diff --git a/index.js b/index.js
index a080448..4a03172 100644
--- a/index.js
+++ b/index.js
@@ -1,25 +1,27 @@
-const readPkg = require('read-pkg');
-const { walk } = require('estree-walker');
-const MagicString = require('magic-string');
+const readPkg = require("read-pkg");
+const { walk } = require("estree-walker");
+const MagicString = require("magic-string");
 const padStr = str => `'${str}'`;
 
-module.exports = ({ transform = (name, version) => `https://unpkg.com/${name}@${version}?type=module` } = {}) => {
+module.exports = ({ transform = (name, version) => `https://unpkg.com/${name}@${version}?type=module`, autoDiscoverExternals = true } = {}) => {
   const pkg = readPkg.sync();
   const cache = {};
   return {
-    name: 'unpkg',
+    name: "unpkg",
     options(opts) {
       let deps = (pkg && pkg.dependencies) || {};
-
+      let userDefinedExternal = Array.isArray(opts.external) ? opts.external : [];
       Object.keys(deps).forEach(dep => {
-        const manifest = readPkg.sync(require.resolve(`${dep}/package.json`));
-        if (manifest.module) cache[manifest.name] = transform(manifest.name, manifest.version);
+        // If auto discover is set to false only add to cache if dep is in external list.
+        if (autoDiscoverExternals || userDefinedExternal.includes(dep)) {
+          const manifest = readPkg.sync({ cwd: `${process.cwd()}/node_modules/${dep}` });
+          if (manifest.module) cache[manifest.name] = transform(manifest.name, manifest.version);
+        }
       });
 
       let external = Object.values(cache);
-      if (Array.isArray(opts.external)) {
-        external = Array.from(new Set(opts.external.concat(external)));
-      }
+      external = Array.from(new Set(userDefinedExternal.concat(external)));
+
       return Object.assign({}, opts, { external });
     },
     transform(code, id) {
@@ -27,7 +29,7 @@ module.exports = ({ transform = (name, version) => `https://unpkg.com/${name}@${
       const magicString = new MagicString(code);
       walk(ast, {
         enter(node, parent) {
-          if (node.type === 'Literal' && parent.type === 'ImportDeclaration') {
+          if (node.type === "Literal" && parent.type === "ImportDeclaration") {
             if (cache[node.value])
               magicString.overwrite(node.start, node.end, padStr(cache[node.value]), {
                 storeName: false