diff --git a/CHANGELOG.md b/CHANGELOG.md
index 66ba0436d47..eefce071c0c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+* JavaScript build API can now avoid writing to the file system ([#139](https://github.com/evanw/esbuild/issues/139) and [#220](https://github.com/evanw/esbuild/issues/220))
+
+    You can now pass `write: false` to the JavaScript build API to avoid writing to the file system. Instead, the returned object will have the `outputFiles` property with an array of output files, each of which has a string `path` property and a Uint8Array `contents` property. This brings the JavaScript API to parity with the Go API, which already had this feature.
+
 ## 0.5.21
 
 * Binaries for FreeBSD ([#217](https://github.com/evanw/esbuild/pull/217))
diff --git a/cmd/esbuild/service.go b/cmd/esbuild/service.go
index 62e6ea5eb3c..b5bc38b38b8 100644
--- a/cmd/esbuild/service.go
+++ b/cmd/esbuild/service.go
@@ -183,6 +183,17 @@ func handlePingRequest(responses chan responseType, id string, rawArgs []string)
 }
 
 func handleBuildRequest(responses chan responseType, id string, rawArgs []string) {
+	// Special-case the service-only write flag
+	write := true
+	for i, arg := range rawArgs {
+		if arg == "--write=false" {
+			write = false
+			copy(rawArgs[i:], rawArgs[i+1:])
+			rawArgs = rawArgs[:len(rawArgs)-1]
+			break
+		}
+	}
+
 	options, err := cli.ParseBuildOptions(rawArgs)
 	if err != nil {
 		responses <- responseType{
@@ -193,21 +204,41 @@ func handleBuildRequest(responses chan responseType, id string, rawArgs []string
 	}
 
 	result := api.Build(options)
-	for _, outputFile := range result.OutputFiles {
-		if err := os.MkdirAll(filepath.Dir(outputFile.Path), 0755); err != nil {
-			result.Errors = append(result.Errors, api.Message{Text: fmt.Sprintf(
-				"Failed to create output directory: %s", err.Error())})
-		} else if err := ioutil.WriteFile(outputFile.Path, outputFile.Contents, 0644); err != nil {
-			result.Errors = append(result.Errors, api.Message{Text: fmt.Sprintf(
-				"Failed to write to output file: %s", err.Error())})
-		}
-	}
-
-	responses <- responseType{
+	response := responseType{
 		"id":       []byte(id),
 		"errors":   messagesToJSON(result.Errors),
 		"warnings": messagesToJSON(result.Warnings),
 	}
+
+	if write {
+		// Write the output files to disk
+		for _, outputFile := range result.OutputFiles {
+			if err := os.MkdirAll(filepath.Dir(outputFile.Path), 0755); err != nil {
+				result.Errors = append(result.Errors, api.Message{Text: fmt.Sprintf(
+					"Failed to create output directory: %s", err.Error())})
+			} else if err := ioutil.WriteFile(outputFile.Path, outputFile.Contents, 0644); err != nil {
+				result.Errors = append(result.Errors, api.Message{Text: fmt.Sprintf(
+					"Failed to write to output file: %s", err.Error())})
+			}
+		}
+	} else {
+		// Pass the output files back to the caller
+		length := 4
+		for _, outputFile := range result.OutputFiles {
+			length += 4 + len(outputFile.Path) + 4 + len(outputFile.Contents)
+		}
+		bytes := make([]byte, 0, length)
+		bytes = writeUint32(bytes, uint32(len(result.OutputFiles)))
+		for _, outputFile := range result.OutputFiles {
+			bytes = writeUint32(bytes, uint32(len(outputFile.Path)))
+			bytes = append(bytes, outputFile.Path...)
+			bytes = writeUint32(bytes, uint32(len(outputFile.Contents)))
+			bytes = append(bytes, outputFile.Contents...)
+		}
+		response["outputFiles"] = bytes
+	}
+
+	responses <- response
 }
 
