tools: lint for alignment of variable assignments

Enforce alignment/indentation on variable assignments that span multiple
lines.

PR-URL: https://github.com/nodejs/node/pull/6242
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>

.eslintrc

@@ -86,7 +86,7 @@ rules:
 
   # Custom rules in tools/eslint-rules
   new-with-error: [2, "Error", "RangeError", "TypeError", "SyntaxError", "ReferenceError"]
-
+  align-multiline-assignment: 2
 
 # Global scoped method and vars
 globals:

tools/eslint-rules/align-multiline-assignment.js

@@ -0,0 +1,68 @@
+/**
+ * @fileoverview Align multiline variable assignments
+ * @author Rich Trott
+ */
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+function getBinaryExpressionStarts(binaryExpression, starts = []) {
+  function getStartsFromOneSide(side, starts) {
+    starts.push(side.loc.start);
+    if (side.type === 'BinaryExpression') {
+      starts = getBinaryExpressionStarts(side, starts);
+    }
+    return starts;
+  }
+
+  starts = getStartsFromOneSide(binaryExpression.left, starts);
+  starts = getStartsFromOneSide(binaryExpression.right, starts);
+  return starts;
+}
+
+function checkExpressionAlignment(expression) {
+  if (!expression)
+    return;
+
+  var msg = '';
+
+  switch (expression.type) {
+    case 'BinaryExpression':
+      var starts = getBinaryExpressionStarts(expression);
+      var startLine = starts[0].line;
+      const startColumn = starts[0].column;
+      starts.forEach((loc) => {
+        if (loc.line > startLine) {
+          startLine = loc.line;
+          if (loc.column !== startColumn) {
+            msg = 'Misaligned multiline assignment';
+          }
+        }
+      });
+      break;
+  }
+  return msg;
+}
+
+function testAssignment(context, node) {
+  const msg = checkExpressionAlignment(node.right);
+  if (msg)
+    context.report(node, msg);
+}
+
+function testDeclaration(context, node) {
+  node.declarations.forEach((declaration) => {
+    const msg = checkExpressionAlignment(declaration.init);
+    // const start = declaration.init.loc.start;
+    if (msg)
+      context.report(node, msg);
+  });
+}
+
+module.exports = function(context) {
+  return {
+    'AssignmentExpression': (node) => testAssignment(context, node),
+    'VariableDeclaration': (node) => testDeclaration(context, node)
+  };
+};