diff --git a/cli/js/globals.ts b/cli/js/globals.ts
index caf069ffd0..89f6075a3b 100644
--- a/cli/js/globals.ts
+++ b/cli/js/globals.ts
@@ -23,6 +23,7 @@ import * as workers from "./web/workers.ts";
 import * as performanceUtil from "./web/performance.ts";
 import * as request from "./web/request.ts";
 import * as readableStream from "./web/streams/readable_stream.ts";
+import * as transformStream from "./web/streams/transform_stream.ts";
 import * as queuingStrategy from "./web/streams/queuing_strategy.ts";
 import * as writableStream from "./web/streams/writable_stream.ts";
 
@@ -234,6 +235,7 @@ export const windowOrWorkerGlobalScopeProperties = {
   TextEncoder: nonEnumerable(textEncoding.TextEncoder),
   TextDecoder: nonEnumerable(textEncoding.TextDecoder),
   ReadableStream: nonEnumerable(readableStream.ReadableStreamImpl),
+  TransformStream: nonEnumerable(transformStream.TransformStreamImpl),
   Request: nonEnumerable(request.Request),
   Response: nonEnumerable(fetchTypes.Response),
   performance: writable(new performanceUtil.Performance()),
diff --git a/cli/js/lib.deno.shared_globals.d.ts b/cli/js/lib.deno.shared_globals.d.ts
index 8f73f0585c..d962304473 100644
--- a/cli/js/lib.deno.shared_globals.d.ts
+++ b/cli/js/lib.deno.shared_globals.d.ts
@@ -423,6 +423,42 @@ interface WritableStreamDefaultWriter<W = any> {
   write(chunk: W): Promise<void>;
 }
 
+declare class TransformStream<I = any, O = any> {
+  constructor(
+    transformer?: Transformer<I, O>,
+    writableStrategy?: QueuingStrategy<I>,
+    readableStrategy?: QueuingStrategy<O>
+  );
+  readonly readable: ReadableStream<O>;
+  readonly writable: WritableStream<I>;
+}
+
+interface TransformStreamDefaultController<O = any> {
+  readonly desiredSize: number | null;
+  enqueue(chunk: O): void;
+  error(reason?: any): void;
+  terminate(): void;
+}
+
+interface Transformer<I = any, O = any> {
+  flush?: TransformStreamDefaultControllerCallback<O>;
+  readableType?: undefined;
+  start?: TransformStreamDefaultControllerCallback<O>;
+  transform?: TransformStreamDefaultControllerTransformCallback<I, O>;
+  writableType?: undefined;
+}
+
+interface TransformStreamDefaultControllerCallback<O> {
+  (controller: TransformStreamDefaultController<O>): void | PromiseLike<void>;
+}
+
+interface TransformStreamDefaultControllerTransformCallback<I, O> {
+  (
+    chunk: I,
+    controller: TransformStreamDefaultController<O>
+  ): void | PromiseLike<void>;
+}
+
 interface DOMStringList {
   /** Returns the number of strings in strings. */
   readonly length: number;
diff --git a/cli/js/tests/streams_transform_test.ts b/cli/js/tests/streams_transform_test.ts
new file mode 100644
index 0000000000..f3ec148ae5
--- /dev/null
+++ b/cli/js/tests/streams_transform_test.ts
@@ -0,0 +1,562 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import {
+  unitTest,
+  assert,
+  assertEquals,
+  assertNotEquals,
+  assertThrows,
+} from "./test_util.ts";
+
+function delay(seconds: number): Promise<void> {
+  return new Promise<void>((resolve) => {
+    setTimeout(() => {
+      resolve();
+    }, seconds);
+  });
+}
+
+function readableStreamToArray<R>(
+  readable: { getReader(): ReadableStreamDefaultReader<R> },
+  reader?: ReadableStreamDefaultReader<R>
+): Promise<R[]> {
+  if (reader === undefined) {
+    reader = readable.getReader();
+  }
+
+  const chunks: R[] = [];
+
+  return pump();
+
+  function pump(): Promise<R[]> {
+    return reader!.read().then((result) => {
+      if (result.done) {
+        return chunks;
+      }
+
+      chunks.push(result.value);
+      return pump();
+    });
+  }
+}
+
+unitTest(function transformStreamConstructedWithTransformFunction() {
+  new TransformStream({ transform(): void {} });
+});
+
+unitTest(function transformStreamConstructedNoTransform() {
+  new TransformStream();
+  new TransformStream({});
+});
+
+unitTest(function transformStreamIntstancesHaveProperProperties() {
+  const ts = new TransformStream({ transform(): void {} });
+  const proto = Object.getPrototypeOf(ts);
+
+  const writableStream = Object.getOwnPropertyDescriptor(proto, "writable");
+  assert(writableStream !== undefined, "it has a writable property");
+  assert(!writableStream.enumerable, "writable should be non-enumerable");
+  assertEquals(
+    typeof writableStream.get,
+    "function",
+    "writable should have a getter"
+  );
+  assertEquals(
+    writableStream.set,
+    undefined,
+    "writable should not have a setter"
+  );
+  assert(writableStream.configurable, "writable should be configurable");
+  assert(
+    ts.writable instanceof WritableStream,
+    "writable is an instance of WritableStream"
+  );
+  assert(
+    WritableStream.prototype.getWriter.call(ts.writable),
+    "writable should pass WritableStream brand check"
+  );
+
+  const readableStream = Object.getOwnPropertyDescriptor(proto, "readable");
+  assert(readableStream !== undefined, "it has a readable property");
+  assert(!readableStream.enumerable, "readable should be non-enumerable");
+  assertEquals(
+    typeof readableStream.get,
+    "function",
+    "readable should have a getter"
+  );
+  assertEquals(
+    readableStream.set,
+    undefined,
+    "readable should not have a setter"
+  );
+  assert(readableStream.configurable, "readable should be configurable");
+  assert(
+    ts.readable instanceof ReadableStream,
+    "readable is an instance of ReadableStream"
+  );
+  assertNotEquals(
