diff --git a/.eslintignore b/.eslintignore
index 254390449..1f9f25268 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -1,2 +1,3 @@
 **/node_modules
 build/
+test/data/esm_*
diff --git a/package-lock.json b/package-lock.json
index 312bfcbeb..04948517f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,6 +1,6 @@
 {
   "name": "@google-cloud/functions-framework",
-  "version": "1.7.1",
+  "version": "1.8.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -8,7 +8,6 @@
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       "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.12.11.tgz",
       "integrity": "sha512-Zt1yodBx1UcyiePMSkWnU4hPqhwq7hGi2nFL1LeA3EUl+q2LQx16MISgJ0+z7dnmgvP9QtIleuETGOiOH1RcIw==",
-      "dev": true,
       "requires": {
         "@babel/highlight": "^7.10.4"
       }
@@ -16,14 +15,12 @@
     "@babel/helper-validator-identifier": {
       "version": "7.12.11",
       "resolved": "https://registry.npmjs.org/@babel/helper-validator-identifier/-/helper-validator-identifier-7.12.11.tgz",
-      "integrity": "sha512-np/lG3uARFybkoHokJUmf1QfEvRVCPbmQeUQpKow5cQ3xWrV9i3rUHodKDJPQfTVX61qKi+UdYk8kik84n7XOw==",
-      "dev": true
+      "integrity": "sha512-np/lG3uARFybkoHokJUmf1QfEvRVCPbmQeUQpKow5cQ3xWrV9i3rUHodKDJPQfTVX61qKi+UdYk8kik84n7XOw=="
     },
     "@babel/highlight": {
       "version": "7.10.4",
       "resolved": "https://registry.npmjs.org/@babel/highlight/-/highlight-7.10.4.tgz",
       "integrity": "sha512-i6rgnR/YgPEQzZZnbTHHuZdlE8qyoBNalD6F+q4vAFlcMEcqmkoG+mPqJYJCo63qPf74+Y1UZsl3l6f7/RIkmA==",
-      "dev": true,
       "requires": {
         "@babel/helper-validator-identifier": "^7.10.4",
         "chalk": "^2.0.0",
@@ -34,7 +31,6 @@
           "version": "2.4.2",
           "resolved": "https://registry.npmjs.org/chalk/-/chalk-2.4.2.tgz",
           "integrity": "sha512-Mti+f9lpJNcwF4tWV8/OrTTtF1gZi+f8FqlyAdouralcFWFQWF2+NgCHShjkCb+IFBLq9buZwE1xckQU4peSuQ==",
-          "dev": true,
           "requires": {
             "ansi-styles": "^3.2.1",
             "escape-string-regexp": "^1.0.5",
@@ -44,14 +40,12 @@
         "has-flag": {
           "version": "3.0.0",
           "resolved": "https://registry.npmjs.org/has-flag/-/has-flag-3.0.0.tgz",
-          "integrity": "sha1-tdRU3CGZriJWmfNGfloH87lVuv0=",
-          "dev": true
+          "integrity": "sha1-tdRU3CGZriJWmfNGfloH87lVuv0="
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         "supports-color": {
           "version": "5.5.0",
           "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-5.5.0.tgz",
           "integrity": "sha512-QjVjwdXIt408MIiAqCX4oUKsgU2EqAGzs2Ppkm4aQYbjm+ZEWEcW4SfFNTr4uMNZma0ey4f5lgLrkB0aX0QMow==",
-          "dev": true,
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@@ -256,8 +250,7 @@
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       "version": "2.4.0",
       "resolved": "https://registry.npmjs.org/@types/normalize-package-data/-/normalize-package-data-2.4.0.tgz",
-      "integrity": "sha512-f5j5b/Gf71L+dbqxIpQ4Z2WlmI/mPJ0fOkGGmFgtb6sAu97EPczzbS3/tJKxmcYDj55OX6ssqwDAWOHIYDRDGA==",
-      "dev": true
+      "integrity": "sha512-f5j5b/Gf71L+dbqxIpQ4Z2WlmI/mPJ0fOkGGmFgtb6sAu97EPczzbS3/tJKxmcYDj55OX6ssqwDAWOHIYDRDGA=="
     },
     "@types/on-finished": {
