Add reduce() method

src/index.ts

@@ -137,6 +137,43 @@ export function filter<T>(
     });
 }
 
+export function reduce<T, R>(
+    iteratee:
+        | ((previousValue: R, chunk: T, encoding: string) => R)
+        | ((previousValue: R, chunk: T, encoding: string) => Promise<R>),
+    initialValue: R,
+    options: TransformOptions = {
+        readableObjectMode: true,
+        writableObjectMode: true,
+    },
+) {
+    let value = initialValue;
+    return new Transform({
+        readableObjectMode: options.readableObjectMode,
+        writableObjectMode: options.writableObjectMode,
+        async transform(chunk: T, encoding, callback) {
+            let isPromise = false;
+            try {
+                const result = iteratee(value, chunk, encoding);
+                isPromise = result instanceof Promise;
+                value = await result;
+                callback();
+            } catch (err) {
+                if (isPromise) {
+                    // Calling the callback asynchronously with an error wouldn't emit the error, so emit directly
+                    this.emit("error", err);
+                    callback();
+                } else {
+                    callback(err);
+                }
+            }
+        },
+        flush(callback) {
+            callback(undefined, value);
+        },
+    });
+}
+
 /**
  * Return a ReadWrite stream that splits streamed chunks using the given separator
  * @param separator Separator to split by, defaulting to "\n"