+    ReadableStream.prototype.getReader.call(ts.readable),
+    undefined,
+    "readable should pass ReadableStream brand check"
+  );
+});
+
+unitTest(function transformStreamWritableStartsAsWritable() {
+  const ts = new TransformStream({ transform(): void {} });
+
+  const writer = ts.writable.getWriter();
+  assertEquals(writer.desiredSize, 1, "writer.desiredSize should be 1");
+});
+
+unitTest(async function transformStreamReadableCanReadOutOfWritable() {
+  const ts = new TransformStream();
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+  assertEquals(
+    writer.desiredSize,
+    0,
+    "writer.desiredSize should be 0 after write()"
+  );
+
+  const result = await ts.readable.getReader().read();
+  assertEquals(
+    result.value,
+    "a",
+    "result from reading the readable is the same as was written to writable"
+  );
+  assert(!result.done, "stream should not be done");
+
+  await delay(0);
+  assert(writer.desiredSize === 1, "desiredSize should be 1 again");
+});
+
+unitTest(async function transformStreamCanReadWhatIsWritten() {
+  let c: TransformStreamDefaultController;
+  const ts = new TransformStream({
+    start(controller: TransformStreamDefaultController): void {
+      c = controller;
+    },
+    transform(chunk: string): void {
+      c.enqueue(chunk.toUpperCase());
+    },
+  });
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+
+  const result = await ts.readable.getReader().read();
+  assertEquals(
+    result.value,
+    "A",
+    "result from reading the readable is the transformation of what was written to writable"
+  );
+  assert(!result.done, "stream should not be done");
+});
+
+unitTest(async function transformStreamCanReadBothChunks() {
+  let c: TransformStreamDefaultController;
+  const ts = new TransformStream({
+    start(controller: TransformStreamDefaultController): void {
+      c = controller;
+    },
+    transform(chunk: string): void {
+      c.enqueue(chunk.toUpperCase());
+      c.enqueue(chunk.toUpperCase());
+    },
+  });
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+
+  const reader = ts.readable.getReader();
+
+  const result1 = await reader.read();
+  assertEquals(
+    result1.value,
+    "A",
+    "the first chunk read is the transformation of the single chunk written"
+  );
+  assert(!result1.done, "stream should not be done");
+
+  const result2 = await reader.read();
+  assertEquals(
+    result2.value,
+    "A",
+    "the second chunk read is also the transformation of the single chunk written"
+  );
+  assert(!result2.done, "stream should not be done");
+});
+
+unitTest(async function transformStreamCanReadWhatIsWritten() {
+  let c: TransformStreamDefaultController;
+  const ts = new TransformStream({
+    start(controller: TransformStreamDefaultController): void {
+      c = controller;
+    },
+    transform(chunk: string): Promise<void> {
+      return delay(0).then(() => c.enqueue(chunk.toUpperCase()));
+    },
+  });
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+
+  const result = await ts.readable.getReader().read();
+  assertEquals(
+    result.value,
+    "A",
+    "result from reading the readable is the transformation of what was written to writable"
+  );
+  assert(!result.done, "stream should not be done");
+});
+
+unitTest(async function transformStreamAsyncReadMultipleChunks() {
+  let doSecondEnqueue: () => void;
+  let returnFromTransform: () => void;
+  const ts = new TransformStream({
+    transform(
+      chunk: string,
+      controller: TransformStreamDefaultController
+    ): Promise<void> {
+      delay(0).then(() => controller.enqueue(chunk.toUpperCase()));
+      doSecondEnqueue = (): void => controller.enqueue(chunk.toUpperCase());
+      return new Promise((resolve) => {
+        returnFromTransform = resolve;
+      });
+    },
+  });
+
+  const reader = ts.readable.getReader();
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+
+  const result1 = await reader.read();
+  assertEquals(
+    result1.value,
+    "A",
+    "the first chunk read is the transformation of the single chunk written"
+  );
+  assert(!result1.done, "stream should not be done");
+  doSecondEnqueue!();
+
+  const result2 = await reader.read();
+  assertEquals(
+    result2.value,
+    "A",
+    "the second chunk read is also the transformation of the single chunk written"
+  );
+  assert(!result2.done, "stream should not be done");
+  returnFromTransform!();
+});
+
+unitTest(function transformStreamClosingWriteClosesRead() {
+  const ts = new TransformStream({ transform(): void {} });
+
+  const writer = ts.writable.getWriter();
+  writer.close();
+
+  return Promise.all([writer.closed, ts.readable.getReader().closed]).then(
+    undefined
+  );
+});
+
+unitTest(async function transformStreamCloseWaitAwaitsTransforms() {
+  let transformResolve: () => void;
+  const transformPromise = new Promise<void>((resolve) => {
+    transformResolve = resolve;
+  });
+  const ts = new TransformStream(
+    {
+      transform(): Promise<void> {
+        return transformPromise;
+      },
+    },
+    undefined,
+    { highWaterMark: 1 }
+  );
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+  writer.close();
+
+  let rsClosed = false;
+  ts.readable.getReader().closed.then(() => {
+    rsClosed = true;
+  });
+
+  await delay(0);
+  assertEquals(rsClosed, false, "readable is not closed after a tick");
+  transformResolve!();
+
+  await writer.closed;
+  // TODO: Is this expectation correct?