 func handleTransformRequest(responses chan responseType, id string, rawArgs []string) {
diff --git a/lib/api-common.ts b/lib/api-common.ts
index 4689754d248..3fb9e95be6a 100644
--- a/lib/api-common.ts
+++ b/lib/api-common.ts
@@ -36,6 +36,7 @@ function flagsForBuildOptions(options: types.BuildOptions, isTTY: boolean): stri
   if (options.resolveExtensions) flags.push(`--resolve-extensions=${options.resolveExtensions.join(',')}`);
   if (options.external) for (let name of options.external) flags.push(`--external:${name}`);
   if (options.loader) for (let ext in options.loader) flags.push(`--loader:${ext}=${options.loader[ext]}`);
+  if (options.write === false) flags.push(`--write=false`);
 
   for (let entryPoint of options.entryPoints) {
     if (entryPoint.startsWith('-')) throw new Error(`Invalid entry point: ${entryPoint}`);
@@ -57,7 +58,7 @@ function flagsForTransformOptions(options: types.TransformOptions, isTTY: boolea
 }
 
 type Request = string[];
-type Response = { [key: string]: string };
+type Response = { [key: string]: Uint8Array };
 type ResponseCallback = (err: string | null, res: Response) => void;
 
 export interface StreamIn {
@@ -198,8 +199,8 @@ export function createChannel(options: StreamIn): StreamOut {
       let keyLength = readUInt32LE(bytes, eat(4));
       let key = codec.decode(bytes.slice(offset, eat(keyLength) + keyLength));
       let valueLength = readUInt32LE(bytes, eat(4));
-      let value = codec.decode(bytes.slice(offset, eat(valueLength) + valueLength));
-      if (key === 'id') id = value;
+      let value = bytes.slice(offset, eat(valueLength) + valueLength);
+      if (key === 'id') id = codec.decode(value);
       else response[key] = value;
     }
 
@@ -207,7 +208,7 @@ export function createChannel(options: StreamIn): StreamOut {
     if (!id) throw new Error('Invalid message');
     let callback = requests.get(id)!;
     requests.delete(id);
-    if (response.error) callback(response.error, {});
+    if (response.error) callback(codec.decode(response.error), {});
     else callback(null, response);
   };
 
@@ -220,10 +221,12 @@ export function createChannel(options: StreamIn): StreamOut {
         let flags = flagsForBuildOptions(options, isTTY);
         sendRequest(['build'].concat(flags), (error, response) => {
           if (error) return callback(new Error(error), null);
-          let errors = jsonToMessages(response.errors);
-          let warnings = jsonToMessages(response.warnings);
+          let errors = jsonToMessages(codec.decode(response.errors));
+          let warnings = jsonToMessages(codec.decode(response.warnings));
           if (errors.length > 0) return callback(failureErrorWithLog('Build failed', errors, warnings), null);
-          callback(null, { warnings });
+          let result: types.BuildResult = { warnings };
+          if (options.write === false) result.outputFiles = decodeOutputFiles(codec, response.outputFiles);
+          callback(null, result);
         });
       },
 
@@ -231,10 +234,10 @@ export function createChannel(options: StreamIn): StreamOut {
         let flags = flagsForTransformOptions(options, isTTY);
         sendRequest(['transform', input].concat(flags), (error, response) => {
           if (error) return callback(new Error(error), null);
-          let errors = jsonToMessages(response.errors);
-          let warnings = jsonToMessages(response.warnings);
+          let errors = jsonToMessages(codec.decode(response.errors));
+          let warnings = jsonToMessages(codec.decode(response.warnings));
           if (errors.length > 0) return callback(failureErrorWithLog('Transform failed', errors, warnings), null);
-          callback(null, { warnings, js: response.js, jsSourceMap: response.jsSourceMap });
+          callback(null, { warnings, js: codec.decode(response.js), jsSourceMap: codec.decode(response.jsSourceMap) });
         });
       },
     },
@@ -287,3 +290,20 @@ function failureErrorWithLog(text: string, errors: types.Message[], warnings: ty
   error.warnings = warnings;
   return error;
 }
+
+function decodeOutputFiles(codec: TextCodec, bytes: Uint8Array): types.OutputFile[] {
+  let outputFiles: types.OutputFile[] = [];
+  let offset = 0;
+  let count = readUInt32LE(bytes, offset);
+  offset += 4;
+  for (let i = 0; i < count; i++) {
+    let pathLength = readUInt32LE(bytes, offset);
+    let path = codec.decode(bytes.slice(offset + 4, offset + 4 + pathLength));
+    offset += 4 + pathLength;
+    let contentsLength = readUInt32LE(bytes, offset);
+    let contents = new Uint8Array(bytes.slice(offset + 4, offset + 4 + contentsLength));
+    offset += 4 + contentsLength;
+    outputFiles.push({ path, contents });
+  }
+  return outputFiles;
+}
diff --git a/lib/api-types.ts b/lib/api-types.ts
index d553a8eb6f3..aa4a1506995 100644
--- a/lib/api-types.ts
+++ b/lib/api-types.ts
@@ -37,6 +37,7 @@ export interface BuildOptions extends CommonOptions {
   external?: string[];
   loader?: { [ext: string]: Loader };
   resolveExtensions?: string[];
+  write?: boolean;
 
