', async(): Promise<void> => {
+      const error = new Error('stream error');
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely<string>(source, {
+        transform(): never {
+          throw error;
+        },
+      });
+      await expect(arrayifyStream(transformed)).rejects.toThrow(error);
+    });
+
+    it('catches synchronous errors on flush.', async(): Promise<void> => {
+      const error = new Error('stream error');
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely<string>(source, {
+        async flush(): Promise<never> {
+          await new Promise((resolve): any => setImmediate(resolve));
+          throw error;
+        },
+      });
+      await expect(arrayifyStream(transformed)).rejects.toThrow(error);
+    });
+
+    it('catches asynchronous errors on transform.', async(): Promise<void> => {
+      const error = new Error('stream error');
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely<string>(source, {
+        transform(): never {
+          throw error;
+        },
+      });
+      await expect(arrayifyStream(transformed)).rejects.toThrow(error);
+    });
+
+    it('catches asynchronous errors on flush.', async(): Promise<void> => {
+      const error = new Error('stream error');
+      const source = streamifyArray([ 'data' ]);
+      const transformed = transformSafely<string>(source, {
+        async flush(): Promise<never> {
+          await new Promise((resolve): any => setImmediate(resolve));
+          throw error;
+        },
+      });
+      await expect(arrayifyStream(transformed)).rejects.toThrow(error);
+    });
+  });
+
   describe('#guardedStreamFrom', (): void => {
     it('converts data to a guarded stream.', async(): Promise<void> => {
       const data = [ 'a', 'b' ];