+  assertEquals(rsClosed, true, "readable is closed at that point");
+});
+
+unitTest(async function transformStreamCloseWriteAfterSyncEnqueues() {
+  let c: TransformStreamDefaultController<string>;
+  const ts = new TransformStream<string, string>({
+    start(controller: TransformStreamDefaultController): void {
+      c = controller;
+    },
+    transform(): Promise<void> {
+      c.enqueue("x");
+      c.enqueue("y");
+      return delay(0);
+    },
+  });
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+  writer.close();
+
+  const readableChunks = readableStreamToArray(ts.readable);
+
+  await writer.closed;
+  const chunks = await readableChunks;
+  assertEquals(
+    chunks,
+    ["x", "y"],
+    "both enqueued chunks can be read from the readable"
+  );
+});
+
+unitTest(async function transformStreamWritableCloseAsyncAfterAsyncEnqueues() {
+  let c: TransformStreamDefaultController<string>;
+  const ts = new TransformStream<string, string>({
+    start(controller: TransformStreamDefaultController<string>): void {
+      c = controller;
+    },
+    transform(): Promise<void> {
+      return delay(0)
+        .then(() => c.enqueue("x"))
+        .then(() => c.enqueue("y"))
+        .then(() => delay(0));
+    },
+  });
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+  writer.close();
+
+  const readableChunks = readableStreamToArray(ts.readable);
+
+  await writer.closed;
+  const chunks = await readableChunks;
+  assertEquals(
+    chunks,
+    ["x", "y"],
+    "both enqueued chunks can be read from the readable"
+  );
+});
+
+unitTest(async function transformStreamTransformerMethodsCalledAsMethods() {
+  let c: TransformStreamDefaultController<string>;
+  const transformer = {
+    suffix: "-suffix",
+
+    start(controller: TransformStreamDefaultController<string>): void {
+      c = controller;
+      c.enqueue("start" + this.suffix);
+    },
+
+    transform(chunk: string): void {
+      c.enqueue(chunk + this.suffix);
+    },
+
+    flush(): void {
+      c.enqueue("flushed" + this.suffix);
+    },
+  };
+  const ts = new TransformStream(transformer);
+
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+  writer.close();
+
+  const readableChunks = readableStreamToArray(ts.readable);
+
+  await writer.closed;
+  const chunks = await readableChunks;
+  assertEquals(
+    chunks,
+    ["start-suffix", "a-suffix", "flushed-suffix"],
+    "all enqueued chunks have suffixes"
+  );
+});
+
+unitTest(async function transformStreamMethodsShouldNotBeAppliedOrCalled() {
+  function functionWithOverloads(): void {}
+  functionWithOverloads.apply = (): void => {
+    throw new Error("apply() should not be called");
+  };
+  functionWithOverloads.call = (): void => {
+    throw new Error("call() should not be called");
+  };
+  const ts = new TransformStream({
+    start: functionWithOverloads,
+    transform: functionWithOverloads,
+    flush: functionWithOverloads,
+  });
+  const writer = ts.writable.getWriter();
+  writer.write("a");
+  writer.close();
+
+  await readableStreamToArray(ts.readable);
+});
+
+unitTest(async function transformStreamCallTransformSync() {
+  let transformCalled = false;
+  const ts = new TransformStream(
+    {
+      transform(): void {
+        transformCalled = true;
+      },
+    },
+    undefined,
+    { highWaterMark: Infinity }
+  );
+  // transform() is only called synchronously when there is no backpressure and
+  // all microtasks have run.
+  await delay(0);
+  const writePromise = ts.writable.getWriter().write(undefined);
+  assert(transformCalled, "transform() should have been called");
+  await writePromise;
+});
+
+unitTest(function transformStreamCloseWriteCloesesReadWithNoChunks() {
+  const ts = new TransformStream({}, undefined, { highWaterMark: 0 });
+
+  const writer = ts.writable.getWriter();
+  writer.close();
+
+  return Promise.all([writer.closed, ts.readable.getReader().closed]).then(
+    undefined
+  );
+});
+
+unitTest(function transformStreamEnqueueThrowsAfterTerminate() {
+  new TransformStream({
+    start(controller: TransformStreamDefaultController): void {
+      controller.terminate();
+      assertThrows(() => {
+        controller.enqueue(undefined);
+      }, TypeError);
+    },
+  });
+});
+
+unitTest(function transformStreamEnqueueThrowsAfterReadableCancel() {
+  let controller: TransformStreamDefaultController;
+  const ts = new TransformStream({
+    start(c: TransformStreamDefaultController): void {
+      controller = c;
+    },
+  });
+  const cancelPromise = ts.readable.cancel();
+  assertThrows(
+    () => controller.enqueue(undefined),
+    TypeError,
+    undefined,
+    "enqueue should throw"
+  );
+  return cancelPromise;
+});
+
+unitTest(function transformStreamSecondTerminateNoOp() {
+  new TransformStream({
+    start(controller: TransformStreamDefaultController): void {
+      controller.terminate();
+      controller.terminate();
+    },
+  });
+});
+
+unitTest(async function transformStreamTerminateAfterReadableCancelIsNoop() {
+  let controller: TransformStreamDefaultController;
+  const ts = new TransformStream({
+    start(c: TransformStreamDefaultController): void {
+      controller = c;
+    },
+  });
+  const cancelReason = { name: "cancelReason" };
+  const cancelPromise = ts.readable.cancel(cancelReason);
+  controller!.terminate();
+  await cancelPromise;
+  try {
+    await ts.writable.getWriter().closed;
+  } catch (e) {
+    assert(e === cancelReason);
+    return;
+  }
+  throw new Error("closed should have rejected");
+});
+
+unitTest(async function transformStreamStartCalledOnce() {
+  let calls = 0;
