Rule fix: no-extend: Only fix if first arg is object literal (#330)

If the first argument is a variable which could be null
or undefined, then Object.assign will throw.

We could convert to `Object.assign( foo || {}, ... )` but in
most cases this is unnecessary, so probably better to leave
it to users to fix manually.

docs/rules/no-extend.md

@@ -27,6 +27,7 @@ $.extend( { myUtil: fn } );
 ❌ Examples of **incorrect** code with `[{"allowDeep":true}]` options:
 ```js
 $.extend( {}, foo );
+$.extend( fooCouldBeNull, doesNotAutofix );
 ```
 
 ✔️ Examples of **correct** code with `[{"allowDeep":true}]` options:

src/rules/no-extend.js

@@ -48,7 +48,9 @@ module.exports = {
 				node,
 				message: 'Prefer Object.assign or the spread operator to $.extend',
 				fix: function ( fixer ) {
-					if ( !isDeep ) {
+					// Only auto-fix if we are sure the first argument is an object.
+					// If it is undefined or null variable, then Object.assign will throw.
+					if ( !isDeep && node.arguments[ 0 ] && node.arguments[ 0 ].type === 'ObjectExpression' ) {
 						return fixer.replaceText( node.callee, 'Object.assign' );
 					}
 				}

tests/rules/no-extend.js

@@ -32,6 +32,11 @@ ruleTester.run( 'no-extend', rule, {
 			options: [ { allowDeep: true } ],
 			errors: [ error ],
 			output: 'Object.assign({}, foo)'
+		},
+		{
+			code: '$.extend(fooCouldBeNull, doesNotAutofix)',
+			options: [ { allowDeep: true } ],
+			errors: [ error ]
 		}
 	]
 } );