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@@ -280,6 +273,12 @@
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       "dev": true
     },
+    "@types/semver": {
+      "version": "7.3.6",
+      "resolved": "https://registry.npmjs.org/@types/semver/-/semver-7.3.6.tgz",
+      "integrity": "sha512-0caWDWmpCp0uifxFh+FaqK3CuZ2SkRR/ZRxAV5+zNdC3QVUi6wyOJnefhPvtNt8NQWXB5OA93BUvZsXpWat2Xw==",
+      "dev": true
+    },
     "@types/serve-static": {
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       "resolved": "https://registry.npmjs.org/@types/serve-static/-/serve-static-1.13.9.tgz",
@@ -563,7 +562,6 @@
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-      "dev": true,
       "requires": {
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       }
@@ -981,7 +979,6 @@
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-      "dev": true,
       "requires": {
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@@ -989,8 +986,7 @@
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-      "integrity": "sha1-p9BVi9icQveV3UIyj3QIMcpTvCU=",
-      "dev": true
+      "integrity": "sha1-p9BVi9icQveV3UIyj3QIMcpTvCU="
     },
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@@ -1272,7 +1268,6 @@
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-      "dev": true,
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@@ -1297,8 +1292,7 @@
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-      "integrity": "sha1-G2HAViGQqN/2rjuyzwIAyhMLhtQ=",
-      "dev": true
+      "integrity": "sha1-G2HAViGQqN/2rjuyzwIAyhMLhtQ="
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@@ -1717,7 +1711,6 @@
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-      "dev": true,
       "requires": {
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@@ -1788,8 +1781,7 @@
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       "resolved": "https://registry.npmjs.org/function-bind/-/function-bind-1.1.1.tgz",
-      "integrity": "sha512-yIovAzMX49sF8Yl58fSCWJ5svSLuaibPxXQJFLmBObTuCr0Mf1KiPopGM9NiFjiYBCbfaa2Fh6breQ6ANVTI0A==",
-      "dev": true
+      "integrity": "sha512-yIovAzMX49sF8Yl58fSCWJ5svSLuaibPxXQJFLmBObTuCr0Mf1KiPopGM9NiFjiYBCbfaa2Fh6breQ6ANVTI0A=="
     },
     "functional-red-black-tree": {
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@@ -1940,7 +1932,6 @@
       "version": "1.0.3",
       "resolved": "https://registry.npmjs.org/has/-/has-1.0.3.tgz",
       "integrity": "sha512-f2dvO0VU6Oej7RkWJGrehjbzMAjFp5/VKPp5tTpWIV4JHHZK1/BxbFRtf/siA2SWTe09caDmVtYYzWEIbBS4zw==",
-      "dev": true,
       "requires": {
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       }
@@ -2134,8 +2125,7 @@
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       "resolved": "https://registry.npmjs.org/is-arrayish/-/is-arrayish-0.2.1.tgz",
-      "integrity": "sha1-d8mYQFJ6qOyxqLppe4BkWnqSap0=",
-      "dev": true
+      "integrity": "sha1-d8mYQFJ6qOyxqLppe4BkWnqSap0="
     },
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@@ -2159,7 +2149,6 @@
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-      "dev": true,
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       }
@@ -2258,8 +2247,7 @@
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       "resolved": "https://registry.npmjs.org/js-tokens/-/js-tokens-4.0.0.tgz",