+  new TransformStream({
+    start(): void {
+      ++calls;
+    },
+  });
+  await delay(0);
+  assertEquals(calls, 1, "start() should have been called exactly once");
+});
+
+unitTest(function transformStreamReadableTypeThrows() {
+  assertThrows(
+    // eslint-disable-next-line
+    () => new TransformStream({ readableType: "bytes" as any }),
+    RangeError,
+    undefined,
+    "constructor should throw"
+  );
+});
+
+unitTest(function transformStreamWirtableTypeThrows() {
+  assertThrows(
+    // eslint-disable-next-line
+    () => new TransformStream({ writableType: "bytes" as any }),
+    RangeError,
+    undefined,
+    "constructor should throw"
+  );
+});
+
+unitTest(function transformStreamSubclassable() {
+  class Subclass extends TransformStream {
+    extraFunction(): boolean {
+      return true;
+    }
+  }
+  assert(
+    Object.getPrototypeOf(Subclass.prototype) === TransformStream.prototype,
+    "Subclass.prototype's prototype should be TransformStream.prototype"
+  );
+  assert(
+    Object.getPrototypeOf(Subclass) === TransformStream,
+    "Subclass's prototype should be TransformStream"
+  );
+  const sub = new Subclass();
+  assert(
+    sub instanceof TransformStream,
+    "Subclass object should be an instance of TransformStream"
+  );
+  assert(
+    sub instanceof Subclass,
+    "Subclass object should be an instance of Subclass"
+  );
+  const readableGetter = Object.getOwnPropertyDescriptor(
+    TransformStream.prototype,
+    "readable"
+  )!.get;
+  assert(
+    readableGetter!.call(sub) === sub.readable,
+    "Subclass object should pass brand check"
+  );
+  assert(
+    sub.extraFunction(),
+    "extraFunction() should be present on Subclass object"
+  );
+});
diff --git a/cli/js/tests/unit_tests.ts b/cli/js/tests/unit_tests.ts
index 40d0124d35..7327bcc051 100644
--- a/cli/js/tests/unit_tests.ts
+++ b/cli/js/tests/unit_tests.ts
@@ -53,6 +53,7 @@ import "./resources_test.ts";
 import "./signal_test.ts";
 import "./stat_test.ts";
 import "./streams_piping_test.ts";
+import "./streams_transform_test.ts";
 import "./streams_writable_test.ts";
 import "./symlink_test.ts";
 import "./text_encoding_test.ts";
diff --git a/cli/js/web/streams/internals.ts b/cli/js/web/streams/internals.ts
index 846db096e6..5ef094afc6 100644
--- a/cli/js/web/streams/internals.ts
+++ b/cli/js/web/streams/internals.ts
@@ -13,6 +13,8 @@ import { ReadableStreamDefaultControllerImpl } from "./readable_stream_default_c
 import { ReadableStreamDefaultReaderImpl } from "./readable_stream_default_reader.ts";
 import { ReadableStreamImpl } from "./readable_stream.ts";
 import * as sym from "./symbols.ts";
+import { TransformStreamImpl } from "./transform_stream.ts";
+import { TransformStreamDefaultControllerImpl } from "./transform_stream_default_controller.ts";
 import { WritableStreamDefaultControllerImpl } from "./writable_stream_default_controller.ts";
 import { WritableStreamDefaultWriterImpl } from "./writable_stream_default_writer.ts";
 import { WritableStreamImpl } from "./writable_stream.ts";
@@ -36,10 +38,12 @@ type Container<R = any> = {
   [sym.queue]: Array<Pair<R> | BufferQueueItem>;
   [sym.queueTotalSize]: number;
 };
+export type FlushAlgorithm = () => Promise<void>;
 export type Pair<R> = { value: R; size: number };
 export type PullAlgorithm = () => PromiseLike<void>;
 export type SizeAlgorithm<T> = (chunk: T) => number;
 export type StartAlgorithm = () => void | PromiseLike<void>;
+export type TransformAlgorithm<I> = (chunk: I) => Promise<void>;
 export type WriteAlgorithm<W> = (chunk: W) => Promise<void>;
 export interface Deferred<T> {
   promise: Promise<T>;
@@ -76,8 +80,16 @@ export function acquireWritableStreamDefaultWriter<W>(
   return new WritableStreamDefaultWriterImpl(stream);
 }
 
+export function call<F extends (...args: any[]) => any>(
+  fn: F,
+  v: ThisType<F>,
+  args: Parameters<F>
+): ReturnType<F> {
+  return Function.prototype.apply.call(fn, v, args);
+}
+
 function createAlgorithmFromUnderlyingMethod<
-  O extends UnderlyingByteSource | UnderlyingSource,
+  O extends UnderlyingByteSource | UnderlyingSource | Transformer,
   P extends keyof O
 >(
   underlyingObject: O,
@@ -86,7 +98,7 @@ function createAlgorithmFromUnderlyingMethod<
   ...extraArgs: any[]
 ): () => Promise<void>;
 function createAlgorithmFromUnderlyingMethod<
-  O extends UnderlyingByteSource | UnderlyingSource,
+  O extends UnderlyingByteSource | UnderlyingSource | Transformer,
   P extends keyof O
 >(
   underlyingObject: O,
@@ -95,7 +107,7 @@ function createAlgorithmFromUnderlyingMethod<
   ...extraArgs: any[]
 ): (arg: any) => Promise<void>;
 function createAlgorithmFromUnderlyingMethod<
-  O extends UnderlyingByteSource | UnderlyingSource,
+  O extends UnderlyingByteSource | UnderlyingSource | Transformer,
   P extends keyof O
 >(
   underlyingObject: O,
@@ -110,11 +122,11 @@ function createAlgorithmFromUnderlyingMethod<
     }
     if (algoArgCount === 0) {
       return async (): Promise<void> =>
-        method.call(underlyingObject, ...extraArgs);
+        call(method, underlyingObject, extraArgs as any);
     } else {
       return async (arg: any): Promise<void> => {
         const fullArgs = [arg, ...extraArgs];
-        return method.call(underlyingObject, ...fullArgs);
+        return call(method, underlyingObject, fullArgs as any);
       };
     }
   }
@@ -148,6 +160,33 @@ function createReadableStream<T>(
   return stream;
 }
 
+function createWritableStream<W>(
+  startAlgorithm: StartAlgorithm,
+  writeAlgorithm: WriteAlgorithm<W>,
+  closeAlgorithm: CloseAlgorithm,
+  abortAlgorithm: AbortAlgorithm,
