Fix last element of an array being null (#232)

This appears in so many of the test262.

src/printer.js

@@ -641,6 +641,18 @@ function genericPrintNoParens(path, options, print) {
       if (n.elements.length === 0) {
         parts.push("[]");
       } else {
+        // JavaScript allows you to have empty elements in an array which
+        // changes its length based on the number of commas. The algorithm
+        // is that if the last argument is null, we need to force insert
+        // a comma to ensure JavaScript recognizes it.
+        //   [,].length === 1
+        //   [1,].length === 1
+        //   [1,,].length === 2
+        //
+        // Note that util.getLast returns null if the array is empty, but
+        // we already check for an empty array just above so we are safe
+        const needsForcedTrailingComma = util.getLast(n.elements) === null;
+
         parts.push(
           multilineGroup(
             concat([
@@ -652,6 +664,7 @@ function genericPrintNoParens(path, options, print) {
                   join(concat([ ",", line ]), path.map(print, "elements"))
                 ])
               ),
+              needsForcedTrailingComma ? "," : "",
               ifBreak(options.trailingComma ? "," : ""),
               options.bracketSpacing ? line : softline,
               "]"

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

@@ -81,6 +81,19 @@ module.exports = \"arrays\";
 "
 `;
 
+exports[`test last.js 1`] = `
+"[,];
+[,,];
+[,,1,];
+[,,1,1];
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+[ , ];
+[ , , ];
+[ , , 1 ];
+[ , , 1, 1 ];
+"
+`;
+
 exports[`test numeric_elem.js 1`] = `
 "var arr = [];
 var day = new Date;

tests/arrays/last.js

@@ -0,0 +1,4 @@
+[,];
+[,,];
+[,,1,];
+[,,1,1];