-      "integrity": "sha512-RdJUflcE3cUzKiMqQgsCu06FPu9UdIJO0beYbPhHN4k6apgJtifcoCtT9bcxOpYBtpD2kCM6Sbzg4CausW/PKQ==",
-      "dev": true
+      "integrity": "sha512-RdJUflcE3cUzKiMqQgsCu06FPu9UdIJO0beYbPhHN4k6apgJtifcoCtT9bcxOpYBtpD2kCM6Sbzg4CausW/PKQ=="
     },
     "js-yaml": {
       "version": "3.14.1",
@@ -2280,8 +2268,7 @@
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       "resolved": "https://registry.npmjs.org/json-parse-even-better-errors/-/json-parse-even-better-errors-2.3.1.tgz",
-      "integrity": "sha512-xyFwyhro/JEof6Ghe2iz2NcXoj2sloNsWr/XsERDK/oiPCfaNhl5ONfp+jQdAZRQQ0IJWNzH9zIZF7li91kh2w==",
-      "dev": true
+      "integrity": "sha512-xyFwyhro/JEof6Ghe2iz2NcXoj2sloNsWr/XsERDK/oiPCfaNhl5ONfp+jQdAZRQQ0IJWNzH9zIZF7li91kh2w=="
     },
     "json-schema-traverse": {
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@@ -2355,14 +2342,12 @@
     "lines-and-columns": {
       "version": "1.1.6",
       "resolved": "https://registry.npmjs.org/lines-and-columns/-/lines-and-columns-1.1.6.tgz",
-      "integrity": "sha1-HADHQ7QzzQpOgHWPe2SldEDZ/wA=",
-      "dev": true
+      "integrity": "sha1-HADHQ7QzzQpOgHWPe2SldEDZ/wA="
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     "locate-path": {
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       "resolved": "https://registry.npmjs.org/locate-path/-/locate-path-5.0.0.tgz",
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-      "dev": true,
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       }
@@ -2398,7 +2383,6 @@
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-      "dev": true,
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@@ -2830,7 +2814,6 @@
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@@ -2839,7 +2822,6 @@
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-      "dev": true,
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@@ -2847,8 +2829,7 @@
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-      "dev": true
+      "integrity": "sha512-R4nPAVTAU0B9D35/Gk3uJf/7XYbQcyohSKdvAxIRSNghFl4e71hVoGnBNQz9cWaXxO2I10KTC+3jMdvvoKw6dQ=="
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@@ -2883,7 +2864,6 @@
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-      "dev": true,
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@@ -2899,8 +2879,7 @@
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-      "dev": true
+      "integrity": "sha512-ak9Qy5Q7jYb2Wwcey5Fpvg2KoAc/ZIhLSLOSBmRmygPsGwkVVt0fZa0qrtMz+m6tJTAHfZQ8FnmB4MG4LWy7/w=="
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     "path-is-absolute": {
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@@ -2917,8 +2896,7 @@
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@@ -3184,7 +3162,6 @@
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@@ -3195,14 +3172,12 @@
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-          "dev": true
+          "integrity": "sha512-mxIDAb9Lsm6DoOJ7xH+5+X4y1LU/4Hi50L9C5sIswK3JzULS4bwk1FvjdBgvYR4bzT4tuUQiC15FE2f5HbLvYw=="
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@@ -3213,14 +3188,12 @@
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@@ -3305,7 +3277,6 @@
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@@ -3383,10 +3354,9 @@