+  highWaterMark = 1,
+  sizeAlgorithm: SizeAlgorithm<W> = (): number => 1
+): WritableStreamImpl<W> {
+  assert(isNonNegativeNumber(highWaterMark));
+  const stream = Object.create(WritableStreamImpl.prototype);
+  initializeWritableStream(stream);
+  const controller = Object.create(
+    WritableStreamDefaultControllerImpl.prototype
+  );
+  setUpWritableStreamDefaultController(
+    stream,
+    controller,
+    startAlgorithm,
+    writeAlgorithm,
+    closeAlgorithm,
+    abortAlgorithm,
+    highWaterMark,
+    sizeAlgorithm
+  );
+  return stream;
+}
+
 export function dequeueValue<R>(container: Container<R>): R {
   assert(sym.queue in container && sym.queueTotalSize in container);
   assert(container[sym.queue].length);
@@ -185,13 +224,61 @@ export function getDeferred<T>(): Required<Deferred<T>> {
   return { promise, resolve: resolve!, reject: reject! };
 }
 
-export function initializeReadableStream(stream: ReadableStreamImpl): void {
+export function initializeReadableStream<R>(
+  stream: ReadableStreamImpl<R>
+): void {
   stream[sym.state] = "readable";
   stream[sym.reader] = stream[sym.storedError] = undefined;
   stream[sym.disturbed] = false;
 }
 
-export function initializeWritableStream(stream: WritableStreamImpl): void {
+export function initializeTransformStream<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  startPromise: Promise<void>,
+  writableHighWaterMark: number,
+  writableSizeAlgorithm: SizeAlgorithm<I>,
+  readableHighWaterMark: number,
+  readableSizeAlgorithm: SizeAlgorithm<O>
+): void {
+  const startAlgorithm = (): Promise<void> => startPromise;
+  const writeAlgorithm = (chunk: any): Promise<void> =>
+    transformStreamDefaultSinkWriteAlgorithm(stream, chunk);
+  const abortAlgorithm = (reason: any): Promise<void> =>
+    transformStreamDefaultSinkAbortAlgorithm(stream, reason);
+  const closeAlgorithm = (): Promise<void> =>
+    transformStreamDefaultSinkCloseAlgorithm(stream);
+  stream[sym.writable] = createWritableStream(
+    startAlgorithm,
+    writeAlgorithm,
+    closeAlgorithm,
+    abortAlgorithm,
+    writableHighWaterMark,
+    writableSizeAlgorithm
+  );
+  const pullAlgorithm = (): PromiseLike<void> =>
+    transformStreamDefaultSourcePullAlgorithm(stream);
+  const cancelAlgorithm = (reason: any): Promise<void> => {
+    transformStreamErrorWritableAndUnblockWrite(stream, reason);
+    return Promise.resolve(undefined);
+  };
+  stream[sym.readable] = createReadableStream(
+    startAlgorithm,
+    pullAlgorithm,
+    cancelAlgorithm,
+    readableHighWaterMark,
+    readableSizeAlgorithm
+  );
+  stream[sym.backpressure] = stream[sym.backpressureChangePromise] = undefined;
+  transformStreamSetBackpressure(stream, true);
+  Object.defineProperty(stream, sym.transformStreamController, {
+    value: undefined,
+    configurable: true,
+  });
+}
+
+export function initializeWritableStream<W>(
+  stream: WritableStreamImpl<W>
+): void {
   stream[sym.state] = "writable";
   stream[sym.storedError] = stream[sym.writer] = stream[
     sym.writableStreamController
@@ -202,7 +289,7 @@ export function initializeWritableStream(stream: WritableStreamImpl): void {
   stream[sym.backpressure] = false;
 }
 
-function invokeOrNoop<O extends any, P extends keyof O>(
+export function invokeOrNoop<O extends Record<string, any>, P extends keyof O>(
   o: O,
   p: P,
   ...args: Parameters<O[P]>
@@ -212,7 +299,7 @@ function invokeOrNoop<O extends any, P extends keyof O>(
   if (!method) {
     return undefined;
   }
-  return method.call(o, ...args);
+  return call(method, o, args);
 }
 
 function isCallable(value: unknown): value is (...args: any) => any {
@@ -299,6 +386,26 @@ export function isReadableStreamLocked(stream: ReadableStreamImpl): boolean {
   return stream[sym.reader] ? true : false;
 }
 
+export function isTransformStream(
+  x: unknown
+): x is TransformStreamImpl<any, any> {
+  return typeof x !== "object" ||
+    x === null ||
+    !(sym.transformStreamController in x)
+    ? false
+    : true;
+}
+
+export function isTransformStreamDefaultController(
+  x: unknown
+): x is TransformStreamDefaultControllerImpl<any, any> {
+  return typeof x !== "object" ||
+    x === null ||
+    !(sym.controlledTransformStream in x)
+    ? false
+    : true;
+}
+
 export function isUnderlyingByteSource(
   underlyingSource: UnderlyingByteSource | UnderlyingSource
 ): underlyingSource is UnderlyingByteSource {
@@ -717,6 +824,14 @@ export function readableStreamDefaultControllerError<T>(
   readableStreamError(stream, e);
 }
 
+function readableStreamDefaultControllerHasBackpressure<T>(
+  controller: ReadableStreamDefaultControllerImpl<T>
+): boolean {
+  return readableStreamDefaultControllerShouldCallPull(controller)
+    ? true
+    : false;
+}
+
 function readableStreamDefaultControllerShouldCallPull<T>(
   controller: ReadableStreamDefaultControllerImpl<T>
 ): boolean {
@@ -1416,6 +1531,62 @@ export function setUpReadableStreamDefaultControllerFromUnderlyingSource<T>(
   );
 }
 
+function setUpTransformStreamDefaultController<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  controller: TransformStreamDefaultControllerImpl<I, O>,
+  transformAlgorithm: TransformAlgorithm<I>,
+  flushAlgorithm: FlushAlgorithm
+): void {
+  assert(isTransformStream(stream));
+  assert(stream[sym.transformStreamController] === undefined);
+  controller[sym.controlledTransformStream] = stream;
+  stream[sym.transformStreamController] = controller;
+  controller[sym.transformAlgorithm] = transformAlgorithm;
+  controller[sym.flushAlgorithm] = flushAlgorithm;
+}
+
