Don't treat spreading an array of objects into Object.assign() as an error (#91)

Author: oBusk

src/rules/prefer-spread-syntax.test.ts

@@ -59,6 +59,11 @@ ruleTester.run('prefer-spread-syntax (untyped)', preferSpreadSyntax as never, {
     'Object.assign({__proto__: null}, obj);',
     'Object.assign({__proto__: proto, foo: 1}, obj);',
 
+    // Object.assign with spread element argument — cannot be safely converted
+    // Object.assign({}, ...objs) !== {...objs} (the latter spreads the array as an object)
+    'Object.assign({}, ...objs);',
+    'Object.assign({}, ...objs, extra);',
+
     // apply with context
     'fn.apply(context, args);',
     'obj.method.apply(obj, args);',

src/rules/prefer-spread-syntax.ts

@@ -120,8 +120,17 @@ export const preferSpreadSyntax: TSESLint.RuleModule<MessageIds, []> = {
             );
 
             if (!hasUnquotedProto) {
-              const spreadArgs = node.arguments
-                .slice(1)
+              const restArgs = node.arguments.slice(1);
+
+              // If any argument is already a spread element (e.g. ...objs),
+              // we can't safely convert since Object.assign({}, ...objs) spreads
+              // array elements as individual arguments, which has no simple
+              // equivalent in object spread syntax.
+              if (restArgs.some((arg) => arg.type === 'SpreadElement')) {
+                return;
+              }
+
+              const spreadArgs = restArgs
                 .map((arg) => `...${sourceCode.getText(arg)}`)
                 .join(', ');