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-      "integrity": "sha512-tCfb2WLjqFAtXn4KEdxIhalnRtoKFN7nAwj0B3ZXCbQloV2tq5eDbcTmT68JJD3nRJq24/XgxtQKFIpQdtvmVw==",
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+      "version": "7.3.5",
+      "resolved": "https://registry.npmjs.org/semver/-/semver-7.3.5.tgz",
+      "integrity": "sha512-PoeGJYh8HK4BTO/a9Tf6ZG3veo/A7ZVsYrSA6J8ny9nb3B1VrpkuN+z9OE5wfE5p6H4LchYZsegiQgbJD94ZFQ==",
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@@ -3556,7 +3526,6 @@
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-      "dev": true,
       "requires": {
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@@ -3565,14 +3534,12 @@
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       "version": "2.3.0",
       "resolved": "https://registry.npmjs.org/spdx-exceptions/-/spdx-exceptions-2.3.0.tgz",
-      "integrity": "sha512-/tTrYOC7PPI1nUAgx34hUpqXuyJG+DTHJTnIULG4rDygi4xu/tfgmq1e1cIRwRzwZgo4NLySi+ricLkZkw4i5A==",
-      "dev": true
+      "integrity": "sha512-/tTrYOC7PPI1nUAgx34hUpqXuyJG+DTHJTnIULG4rDygi4xu/tfgmq1e1cIRwRzwZgo4NLySi+ricLkZkw4i5A=="
     },
     "spdx-expression-parse": {
       "version": "3.0.1",
       "resolved": "https://registry.npmjs.org/spdx-expression-parse/-/spdx-expression-parse-3.0.1.tgz",
       "integrity": "sha512-cbqHunsQWnJNE6KhVSMsMeH5H/L9EpymbzqTQ3uLwNCLZ1Q481oWaofqH7nO6V07xlXwY6PhQdQ2IedWx/ZK4Q==",
-      "dev": true,
       "requires": {
         "spdx-exceptions": "^2.1.0",
         "spdx-license-ids": "^3.0.0"
@@ -3581,8 +3548,7 @@
     "spdx-license-ids": {
       "version": "3.0.7",
       "resolved": "https://registry.npmjs.org/spdx-license-ids/-/spdx-license-ids-3.0.7.tgz",
-      "integrity": "sha512-U+MTEOO0AiDzxwFvoa4JVnMV6mZlJKk2sBLt90s7G0Gd0Mlknc7kxEn3nuDPNZRta7O2uy8oLcZLVT+4sqNZHQ==",
-      "dev": true
+      "integrity": "sha512-U+MTEOO0AiDzxwFvoa4JVnMV6mZlJKk2sBLt90s7G0Gd0Mlknc7kxEn3nuDPNZRta7O2uy8oLcZLVT+4sqNZHQ=="
     },
     "sprintf-js": {
       "version": "1.0.3",
@@ -3727,15 +3693,6 @@
           "resolved": "https://registry.npmjs.org/qs/-/qs-6.9.6.tgz",
           "integrity": "sha512-TIRk4aqYLNoJUbd+g2lEdz5kLWIuTMRagAXxl78Q0RiVjAOugHmeKNGdd3cwo/ktpf9aL9epCfFqWDEKysUlLQ==",
           "dev": true
-        },
-        "semver": {
-          "version": "7.3.4",
-          "resolved": "https://registry.npmjs.org/semver/-/semver-7.3.4.tgz",
-          "integrity": "sha512-tCfb2WLjqFAtXn4KEdxIhalnRtoKFN7nAwj0B3ZXCbQloV2tq5eDbcTmT68JJD3nRJq24/XgxtQKFIpQdtvmVw==",
-          "dev": true,
-          "requires": {
-            "lru-cache": "^6.0.0"
-          }
         }
       }
     },
@@ -3920,8 +3877,7 @@
     "type-fest": {
       "version": "0.8.1",
       "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-0.8.1.tgz",
-      "integrity": "sha512-4dbzIzqvjtgiM5rw1k5rEHtBANKmdudhGyBEajN01fEyhaAIhsoKNy6y7+IN93IfpFtwY9iqi7kD+xwKhQsNJA==",
-      "dev": true
+      "integrity": "sha512-4dbzIzqvjtgiM5rw1k5rEHtBANKmdudhGyBEajN01fEyhaAIhsoKNy6y7+IN93IfpFtwY9iqi7kD+xwKhQsNJA=="
     },
     "type-is": {
       "version": "1.6.18",
@@ -4039,7 +3995,6 @@
       "version": "3.0.4",
       "resolved": "https://registry.npmjs.org/validate-npm-package-license/-/validate-npm-package-license-3.0.4.tgz",