+export function setUpTransformStreamDefaultControllerFromTransformer<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  transformer: Transformer<I, O>
+): void {
+  assert(transformer);
+  const controller = Object.create(
+    TransformStreamDefaultControllerImpl.prototype
+  ) as TransformStreamDefaultControllerImpl<I, O>;
+  let transformAlgorithm: TransformAlgorithm<I> = (chunk) => {
+    try {
+      transformStreamDefaultControllerEnqueue(
+        controller,
+        // it defaults to no tranformation, so I is assumed to be O
+        (chunk as unknown) as O
+      );
+    } catch (e) {
+      return Promise.reject(e);
+    }
+    return Promise.resolve();
+  };
+  const transformMethod = transformer.transform;
+  if (transformMethod) {
+    if (typeof transformMethod !== "function") {
+      throw new TypeError("tranformer.transform must be callable.");
+    }
+    transformAlgorithm = async (chunk): Promise<void> =>
+      call(transformMethod, transformer, [chunk, controller]);
+  }
+  const flushAlgorithm = createAlgorithmFromUnderlyingMethod(
+    transformer,
+    "flush",
+    0,
+    controller
+  );
+  setUpTransformStreamDefaultController(
+    stream,
+    controller,
+    transformAlgorithm,
+    flushAlgorithm
+  );
+}
+
 function setUpWritableStreamDefaultController<W>(
   stream: WritableStreamImpl<W>,
   controller: WritableStreamDefaultControllerImpl<W>,
@@ -1508,6 +1679,181 @@ export function setUpWritableStreamDefaultControllerFromUnderlyingSink<W>(
   );
 }
 
+function transformStreamDefaultControllerClearAlgorithms<I, O>(
+  controller: TransformStreamDefaultControllerImpl<I, O>
+): void {
+  (controller as any)[sym.transformAlgorithm] = undefined;
+  (controller as any)[sym.flushAlgorithm] = undefined;
+}
+
+export function transformStreamDefaultControllerEnqueue<I, O>(
+  controller: TransformStreamDefaultControllerImpl<I, O>,
+  chunk: O
+): void {
+  const stream = controller[sym.controlledTransformStream];
+  const readableController = stream[sym.readable][
+    sym.readableStreamController
+  ] as ReadableStreamDefaultControllerImpl<O>;
+  if (!readableStreamDefaultControllerCanCloseOrEnqueue(readableController)) {
+    throw new TypeError(
+      "TransformStream's readable controller cannot be closed or enqueued."
+    );
+  }
+  try {
+    readableStreamDefaultControllerEnqueue(readableController, chunk);
+  } catch (e) {
+    transformStreamErrorWritableAndUnblockWrite(stream, e);
+    throw stream[sym.readable][sym.storedError];
+  }
+  const backpressure = readableStreamDefaultControllerHasBackpressure(
+    readableController
+  );
+  if (backpressure) {
+    transformStreamSetBackpressure(stream, true);
+  }
+}
+
+export function transformStreamDefaultControllerError<I, O>(
+  controller: TransformStreamDefaultControllerImpl<I, O>,
+  e: any
+): void {
+  transformStreamError(controller[sym.controlledTransformStream], e);
+}
+
+function transformStreamDefaultControllerPerformTransform<I, O>(
+  controller: TransformStreamDefaultControllerImpl<I, O>,
+  chunk: I
+): Promise<void> {
+  const transformPromise = controller[sym.transformAlgorithm](chunk);
+  return transformPromise.then(undefined, (r) => {
+    transformStreamError(controller[sym.controlledTransformStream], r);
+    throw r;
+  });
+}
+
+function transformStreamDefaultSinkAbortAlgorithm<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  reason: any
+): Promise<void> {
+  transformStreamError(stream, reason);
+  return Promise.resolve(undefined);
+}
+
+function transformStreamDefaultSinkCloseAlgorithm<I, O>(
+  stream: TransformStreamImpl<I, O>
+): Promise<void> {
+  const readable = stream[sym.readable];
+  const controller = stream[sym.transformStreamController];
+  const flushPromise = controller[sym.flushAlgorithm]();
+  transformStreamDefaultControllerClearAlgorithms(controller);
+  return flushPromise.then(
+    () => {
+      if (readable[sym.state] === "errored") {
+        throw readable[sym.storedError];
+      }
+      const readableController = readable[
+        sym.readableStreamController
+      ] as ReadableStreamDefaultControllerImpl<O>;
+      if (
+        readableStreamDefaultControllerCanCloseOrEnqueue(readableController)
+      ) {
+        readableStreamDefaultControllerClose(readableController);
+      }
+    },
+    (r) => {
+      transformStreamError(stream, r);
+      throw readable[sym.storedError];
+    }
+  );
+}
+
+function transformStreamDefaultSinkWriteAlgorithm<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  chunk: I
+): Promise<void> {
+  assert(stream[sym.writable][sym.state] === "writable");
+  const controller = stream[sym.transformStreamController];
+  if (stream[sym.backpressure]) {
+    const backpressureChangePromise = stream[sym.backpressureChangePromise];
+    assert(backpressureChangePromise);
+    return backpressureChangePromise.promise.then(() => {
+      const writable = stream[sym.writable];
+      const state = writable[sym.state];
+      if (state === "erroring") {
+        throw writable[sym.storedError];
+      }
+      assert(state === "writable");
+      return transformStreamDefaultControllerPerformTransform(
+        controller,
+        chunk
+      );
+    });
+  }
+  return transformStreamDefaultControllerPerformTransform(controller, chunk);
+}
+
+function transformStreamDefaultSourcePullAlgorithm<I, O>(
+  stream: TransformStreamImpl<I, O>
+): Promise<void> {
+  assert(stream[sym.backpressure] === true);
+  assert(stream[sym.backpressureChangePromise] !== undefined);
+  transformStreamSetBackpressure(stream, false);
+  return stream[sym.backpressureChangePromise]!.promise;
