Swap quotes (#355)

- During the first iteration, we printed the unescaped values which let to printing invalid JavaScript characters and bad things like invisible characters.
- During the second iteration, we escaped everything, which generated valid JavaScript but you lost your emojis and chinese/cyrillic characters

In this iteration, which I hope will be the last one, we maintain the string exactly as encoded and only swap quotes. The swap quotes implementation is a bit convoluted but I think it works.

src/printer.js

@@ -4,7 +4,6 @@ var comments = require("./comments");
 var FastPath = require("./fast-path");
 var util = require("./util");
 var isIdentifierName = require("esutils").keyword.isIdentifierNameES6;
-var jsesc = require("jsesc");
 
 var docBuilders = require("./doc-builders");
 var concat = docBuilders.concat;
@@ -2319,6 +2318,13 @@ function nodeStr(node, options) {
   const str = node.value;
   isString.assert(str);
 
+  // Workaround a bug in the Javascript version of the flow parser where
+  // astral unicode characters like \uD801\uDC28 are incorrectly parsed as
+  // a sequence of \uFFFD.
+  if (options.parser === "flow" && str.indexOf("\uFFFD") !== -1) {
+    return node.raw;
+  }
+
   const containsSingleQuote = str.indexOf("'") !== -1;
   const containsDoubleQuote = str.indexOf('"') !== -1;
 
@@ -2330,19 +2336,48 @@ function nodeStr(node, options) {
     shouldUseSingleQuote = true;
   }
 
-  const result = jsesc(str, {
-    quotes: shouldUseSingleQuote ? "single" : "double",
-    wrap: true
-  });
+  const raw = node.extra ? node.extra.raw : node.raw;
+  const isSingleQuote = raw.charAt(0) === "'";
 
-  // Workaround a bug in the Javascript version of the flow parser where
-  // astral unicode characters like \uD801\uDC28 are incorrectly parsed as
-  // a sequence of \uFFFD.
-  if (options.parser === "flow" && result.indexOf("\\uFFFD") !== -1) {
-    return node.raw;
+  if (isSingleQuote && shouldUseSingleQuote ||
+      !isSingleQuote && !shouldUseSingleQuote) {
+    return raw;
   }
 
-  return result;
+  const nextQuote = shouldUseSingleQuote ? "'" : '"';
+
+  // When we change the outer quotes, we need to change the number of
+  // backslash before. In order to know how many backslash to put,
+  // I manually did the first few examples and extracted a general rule
+  // out of it.
+  //
+  // shouldSwitch === true
+  //
+  //   3 -> 3 "\\\'" -> '\\\''
+  //   2 -> 3 "\\'"  -> '\\\''
+  //   1 -> 1 "\'"   -> '\''
+  //   0 -> 1 "\'"   -> '\''
+  //
+  //   Rule: n -> Math.floor(n / 2) + 1
+  //
+  // shouldSwitch === false
+  //
+  //   3 -> 2 '\\\'' -> "\\'"
+  //   1 -> 0 '\''   -> "'"
+  //
+  //   Rule: n -> n - 1
+  return nextQuote + raw.slice(1, -1)
+    .replace(/\\*['"]/g, (match) => {
+      const quote = match[match.length - 1];
+      const shouldSwitch = quote === "'" && shouldUseSingleQuote ||
+       quote === '"' && !shouldUseSingleQuote;
+      const numBackslash = match.length - 1;
+      const nextNumBackslash = shouldSwitch ?
+        Math.floor(numBackslash / 2) + 1 :
+        numBackslash - 1;
+
+      return '\\'.repeat(nextNumBackslash) + quote;
+    }) + nextQuote;
 }
 
 function isFirstStatement(path) {

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

@@ -57,11 +57,11 @@ exports[`test strings.js 1`] = `
 \"\'\";
 
 \'\"\';
-\'\"\';
+\'\\\"\';
 \'\\\\\"\';
 
 \"\'\";
-\"\'\";
+\"\\\'\";
 \"\\\\\'\";
 
 \"\'\\\"\";
@@ -71,7 +71,7 @@ exports[`test strings.js 1`] = `
 \"\\\\\";
 
 \"\\0\";
-\"\\uD83D\\uDC36\";
+\"🐶\";
 "
 `;
 
@@ -104,11 +104,11 @@ exports[`test strings.js 2`] = `
 \"\'\";
 
 \'\"\';
-\'\"\';
+\'\\\"\';
 \'\\\\\"\';
 
 \"\'\";
-\"\'\";
+\"\\\'\";
 \"\\\\\'\";
 
 \'\\\'\"\';
@@ -118,6 +118,6 @@ exports[`test strings.js 2`] = `
 \'\\\\\';
 
 \'\\0\';
-\'\\uD83D\\uDC36\';
+\'🐶\';
 "
 `;

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

@@ -30,17 +30,17 @@ exports[`test strings.js 1`] = `
   \"abc\",
   \"\'\",
   \'\"\',
-  \'\"\',
+  \'\\\"\',
   \'\\\\\"\',
   \"\'\",
-  \"\'\",
+  \"\\\'\",
   \"\\\\\'\",
   \"\'\\\"\",
   \"\'\\\"\",
   \"\\\\\",
   \"\\\\\",
   \"\\0\",
-  \"\\uD83D\\uDC36\",
+  \"🐶\",
   \'\\uD801\\uDC28\'
 ];
 "

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

@@ -73,8 +73,6 @@ function utf16Length(str, pos) {
 exports[`test keys.js 1`] = `
 "({\'この事はつもり素晴らしいことさ\': \'35jL9V\'})
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-({
-  \"\\u3053\\u306E\\u4E8B\\u306F\\u3064\\u3082\\u308A\\u7D20\\u6674\\u3089\\u3057\\u3044\\u3053\\u3068\\u3055\": \"35jL9V\"
-});
+({ \"この事はつもり素晴らしいことさ\": \"35jL9V\" });
 "
 `;