add no-multi-assign-on-declaration lint rule, phetsims/chipper#794

eslint/.eslintrc.js

@@ -136,6 +136,9 @@ module.exports = {
     'property-visibility-annotation': 0,
     'no-property-in-require-statement': 2,
 
+    // disallow assignment within variable declaration, see https://github.com/phetsims/chipper/issues/794
+    'no-multi-assign-on-declaration': 2,
+
     // permit only one var declaration per line, see #390
     'one-var': [
       2,

eslint/rules/no-multi-assign-on-declaration.js

@@ -0,0 +1,33 @@
+// Copyright 2019, University of Colorado Boulder
+
+/**
+ * @fileoverview Don't allow assignment within a variable declaration
+ * @author Stewart Rand (original author of eslint file)
+ * @author Michael Kauzmann (PhET Interactive Simulations)
+ *
+ * This rule was adapted straight from eslint's no-multi-assign, and adapted/simplified for PhET purposes, see
+ * original file: https://github.com/eslint/eslint/blob/7ad86de/lib/rules/no-multi-assign.js
+ */
+
+/* eslint-env node */
+
+'use strict';
+
+module.exports = {
+  create( context ) {
+    return {
+
+      // run on any AssignmentExpression node
+      AssignmentExpression( node ) {
+
+        // if within a variable declaration, error out
+        if ( node.parent.type === 'VariableDeclarator' ) {
+          context.report( {
+            node: node,
+            message: 'Unexpected chained assignment when declaring variable.'
+          } );
+        }
+      }
+    };
+  }
+};