   entryPoints: string[];
 }
@@ -52,8 +53,14 @@ export interface Message {
   };
 }
 
+export interface OutputFile {
+  path: string;
+  contents: Uint8Array;
+}
+
 export interface BuildResult {
   warnings: Message[];
+  outputFiles?: OutputFile[]; // Only when "write: false"
 }
 
 export interface BuildFailure extends Error {
diff --git a/scripts/js-api-tests.js b/scripts/js-api-tests.js
index e619da858d6..09ef011c78e 100644
--- a/scripts/js-api-tests.js
+++ b/scripts/js-api-tests.js
@@ -14,7 +14,8 @@ let buildTests = {
     const input = path.join(testDir, 'es6_to_cjs-in.js')
     const output = path.join(testDir, 'es6_to_cjs-out.js')
     await util.promisify(fs.writeFile)(input, 'export default 123')
-    await esbuild.build({ entryPoints: [input], bundle: true, outfile: output, format: 'cjs' })
+    const value = await esbuild.build({ entryPoints: [input], bundle: true, outfile: output, format: 'cjs' })
+    assert.strictEqual(value.outputFiles, void 0)
     const result = require(output)
     assert.strictEqual(result.default, 123)
     assert.strictEqual(result.__esModule, true)
@@ -143,6 +144,32 @@ let buildTests = {
     assert.strictEqual(typeof outputInputs[makePath(imported)].bytesInOutput, 'number')
     assert.strictEqual(typeof outputInputs[makePath(text)].bytesInOutput, 'number')
   },
+
+  // Test in-memory output files
+  async writeFalse({ esbuild }) {
+    const input = path.join(testDir, 'writeFalse.js')
+    const output = path.join(testDir, 'writeFalse-out.js')
+    await util.promisify(fs.writeFile)(input, 'console.log()')
+    const value = await esbuild.build({
+      entryPoints: [input],
+      bundle: true,
+      outfile: output,
+      sourcemap: true,
+      format: 'esm',
+      write: false,
+    })
+    assert.strictEqual(await fs.existsSync(output), false)
+    assert.notStrictEqual(value.outputFiles, void 0)
+    assert.strictEqual(value.outputFiles.length, 2)
+    assert.strictEqual(value.outputFiles[0].path, output + '.map')
+    assert.strictEqual(value.outputFiles[0].contents.constructor, Uint8Array)
+    assert.strictEqual(value.outputFiles[1].path, output)
+    assert.strictEqual(value.outputFiles[1].contents.constructor, Uint8Array)
+    const sourceMap = JSON.parse(Buffer.from(value.outputFiles[0].contents).toString())
+    const js = Buffer.from(value.outputFiles[1].contents).toString()
+    assert.strictEqual(sourceMap.version, 3)
+    assert.strictEqual(js, `// scripts/.js-api-tests/writeFalse.js\nconsole.log();\n//# sourceMappingURL=writeFalse-out.js.map\n`)
+  },
 }
 
 async function futureSyntax(service, js, targetBelow, targetAbove) {