Add rule exports-last

exports-last will check that all export statements are at the end of the file

Closes #620

src/index.js

@@ -31,6 +31,9 @@ export const rules = {
   'unambiguous': require('./rules/unambiguous'),
   'no-unassigned-import': require('./rules/no-unassigned-import'),
 
+  // export
+  'exports-last': require('./rules/exports-last'),
+
   // metadata-based
   'no-deprecated': require('./rules/no-deprecated'),
 

src/rules/exports-last.js

@@ -0,0 +1,39 @@
+function isExportStatement({ type }) {
+  // ES Module export statements
+  if (type === 'ExportDefaultDeclaration' || type === 'ExportNamedDeclaration') {
+    return true
+
+  // CommonJS export statements
+  } else if (type === 'ExpressionStatement') {
+    // TODO
+  }
+
+  return false
+}
+
+const rule = {
+  create(context) {
+    return {
+      Program({ body }) {
+        const lastNonExportStatement = body.reduce((acc, node, index) => {
+          if (isExportStatement(node)) {
+            return acc
+          }
+          return index
+        }, 0)
+
+        body.forEach((node, index) => {
+          if (isExportStatement(node) && index < lastNonExportStatement) {
+
+            context.report({
+              node,
+              message: 'Export statements should appear at the end of the file',
+            })
+          }
+        })
+      },
+    }
+  },
+}
+
+export default rule

tests/src/rules/exports-last.js

@@ -0,0 +1,82 @@
+import { test } from '../utils'
+
+import { RuleTester } from 'eslint'
+import rule from 'rules/exports-last'
+
+const ruleTester = new RuleTester()
+
+const errors = ['Export statements should appear at the end of the file']
+
+ruleTester.run('exports-last', rule, {
+  valid: [
+    test({
+      code: `
+        const foo = 'bar';
+        const bar = 'baz';
+      `,
+    }),
+    test({
+      code: `
+        const foo = 'bar';
+        export {foo};
+      `,
+    }),
+    test({
+      code: `
+        const foo = 'bar';
+        export default foo;
+      `,
+    }),
+    test({
+      code: `
+        const foo = 'bar';
+        export default foo;
+        export const bar = true;
+      `,
+    }),
+    // test({
+    //   code: `
+    //     const foo = 'bar';
+    //     module.exports = foo
+    //   `,
+    // }),
+    // test({
+    //   code: `
+    //     const foo = 'bar';
+    //     module.exports = foo;
+    //     exports.bar = true
+    //   `,
+    // }),
+
+  ],
+  invalid: [
+    test({
+      code: `
+        export default 'bar';
+        const bar = true;
+      `,
+      errors,
+    }),
+    test({
+      code: `
+        export const foo = 'bar';
+        const bar = true;
+      `,
+      errors,
+    }),
+    // test({
+    //   code: `
+    //     module.exports = 'bar';
+    //     console.log('hi');
+    //   `,
+    //   errors,
+    // }),
+    // test({
+    //   code: `
+    //     exports.foo = 'bar';
+    //     const bar = true;
+    //   `,
+    //   errors,
+    // }),
+  ],
+})