       "integrity": "sha512-DpKm2Ui/xN7/HQKCtpZxoRWBhZ9Z0kqtygG8XCgNQ8ZlDnxuQmWhj566j8fN4Cu3/JmbhsDo7fcAJq4s9h27Ew==",
-      "dev": true,
       "requires": {
         "spdx-correct": "^3.0.0",
         "spdx-expression-parse": "^3.0.0"
@@ -4243,8 +4198,7 @@
     "yallist": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/yallist/-/yallist-4.0.0.tgz",
-      "integrity": "sha512-3wdGidZyq5PB084XLES5TpOSRA3wjXAlIWMhum2kRcv/41Sn2emQ0dycQW4uZXLejwKvg6EsvbdlVL+FYEct7A==",
-      "dev": true
+      "integrity": "sha512-3wdGidZyq5PB084XLES5TpOSRA3wjXAlIWMhum2kRcv/41Sn2emQ0dycQW4uZXLejwKvg6EsvbdlVL+FYEct7A=="
     },
     "yargs": {
       "version": "16.2.0",
diff --git a/package.json b/package.json
index e8cb84e72..c306ba111 100644
--- a/package.json
+++ b/package.json
@@ -12,7 +12,9 @@
     "body-parser": "^1.18.3",
     "express": "^4.16.4",
     "minimist": "^1.2.5",
-    "on-finished": "^2.3.0"
+    "on-finished": "^2.3.0",
+    "read-pkg-up": "^7.0.1",
+    "semver": "^7.3.5"
   },
   "scripts": {
     "test": "mocha build/test --recursive",
@@ -41,6 +43,7 @@
     "@types/mocha": "8.2.2",
     "@types/node": "11.15.50",
     "@types/on-finished": "2.3.1",
+    "@types/semver": "^7.3.6",
     "@types/sinon": "^10.0.0",
     "@types/supertest": "2.0.11",
     "gts": "3.1.0",
diff --git a/src/index.ts b/src/index.ts
index 3175263a1..7a70aa49d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -96,22 +96,23 @@ Documentation:
   process.exit(0);
 }
 
-const USER_FUNCTION = getUserFunction(CODE_LOCATION, TARGET);
-if (!USER_FUNCTION) {
-  console.error('Could not load the function, shutting down.');
-  // eslint-disable-next-line no-process-exit
-  process.exit(1);
-}
+getUserFunction(CODE_LOCATION, TARGET).then(userFunction => {
+  if (!userFunction) {
+    console.error('Could not load the function, shutting down.');
+    // eslint-disable-next-line no-process-exit
+    process.exit(1);
+  }
 
-const SERVER = getServer(USER_FUNCTION!, SIGNATURE_TYPE!);
-const ERROR_HANDLER = new ErrorHandler(SERVER);
+  const SERVER = getServer(userFunction!, SIGNATURE_TYPE!);
+  const ERROR_HANDLER = new ErrorHandler(SERVER);
 
-SERVER.listen(PORT, () => {
-  ERROR_HANDLER.register();
-  if (process.env.NODE_ENV !== NodeEnv.PRODUCTION) {
-    console.log('Serving function...');
-    console.log(`Function: ${TARGET}`);
-    console.log(`Signature type: ${SIGNATURE_TYPE}`);
-    console.log(`URL: http://localhost:${PORT}/`);
-  }
-}).setTimeout(0); // Disable automatic timeout on incoming connections.
+  SERVER.listen(PORT, () => {
+    ERROR_HANDLER.register();
+    if (process.env.NODE_ENV !== NodeEnv.PRODUCTION) {
+      console.log('Serving function...');
+      console.log(`Function: ${TARGET}`);
+      console.log(`Signature type: ${SIGNATURE_TYPE}`);
+      console.log(`URL: http://localhost:${PORT}/`);
+    }
+  }).setTimeout(0); // Disable automatic timeout on incoming connections.
+});
diff --git a/src/loader.ts b/src/loader.ts
index e7f5c9a64..fd24c6d85 100644
--- a/src/loader.ts
+++ b/src/loader.ts
@@ -18,20 +18,75 @@
  * @packageDocumentation
  */
 
+import * as path from 'path';
+import * as semver from 'semver';
+import * as readPkgUp from 'read-pkg-up';
+import {pathToFileURL} from 'url';
 /**
  * Import function signature type's definition.
