diff --git a/src/index.js b/src/index.js
index f6d1e99..c35391b 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,5 +1,7 @@
-const { exec } = require("child_process");
 const fs = require("fs");
+const util = require("util")
+
+const pWriteFile = util.promisify(fs.writeFile)
 
 /**
  * Overrides an ENV var with a value if it exists
@@ -8,52 +10,29 @@ const fs = require("fs");
  * @param {*} mode the mode to use (prefix or suffix)
  */
 function setEnvWithValue(key, contextOrBranch, mode) {
-  let foundOne = false;
-  let found;
-
-  if (mode === "prefix") {
-    const prefixedEnvVar = `${contextOrBranch}_${key}`;
-
-    if (process.env[prefixedEnvVar]) {
-      console.log(`Setting ${key} to the value from ${prefixedEnvVar}.`);
-      process.env[key] = process.env[prefixedEnvVar];
-      found = `${key}=${process.env[prefixedEnvVar]}`;
-    }
-  } else {
-    const suffixedEnvVar = `${key}_${contextOrBranch}`;
+  const envVar = mode === 'prefix' ? `${contextOrBranch}_${key}` : `${key}_${contextOrBranch}`;
 
-    if (process.env[suffixedEnvVar]) {
-      console.log(`Setting ${key} to the value from ${suffixedEnvVar}.`);
-      process.env[key] = process.env[suffixedEnvVar];
-      found = `${key}=${process.env[suffixedEnvVar]}`;
-    }
+  if (!process.env[envVar]) {
+    return;
   }
 
-  return found;
+  console.log(`Setting ${key} to the value from ${envVar}.`);
+  process.env[key] = process.env[envVar];
+  return `${key}=${process.env[envVar]}\n`;
 }
 
 module.exports = {
-  onPreBuild: ({ inputs }) => {
+  onPreBuild: async ({ inputs }) => {
     const context = `${process.env.CONTEXT}`.toUpperCase().replace(/-/g, "_");
     const branch = `${process.env.BRANCH}`.toUpperCase().replace(/-/g, "_");
-    const replaced = [];
-
-    Object.keys(process.env).forEach((key) => {
-      const foundContext = setEnvWithValue(key, context, inputs.mode);
-      const foundBranch = setEnvWithValue(key, branch, inputs.mode);
 
-      if (foundContext) replaced.push(foundContext);
-      if (foundBranch) replaced.push(foundBranch);
-    });
+    const replaced = [].concat(...Object.keys(process.env)
+      .map((key) => [setEnvWithValue(key, context, inputs.mode), setEnvWithValue(key, branch, inputs.mode)])
+    ).filter(Boolean)
 
     if (replaced.length) {
       // Write an env file so we can source it during build
-      const file = fs.createWriteStream(".env");
-      replaced.forEach(function (v) {
-        file.write(`${v}\n`);
-      });
-      file.end();
-
+      await pWriteFile(".env", replaced.join(""))
       console.log(`Replaced ${replaced.length} ENVs and wrote .env file`);
     } else {
       console.log(`Nothing found... keeping default ENVs`);