Don't add trailing commas in JSXAttribute for arrow functions (#3181)

src/printer.js

@@ -556,6 +556,9 @@ function genericPrintNoParens(path, options, print, args) {
         (args && args.expandLastArg) ||
         path.getParentNode().type === "JSXExpressionContainer";
 
+      const printTrailingComma =
+        args && args.expandLastArg && shouldPrintComma(options, "all");
+
       // In order to avoid confusion between
       // a => a ? a : a
       // a <= a ? a : a
@@ -580,10 +583,7 @@ function genericPrintNoParens(path, options, print, args) {
                 ])
               ),
               shouldAddSoftLine
-                ? concat([
-                    ifBreak(shouldPrintComma(options, "all") ? "," : ""),
-                    softline
-                  ])
+                ? concat([ifBreak(printTrailingComma ? "," : ""), softline])
                 : ""
             ])
           )

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

@@ -210,6 +210,63 @@ a(
 
 `;
 
+exports[`jsx.js 1`] = `
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar
+    })
+  }
+/>;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar
+    })
+  }
+/>;
+
+`;
+
+exports[`jsx.js 2`] = `
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar
+    })
+  }
+/>;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar,
+    })
+  }
+/>;
+
+`;
+
+exports[`jsx.js 3`] = `
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar
+    })
+  }
+/>;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar,
+    })
+  }
+/>;
+
+`;
+
 exports[`object.js 1`] = `
 const a = {
   b: true,

tests/trailing_comma/jsx.js

@@ -0,0 +1,7 @@
+<div
+  onClick={() =>
+    doSomething({
+      foo: bar
+    })
+  }
+/>;