  */
 import {HandlerFunction} from './functions';
 
+// Dynamic import function required to load user code packaged as an
+// ES module is only available on Node.js v13.2.0 and up.
+//   https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#browser_compatibility
+// Exported for testing.
+export const MIN_NODE_VERSION_ESMODULES = '13.2.0';
+
+/**
+ * Determines whether the given module is an ES module.
+ *
+ * Implements "algorithm" described at:
+ *   https://nodejs.org/api/packages.html#packages_type
+ *
+ * In words:
+ *   1. A module with .mjs extension is an ES module.
+ *   2. A module with .clj extension is not an ES module.
+ *   3. A module with .js extensions where the nearest package.json's
+ *      with "type": "module" is an ES module.
+ *   4. Otherwise, it is not an ES module.
+ *
+ * @returns {Promise<boolean>} True if module is an ES module.
+ */
+async function isEsModule(modulePath: string): Promise<boolean> {
+  const ext = path.extname(modulePath);
+  if (ext === '.mjs') {
+    return true;
+  }
+  if (ext === '.cjs') {
+    return false;
+  }
+
+  const pkg = await readPkgUp({
+    cwd: path.dirname(modulePath),
+    normalize: false,
+  });
+
+  // If package.json specifies type as 'module', it's an ES module.
+  return pkg?.packageJson.type === 'module';
+}
+
+/**
+ * Dynamically load import function to prevent TypeScript from
+ * transpiling into a require.
+ *
+ * See https://github.com/microsoft/TypeScript/issues/43329.
+ */
+const dynamicImport = new Function(
+  'modulePath',
+  'return import(modulePath)'
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+) as (modulePath: string) => Promise<any>;
+
 /**
  * Returns user's function from function file.
  * Returns null if function can't be retrieved.
  * @return User's function or null.
  */
-export function getUserFunction(
+export async function getUserFunction(
   codeLocation: string,
   functionTarget: string
-): HandlerFunction | null {
+): Promise<HandlerFunction | null> {
   try {
     const functionModulePath = getFunctionModulePath(codeLocation);
     if (functionModulePath === null) {
@@ -39,8 +94,23 @@ export function getUserFunction(
       return null;
     }
 
-    // eslint-disable-next-line @typescript-eslint/no-var-requires
-    const functionModule = require(functionModulePath);
+    let functionModule;
+    const esModule = await isEsModule(functionModulePath);
+    if (esModule) {
+      if (semver.lt(process.version, MIN_NODE_VERSION_ESMODULES)) {
+        console.error(
+          `Cannot load ES Module on Node.js ${process.version}. ` +
+            `Please upgrade to Node.js v${MIN_NODE_VERSION_ESMODULES} and up.`
+        );
+        return null;
+      }
+      // Resolve module path to file:// URL. Required for windows support.
+      const fpath = pathToFileURL(functionModulePath);
+      functionModule = await dynamicImport(fpath.href);
+    } else {
+      functionModule = require(functionModulePath);
+    }
+
     let userFunction = functionTarget
       .split('.')
       .reduce((code, functionTargetPart) => {
diff --git a/test/data/esm_mjs/foo.mjs b/test/data/esm_mjs/foo.mjs
new file mode 100644
index 000000000..b20ba2b9f
--- /dev/null
+++ b/test/data/esm_mjs/foo.mjs
@@ -0,0 +1,12 @@
+/*eslint no-unused-vars: "off"*/
+/**
+ * Test HTTP function to test function loading.
+ *
+ * @param {!Object} req request context.
+ * @param {!Object} res response context.
+ */
+function testFunction(req, res) {
+  return 'PASS';
+}
+
+export {testFunction};
diff --git a/test/data/esm_mjs/package.json b/test/data/esm_mjs/package.json
new file mode 100644
index 000000000..cbf76e822
--- /dev/null
+++ b/test/data/esm_mjs/package.json
@@ -0,0 +1,3 @@
+{
+  "main": "foo.mjs"
+}
diff --git a/test/data/esm_nested/nested/foo.js b/test/data/esm_nested/nested/foo.js
new file mode 100644
index 000000000..b20ba2b9f
--- /dev/null
+++ b/test/data/esm_nested/nested/foo.js
@@ -0,0 +1,12 @@
+/*eslint no-unused-vars: "off"*/
+/**
+ * Test HTTP function to test function loading.
