Do not make variable multilines inside of for loop (#1912)

Fixes #1876

src/printer.js

@@ -1166,6 +1166,16 @@ function genericPrintNoParens(path, options, print, args) {
         return print(childPath);
       }, "declarations");
 
+      // We generally want to terminate all variable declarations with a
+      // semicolon, except when they in the () part of for loops.
+      const parentNode = path.getParentNode();
+
+      const isParentForLoop =
+        parentNode.type === "ForStatement" ||
+        parentNode.type === "ForInStatement" ||
+        parentNode.type === "ForOfStatement" ||
+        parentNode.type === "ForAwaitStatement";
+
       const hasValue = n.declarations.some(decl => decl.init);
 
       parts = [
@@ -1176,21 +1186,13 @@ function genericPrintNoParens(path, options, print, args) {
           concat(
             printed
               .slice(1)
-              .map(p => concat([",", hasValue ? hardline : line, p]))
+              .map(p =>
+                concat([",", hasValue && !isParentForLoop ? hardline : line, p])
+              )
           )
         )
       ];
 
-      // We generally want to terminate all variable declarations with a
-      // semicolon, except when they in the () part of for loops.
-      const parentNode = path.getParentNode();
-
-      const isParentForLoop =
-        parentNode.type === "ForStatement" ||
-        parentNode.type === "ForInStatement" ||
-        parentNode.type === "ForOfStatement" ||
-        parentNode.type === "ForAwaitStatement";
-
       if (!(isParentForLoop && parentNode.body !== n)) {
         parts.push(semi);
       }

tests/variable_declarator/__snapshots__/jsfmt.spec.js.snap

@@ -11,6 +11,8 @@ const eloBar     = require("elo-bar")
 var a, b, c;
 
 let superSuperSuperLong1, superSuperSuperLong2, superSuperSuperLong3, superSuperSuperLong4;
+
+for (var i = 0, len = arr.length; i < len; i++) {}
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 var assert = require("assert"),
   lookup = require("../lookup");
@@ -26,6 +28,8 @@ let superSuperSuperLong1,
   superSuperSuperLong3,
   superSuperSuperLong4;
 
+for (var i = 0, len = arr.length; i < len; i++) {}
+
 `;
 
 exports[`string.js 1`] = `

tests/variable_declarator/multiple.js

@@ -8,3 +8,5 @@ const eloBar     = require("elo-bar")
 var a, b, c;
 
 let superSuperSuperLong1, superSuperSuperLong2, superSuperSuperLong3, superSuperSuperLong4;
+
+for (var i = 0, len = arr.length; i < len; i++) {}