diff --git a/package.json b/package.json
index 3fd6bc0..a9dbe11 100644
--- a/package.json
+++ b/package.json
@@ -2,8 +2,16 @@
   "name": "@andrewmacmurray/elm-concurrent-task",
   "version": "1.0.0",
   "description": "Run a tree of Tasks concurrently, call JS functions as Tasks (Task Ports)",
-  "main": "lib/index.js",
-  "types": "lib/index.d.ts",
+  "main": "lib/cjs/index.js",
+  "module": "lib/mjs/index.js",
+  "types": "lib/mjs/index.d.ts",
+  "exports": {
+    ".": {
+      "import": "./lib/mjs/index.js",
+      "require": "./lib/cjs/index.js",
+      "types": "./lib/mjs/index.d.ts"
+    }
+  },
   "files": [
     "lib"
   ],
@@ -14,7 +22,7 @@
     "review": "elm-review --fix",
     "review:ci": "elm-review",
     "review:watch": "elm-review --watch",
-    "runner:compile": "tsc",
+    "runner:compile": "rm -rf lib/* && tsc -p tsconfig-mjs.json && tsc -p tsconfig-cjs.json && ./scripts/package-fixup.sh",
     "check-versions": "node ./scripts/versions.js"
   },
   "author": "Andrew MacMurray",
diff --git a/runner/http/fetch.ts b/runner/http/fetch.ts
index 48663bb..dc5dd06 100644
--- a/runner/http/fetch.ts
+++ b/runner/http/fetch.ts
@@ -15,7 +15,11 @@ export function http(request: HttpRequest): Promise<HttpResponse> {
     signal: controller?.signal,
   })
     .then((res: Response) => {
-      const headers = Object.fromEntries(res.headers.entries());
+      const headers: { [key: string]: string } = {};
+      res.headers.forEach((val, key) => {
+        headers[key] = val;
+      });
+
       switch (request.expect) {
         case "STRING": {
           return res.text().then((x) => ({
diff --git a/scripts/package-fixup.sh b/scripts/package-fixup.sh
new file mode 100755
index 0000000..c0c82bd
--- /dev/null
+++ b/scripts/package-fixup.sh
@@ -0,0 +1,15 @@
+#! /bin/bash
+
+# Adds package.json files to cjs/mjs subtrees
+
+cat >lib/cjs/package.json <<!EOF
+{
+    "type": "commonjs"
+}
+!EOF
+
+cat >lib/mjs/package.json <<!EOF
+{
+    "type": "module"
+}
+!EOF
\ No newline at end of file
diff --git a/tsconfig.json b/tsconfig-base.json
similarity index 73%
rename from tsconfig.json
rename to tsconfig-base.json
index 528fc12..5983a7d 100644
--- a/tsconfig.json
+++ b/tsconfig-base.json
@@ -1,33 +1,21 @@
 {
   "compilerOptions": {
-    "outDir": "lib",
     "removeComments": true,
-    "target": "ESNext",
     "declaration": true,
-
-    // Module resolution
     "baseUrl": "runner",
     "esModuleInterop": true,
     "moduleResolution": "node",
-
-    // Source Map
     "sourceMap": true,
     "sourceRoot": "./runner",
-
-    // Strict Checks
     "alwaysStrict": true,
     "allowUnreachableCode": false,
     "noImplicitAny": true,
     "strictNullChecks": true,
-
-    // Linter Checks
     "noImplicitReturns": true,
     "noUncheckedIndexedAccess": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true
   },
   "include": ["runner/**/*.ts"],
-  "exclude": [
-    "node_modules/**/*"
-  ]
-}
\ No newline at end of file
+  "exclude": ["node_modules/**/*"]
+}
diff --git a/tsconfig-cjs.json b/tsconfig-cjs.json
new file mode 100644
index 0000000..9036be0
--- /dev/null
+++ b/tsconfig-cjs.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "module": "commonjs",
+    "outDir": "lib/cjs",
+    "target": "ES2015"
+  }
+}
diff --git a/tsconfig-mjs.json b/tsconfig-mjs.json
new file mode 100644
index 0000000..20fa2c5
--- /dev/null
+++ b/tsconfig-mjs.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "module": "esnext",
+    "outDir": "lib/mjs",
+    "target": "ESNext"
+  }
+}