+ *
+ * @param {!Object} req request context.
+ * @param {!Object} res response context.
+ */
+function testFunction(req, res) {
+  return 'PASS';
+}
+
+export {testFunction};
diff --git a/test/data/esm_nested/package.json b/test/data/esm_nested/package.json
new file mode 100644
index 000000000..9ee8b9d5c
--- /dev/null
+++ b/test/data/esm_nested/package.json
@@ -0,0 +1,4 @@
+{
+  "main": "nested/foo.js",
+  "type": "module"
+}
diff --git a/test/data/esm_type/foo.js b/test/data/esm_type/foo.js
new file mode 100644
index 000000000..b20ba2b9f
--- /dev/null
+++ b/test/data/esm_type/foo.js
@@ -0,0 +1,12 @@
+/*eslint no-unused-vars: "off"*/
+/**
+ * Test HTTP function to test function loading.
+ *
+ * @param {!Object} req request context.
+ * @param {!Object} res response context.
+ */
+function testFunction(req, res) {
+  return 'PASS';
+}
+
+export {testFunction};
diff --git a/test/data/esm_type/package.json b/test/data/esm_type/package.json
new file mode 100644
index 000000000..e10c6cb43
--- /dev/null
+++ b/test/data/esm_type/package.json
@@ -0,0 +1,4 @@
+{
+  "main": "foo.js",
+  "type": "module"
+}
diff --git a/test/loader.ts b/test/loader.ts
index 4da92998b..b8b3fe198 100644
--- a/test/loader.ts
+++ b/test/loader.ts
@@ -14,6 +14,7 @@
 
 import * as assert from 'assert';
 import * as express from 'express';
+import * as semver from 'semver';
 import * as functions from '../src/functions';
 import * as loader from '../src/loader';
 
@@ -38,13 +39,51 @@ describe('loading function', () => {
   ];
 
   for (const test of testData) {
-    it(`should load ${test.name}`, () => {
-      const loadedFunction = loader.getUserFunction(
+    it(`should load ${test.name}`, async () => {
+      const loadedFunction = (await loader.getUserFunction(
         process.cwd() + test.codeLocation,
         test.target
-      ) as functions.HttpFunction;
+      )) as functions.HttpFunction;
       const returned = loadedFunction(express.request, express.response);
       assert.strictEqual(returned, 'PASS');
     });
   }
+
+  const esmTestData: TestData[] = [
+    {
+      name: 'specified in package.json type field',
+      codeLocation: '/test/data/esm_type',
+      target: 'testFunction',
+    },
+    {
+      name: 'nested dir, specified in package.json type field',
+      codeLocation: '/test/data/esm_nested',
+      target: 'testFunction',
+    },
+    {
+      name: '.mjs extension',
+      codeLocation: '/test/data/esm_mjs',
+      target: 'testFunction',
+    },
+  ];
+
+  for (const test of esmTestData) {
+    const loadFn: () => Promise<functions.HttpFunction> = async () => {
+      return loader.getUserFunction(
+        process.cwd() + test.codeLocation,
+        test.target
+      ) as Promise<functions.HttpFunction>;
+    };
+    if (semver.lt(process.version, loader.MIN_NODE_VERSION_ESMODULES)) {
+      it(`should fail to load function in an ES module ${test.name}`, async () => {
+        assert.rejects(loadFn);
+      });
+    } else {
+      it(`should load function in an ES module ${test.name}`, async () => {
+        const loadedFunction = await loadFn();
+        const returned = loadedFunction(express.request, express.response);
+        assert.strictEqual(returned, 'PASS');
+      });
+    }
+  }
 });