+}
+
+function transformStreamError<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  e: any
+): void {
+  readableStreamDefaultControllerError(
+    stream[sym.readable][
+      sym.readableStreamController
+    ] as ReadableStreamDefaultControllerImpl<O>,
+    e
+  );
+  transformStreamErrorWritableAndUnblockWrite(stream, e);
+}
+
+export function transformStreamDefaultControllerTerminate<I, O>(
+  controller: TransformStreamDefaultControllerImpl<I, O>
+): void {
+  const stream = controller[sym.controlledTransformStream];
+  const readableController = stream[sym.readable][
+    sym.readableStreamController
+  ] as ReadableStreamDefaultControllerImpl<O>;
+  readableStreamDefaultControllerClose(readableController);
+  const error = new TypeError("TransformStream is closed.");
+  transformStreamErrorWritableAndUnblockWrite(stream, error);
+}
+
+function transformStreamErrorWritableAndUnblockWrite<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  e: any
+): void {
+  transformStreamDefaultControllerClearAlgorithms(
+    stream[sym.transformStreamController]
+  );
+  writableStreamDefaultControllerErrorIfNeeded(
+    stream[sym.writable][sym.writableStreamController]!,
+    e
+  );
+  if (stream[sym.backpressure]) {
+    transformStreamSetBackpressure(stream, false);
+  }
+}
+
+function transformStreamSetBackpressure<I, O>(
+  stream: TransformStreamImpl<I, O>,
+  backpressure: boolean
+): void {
+  assert(stream[sym.backpressure] !== backpressure);
+  if (stream[sym.backpressureChangePromise] !== undefined) {
+    stream[sym.backpressureChangePromise]!.resolve!(undefined);
+  }
+  stream[sym.backpressureChangePromise] = getDeferred<void>();
+  stream[sym.backpressure] = backpressure;
+}
+
 function transferArrayBuffer(buffer: ArrayBuffer): ArrayBuffer {
   assert(!isDetachedBuffer(buffer));
   const transferredIshVersion = buffer.slice(0);
diff --git a/cli/js/web/streams/symbols.ts b/cli/js/web/streams/symbols.ts
index 9e5cb57157..9c0a336e55 100644
--- a/cli/js/web/streams/symbols.ts
+++ b/cli/js/web/streams/symbols.ts
@@ -11,6 +11,7 @@ export const abortSteps = Symbol("abortSteps");
 export const asyncIteratorReader = Symbol("asyncIteratorReader");
 export const autoAllocateChunkSize = Symbol("autoAllocateChunkSize");
 export const backpressure = Symbol("backpressure");
+export const backpressureChangePromise = Symbol("backpressureChangePromise");
 export const byobRequest = Symbol("byobRequest");
 export const cancelAlgorithm = Symbol("cancelAlgorithm");
 export const cancelSteps = Symbol("cancelSteps");
@@ -22,9 +23,11 @@ export const controlledReadableByteStream = Symbol(
   "controlledReadableByteStream"
 );
 export const controlledReadableStream = Symbol("controlledReadableStream");
+export const controlledTransformStream = Symbol("controlledTransformStream");
 export const controlledWritableStream = Symbol("controlledWritableStream");
 export const disturbed = Symbol("disturbed");
 export const errorSteps = Symbol("errorSteps");
+export const flushAlgorithm = Symbol("flushAlgorithm");
 export const forAuthorCode = Symbol("forAuthorCode");
 export const inFlightWriteRequest = Symbol("inFlightWriteRequest");
 export const inFlightCloseRequest = Symbol("inFlightCloseRequest");
@@ -39,6 +42,7 @@ export const pulling = Symbol("pulling");
 export const pullSteps = Symbol("pullSteps");
 export const queue = Symbol("queue");
 export const queueTotalSize = Symbol("queueTotalSize");
+export const readable = Symbol("readable");
 export const readableStreamController = Symbol("readableStreamController");
 export const reader = Symbol("reader");
 export const readRequests = Symbol("readRequests");
@@ -48,7 +52,10 @@ export const state = Symbol("state");
 export const storedError = Symbol("storedError");
 export const strategyHWM = Symbol("strategyHWM");
 export const strategySizeAlgorithm = Symbol("strategySizeAlgorithm");
+export const transformAlgorithm = Symbol("transformAlgorithm");
+export const transformStreamController = Symbol("transformStreamController");
 export const writableStreamController = Symbol("writableStreamController");
 export const writeAlgorithm = Symbol("writeAlgorithm");
+export const writable = Symbol("writable");
 export const writer = Symbol("writer");
 export const writeRequests = Symbol("writeRequests");
diff --git a/cli/js/web/streams/transform_stream.ts b/cli/js/web/streams/transform_stream.ts
new file mode 100644
index 0000000000..ac08fea3fa
--- /dev/null
+++ b/cli/js/web/streams/transform_stream.ts
@@ -0,0 +1,118 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+import {
+  Deferred,
+  getDeferred,
+  initializeTransformStream,
+  invokeOrNoop,
+  isTransformStream,
+  makeSizeAlgorithmFromSizeFunction,
+  setFunctionName,
+  setUpTransformStreamDefaultControllerFromTransformer,
+  validateAndNormalizeHighWaterMark,
+} from "./internals.ts";
+import { ReadableStreamImpl } from "./readable_stream.ts";
+import * as sym from "./symbols.ts";
+import { TransformStreamDefaultControllerImpl } from "./transform_stream_default_controller.ts";
+import { WritableStreamImpl } from "./writable_stream.ts";
+import { customInspect, inspect } from "../console.ts";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export class TransformStreamImpl<I = any, O = any>
+  implements TransformStream<I, O> {
+  [sym.backpressure]?: boolean;
+  [sym.backpressureChangePromise]?: Deferred<void>;
+  [sym.readable]: ReadableStreamImpl<O>;
+  [sym.transformStreamController]: TransformStreamDefaultControllerImpl<I, O>;
+  [sym.writable]: WritableStreamImpl<I>;
+
+  constructor(
+    transformer: Transformer<I, O> = {},
