Add tests and implementation for a "quotemark" printer option

lib/options.js

@@ -49,7 +49,13 @@ var defaults = {
 
     // If you want esprima not to throw exceptions when it encounters
     // non-fatal errors, keep this option true.
-    tolerant: true
+    tolerant: true,
+
+    // If you want to override the quotes used in string literals, specify
+    // either "single", "double", or "auto" here ("auto" will select the one 
+    // which results in the shorter literal)
+    // Otherwise, the input marks will be preserved
+    quote: null,
 }, hasOwn = defaults.hasOwnProperty;
 
 // Copy options and fill in default values.
@@ -73,6 +79,7 @@ exports.normalize = function(options) {
         inputSourceMap: get("inputSourceMap"),
         esprima: get("esprima"),
         range: get("range"),
-        tolerant: get("tolerant")
+        tolerant: get("tolerant"),
+        quote: get("quote"),
     };
 };

lib/printer.js

@@ -543,7 +543,7 @@ function genericPrintNoParens(path, options, print) {
 
     case "ModuleSpecifier":
         // A ModuleSpecifier is a string-valued Literal.
-        return fromString(nodeStr(n), options);
+        return fromString(nodeStr(n, options), options);
 
     case "UnaryExpression":
         var parts = [n.operator];
@@ -1283,10 +1283,26 @@ function endsWithBrace(lines) {
     return lastNonSpaceCharacter(lines) === "}";
 }
 
-function nodeStr(n) {
+function swapQuotes(str) {
+    return str.replace(/['"]/g, function(m) {
+        return m === '"' ? '\'' : '"';
+    });
+}
+
+function nodeStr(n, options) {
     namedTypes.Literal.assert(n);
     isString.assert(n.value);
-    return JSON.stringify(n.value);
+    switch (options.quote) {
+    case "auto":
+        var double = JSON.stringify(n.value);
+        var single = swapQuotes(JSON.stringify(swapQuotes(n.value)));
+        return double.length > single.length ? single : double;
+    case "single":
+        return swapQuotes(JSON.stringify(swapQuotes(n.value)));
+    case "double":
+    default:
+        return JSON.stringify(n.value);
+    }
 }
 
 function maybeAddSemicolon(lines) {

test/printer.js

@@ -622,4 +622,40 @@ describe("printer", function() {
             "}"
         ].join("\n"));
     });
+
+    it("should print string literals with the specified delimiter", function() {
+        var ast = parse([
+            "var obj = {",
+            "    \"foo's\": 'bar',",
+            "    '\"bar\\'s\"': /regex/m",
+            "};"
+        ].join("\n"));
+
+        var variableDeclaration = ast.program.body[0];
+        n.VariableDeclaration.assert(variableDeclaration);
+
+        var printer = new Printer({ quote: "single" });
+        assert.strictEqual(printer.printGenerically(ast).code, [
+            "var obj = {",
+            "    'foo\\'s': 'bar',",
+            "    '\"bar\\'s\"': /regex/m",
+            "};"
+        ].join("\n"));
+
+        var printer2 = new Printer({ quote: "double" });
+        assert.strictEqual(printer2.printGenerically(ast).code, [
+            "var obj = {",
+            "    \"foo's\": \"bar\",",
+            '    "\\"bar\'s\\"": /regex/m',
+            "};"
+        ].join("\n"));
+
+        var printer3 = new Printer({ quote: "auto" });
+        assert.strictEqual(printer3.printGenerically(ast).code, [
+            "var obj = {",
+            '    "foo\'s": "bar",',
+            '    \'"bar\\\'s"\': /regex/m',
+            "};"
+        ].join("\n"));        
+    });
 });