[Feature proposal] Use .d.ts as schema for type safety

dotenv-extended.d.ts

@@ -4,7 +4,7 @@
  * The result of a call to load() or parse()
  */
 export interface IEnvironmentMap {
-    [name: string]: string;
+    [name: string]: string | undefined;
 }
 
 /**

package.json

@@ -52,6 +52,7 @@
     "auto-parse": "^1.3.0",
     "camelcase": "^5.3.1",
     "cross-spawn": "^7.0.1",
-    "dotenv": "^8.2.0"
+    "dotenv": "^8.2.0",
+    "typescript": "^3.8.3"
   }
 }

src/index.js

@@ -4,6 +4,7 @@
 import dotenv from 'dotenv';
 import getConfigFromEnv from './utils/config-from-env';
 import loadEnvironmentFile from './utils/load-environment-file';
+import loadEnvTypeDeclaration from './utils/load-env-type-declaration';
 
 export const parse = dotenv.parse.bind(dotenv);
 export const config = options => {
@@ -35,7 +36,9 @@ export const config = options => {
     const configKeys = Object.keys(config);
 
     if (options.errorOnMissing || options.errorOnExtra || options.errorOnRegex) {
-        const schema = loadEnvironmentFile(options.schema, options.encoding, options.silent);
+        const schema = options.schema.endsWith('.d.ts') ? 
+            loadEnvTypeDeclaration(options.schema, options.encoding, options.silent)
+            : loadEnvironmentFile(options.schema, options.encoding, options.silent);
         const schemaKeys = Object.keys(schema);
 
         let missingKeys = schemaKeys.filter(function (key) {

src/utils/load-env-type-declaration.js

@@ -0,0 +1,25 @@
+import fs from 'fs';
+import ts from 'typescript';
+
+export const loadEnvTypeDeclaration = (path, encoding, silent) => {
+    try {
+        const data = fs.readFileSync(path, encoding);
+        const source = ts.createSourceFile(path, data, ts.ScriptTarget.ES2015, true);
+        const schema = {};
+        source.getChildren().find(c => c.kind === ts.SyntaxKind.SyntaxList).getChildren()
+            .find(c => c.kind === ts.SyntaxKind.ModuleDeclaration && c.name.text === 'dotenv-extended').body.getChildren()
+            .find(c => c.kind === ts.SyntaxKind.SyntaxList).getChildren()
+            .find(c => c.kind === ts.SyntaxKind.InterfaceDeclaration && c.name.escapedText === 'IEnvironmentMap').getChildren()
+            .find(c => c.kind === ts.SyntaxKind.SyntaxList).getChildren()
+            // Only accepts members of type "string"
+            .filter(c => c.type.kind === ts.SyntaxKind.StringKeyword)
+            .forEach(c => schema[c.name.escapedText] = '');
+        return schema;
+    } catch (err) {
+        if (!silent) {
+            console.error(err.message);
+        }
+        return {};
+    }
+};
+export default loadEnvTypeDeclaration;

test/EnvSchema.d.ts

@@ -0,0 +1,15 @@
+import { IEnvironmentMap } from "dotenv-extended";
+
+declare module "dotenv-extended" {
+  interface IEnvironmentMap {
+    TEST_ONE: string;
+    TEST_TWO: string;
+    TEST_THREE: string;
+  }
+}
+
+declare global{
+  namespace NodeJS {
+    interface ProcessEnv extends IEnvironmentMap {}
+  }
+}

test/test.spec.js

@@ -167,6 +167,22 @@ describe('dotenv-extended tests', () => {
         dotenvex.load({silent: false});
         expect(console.error).to.have.been.calledOnce;
     });
+
+    it('Should load .d.ts schema, defaults and env into correct values in process.env and returned object', function () {
+        const config = dotenvex.load({
+            schema: 'EnvSchema.d.ts',
+            defaults: '.env.defaults.example',
+            path: '.env.override',
+            errorOnExtra: true,
+            errorOnMissing: true
+        });
+        expect(config.TEST_ONE).to.equal('one overridden');
+        expect(config.TEST_TWO).to.equal('two');
+        expect(config.TEST_THREE).to.equal('three');
+        expect(process.env.TEST_ONE).to.equal('one overridden');
+        expect(process.env.TEST_TWO).to.equal('two');
+        expect(process.env.TEST_THREE).to.equal('three');
+    });
 });
 
 describe('Supporting libraries tests', () => {