fix: empty destructuring with var should be converted to const

src/transform/let.js

@@ -44,6 +44,17 @@ export default function(ast, loggerInstance) {
             variableMarker.markModified(decl.id.name);
           }
         });
+
+
+        // Empty destructuring should be converted to const.
+        //
+        //     var [] = foo;
+        //     var {} = foo;
+        //
+        // Mix of empty and non-empty destructuring should be left as is.
+        if (node.kind === 'var' && node.declarations.every(isEmptyDestructuring)) {
+          node.kind = 'const';
+        }
       }
       else if (node.type === 'AssignmentExpression') {
         destructuring.extractVariableNames(node.left).forEach(name => {
@@ -90,6 +101,16 @@ function isAnyWhileStatement(node) {
   node.type === 'DoWhileStatement';
 }
 
+function isEmptyDestructuring(node) {
+  if (node.id.type === 'ObjectPattern') {
+    return node.id.properties.length === 0;
+  }
+  if (node.id.type === 'ArrayPattern') {
+    return node.id.elements.length === 0;
+  }
+  return false;
+}
+
 // Program node works almost like a function:
 // it hoists all variables which can be transformed to block-scoped let/const.
 // It just doesn't have name and parameters.
@@ -190,7 +211,7 @@ function transformVarsToLetOrConst() {
 
 // let and const declarations aren't allowed in all the same places where
 // var declarations are allowed. Notably, only var-declaration can occur
-// directlt in if-statement (and other similar statements) body:
+// directly in if-statement (and other similar statements) body:
 //
 //     if (true) var x = 10;
 //

test/transform/letTest.js

@@ -161,6 +161,19 @@ describe('Let/const', () => {
         'for (let i=0, len=arr.length; i<len; i++) {}'
       );
     });
+
+    it('should split to let & const when mixed with empty destructuring', () => {
+      expectTransform(
+        'var [] = [1, 2], {} = {foo: 1, bar: 2}, x = 1, y = 2;\n' +
+        'y = 3;'
+      ).toReturn(
+        'const [] = [1, 2];\n' +
+        'const {} = {foo: 1, bar: 2};\n' +
+        'const x = 1;\n' +
+        'let y = 2;\n' +
+        'y = 3;'
+      );
+    });
   });
 
   describe('with variable declaration in restrictive parent', () => {
@@ -278,6 +291,18 @@ describe('Let/const', () => {
         'console.log(x);'
       );
     });
+
+    it('should use const when no variables are destructured', () => {
+      expectTransform(
+        'var [] = [1, 2];\n' +
+        'var {} = {foo: 1, bar: 2};\n' +
+        'var [] = [1, 2], {} = {foo: 1, bar: 2};'
+      ).toReturn(
+        'const [] = [1, 2];\n' +
+        'const {} = {foo: 1, bar: 2};\n' +
+        'const [] = [1, 2], {} = {foo: 1, bar: 2};'
+      );
+    });
   });
 
   describe('with nested function', () => {