+    writableStrategy: QueuingStrategy<I> = {},
+    readableStrategy: QueuingStrategy<O> = {}
+  ) {
+    const writableSizeFunction = writableStrategy.size;
+    let writableHighWaterMark = writableStrategy.highWaterMark;
+    const readableSizeFunction = readableStrategy.size;
+    let readableHighWaterMark = readableStrategy.highWaterMark;
+    const writableType = transformer.writableType;
+    if (writableType !== undefined) {
+      throw new RangeError(
+        `Expected transformer writableType to be undefined, received "${String(
+          writableType
+        )}"`
+      );
+    }
+    const writableSizeAlgorithm = makeSizeAlgorithmFromSizeFunction(
+      writableSizeFunction
+    );
+    if (writableHighWaterMark === undefined) {
+      writableHighWaterMark = 1;
+    }
+    writableHighWaterMark = validateAndNormalizeHighWaterMark(
+      writableHighWaterMark
+    );
+    const readableType = transformer.readableType;
+    if (readableType !== undefined) {
+      throw new RangeError(
+        `Expected transformer readableType to be undefined, received "${String(
+          readableType
+        )}"`
+      );
+    }
+    const readableSizeAlgorithm = makeSizeAlgorithmFromSizeFunction(
+      readableSizeFunction
+    );
+    if (readableHighWaterMark === undefined) {
+      readableHighWaterMark = 1;
+    }
+    readableHighWaterMark = validateAndNormalizeHighWaterMark(
+      readableHighWaterMark
+    );
+    const startPromise = getDeferred<void>();
+    initializeTransformStream(
+      this,
+      startPromise.promise,
+      writableHighWaterMark,
+      writableSizeAlgorithm,
+      readableHighWaterMark,
+      readableSizeAlgorithm
+    );
+    // the brand check expects this, and the brand check occurs in the following
+    // but the property hasn't been defined.
+    Object.defineProperty(this, sym.transformStreamController, {
+      value: undefined,
+      writable: true,
+      configurable: true,
+    });
+    setUpTransformStreamDefaultControllerFromTransformer(this, transformer);
+    const startResult: void | PromiseLike<void> = invokeOrNoop(
+      transformer,
+      "start",
+      this[sym.transformStreamController]
+    );
+    startPromise.resolve(startResult);
+  }
+
+  get readable(): ReadableStream<O> {
+    if (!isTransformStream(this)) {
+      throw new TypeError("Invalid TransformStream.");
+    }
+    return this[sym.readable];
+  }
+
+  get writable(): WritableStream<I> {
+    if (!isTransformStream(this)) {
+      throw new TypeError("Invalid TransformStream.");
+    }
+    return this[sym.writable];
+  }
+
+  [customInspect](): string {
+    return `${this.constructor.name} {\n  readable: ${inspect(
+      this.readable
+    )}\n  writable: ${inspect(this.writable)}\n}`;
+  }
+}
+
+setFunctionName(TransformStreamImpl, "TransformStream");
diff --git a/cli/js/web/streams/transform_stream_default_controller.ts b/cli/js/web/streams/transform_stream_default_controller.ts
new file mode 100644
index 0000000000..2fc8d21600
--- /dev/null
+++ b/cli/js/web/streams/transform_stream_default_controller.ts
@@ -0,0 +1,75 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+import {
+  FlushAlgorithm,
+  isTransformStreamDefaultController,
+  readableStreamDefaultControllerGetDesiredSize,
+  setFunctionName,
+  TransformAlgorithm,
+  transformStreamDefaultControllerEnqueue,
+  transformStreamDefaultControllerError,
+  transformStreamDefaultControllerTerminate,
+} from "./internals.ts";
+import { ReadableStreamDefaultControllerImpl } from "./readable_stream_default_controller.ts";
+import * as sym from "./symbols.ts";
+import { TransformStreamImpl } from "./transform_stream.ts";
+import { customInspect } from "../console.ts";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export class TransformStreamDefaultControllerImpl<I = any, O = any>
+  implements TransformStreamDefaultController<O> {
+  [sym.controlledTransformStream]: TransformStreamImpl<I, O>;
+  [sym.flushAlgorithm]: FlushAlgorithm;
+  [sym.transformAlgorithm]: TransformAlgorithm<I>;
+
+  private constructor() {
+    throw new TypeError(
+      "TransformStreamDefaultController's constructor cannot be called."
+    );
+  }
+
+  get desiredSize(): number | null {
+    if (!isTransformStreamDefaultController(this)) {
+      throw new TypeError("Invalid TransformStreamDefaultController.");
+    }
+    const readableController = this[sym.controlledTransformStream][
+      sym.readable
+    ][sym.readableStreamController];
+    return readableStreamDefaultControllerGetDesiredSize(
+      readableController as ReadableStreamDefaultControllerImpl<O>
+    );
+  }
+
+  enqueue(chunk: O): void {
+    if (!isTransformStreamDefaultController(this)) {
+      throw new TypeError("Invalid TransformStreamDefaultController.");
+    }
+    transformStreamDefaultControllerEnqueue(this, chunk);
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  error(reason?: any): void {
+    if (!isTransformStreamDefaultController(this)) {
+      throw new TypeError("Invalid TransformStreamDefaultController.");
+    }
+    transformStreamDefaultControllerError(this, reason);
+  }
+
+  terminate(): void {
+    if (!isTransformStreamDefaultController(this)) {
+      throw new TypeError("Invalid TransformStreamDefaultController.");
+    }
+    transformStreamDefaultControllerTerminate(this);
+  }
+
+  [customInspect](): string {
+    return `${this.constructor.name} { desiredSize: ${String(
+      this.desiredSize
+    )} }`;
+  }
+}
+
+setFunctionName(
+  TransformStreamDefaultControllerImpl,
+  "TransformStreamDefaultController"
+);