Output strings with the minimum amount of escaped quotes (#390)

* Add tests for quotes

* Update test snapshots

* Output strings with the minimum amount of escaped quotes

* Update test snapshots

* Move tests/prettier/quotes.js into tests/quotes/strings.js

* Update test snapshots

src/printer.js

@@ -2330,59 +2330,68 @@ function nodeStr(node, options) {
     return node.raw;
   }
 
-  const containsSingleQuote = str.indexOf("'") !== -1;
-  const containsDoubleQuote = str.indexOf('"') !== -1;
-
-  let shouldUseSingleQuote = options.singleQuote;
-  if (options.singleQuote && containsSingleQuote && !containsDoubleQuote) {
-    shouldUseSingleQuote = false;
-  }
-  if (!options.singleQuote && !containsSingleQuote && containsDoubleQuote) {
-    shouldUseSingleQuote = true;
-  }
-
   const raw = node.extra ? node.extra.raw : node.raw;
-  const isSingleQuote = raw.charAt(0) === "'";
+  // `rawContent` is the string exactly like it appeared in the input source
+  // code, with its enclosing quote.
+  const rawContent = raw.slice(1, -1);
 
-  if (isSingleQuote && shouldUseSingleQuote ||
-      !isSingleQuote && !shouldUseSingleQuote) {
-    return raw;
+  const double = { quote: '"', regex: /"/g };
+  const single = { quote: "'", regex: /'/g };
+
+  const preferred = options.singleQuote ? single : double;
+  const alternate = preferred === single ? double : single;
+
+  let shouldUseAlternateQuote = false;
+
+  // If `rawContent` contains at least one of the quote preferred for enclosing
+  // the string, we might want to enclose with the alternate quote instead, to
+  // minimize the number of escaped quotes.
+  if (rawContent.includes(preferred.quote)) {
+    const numPreferredQuotes = (rawContent.match(preferred.regex) || []).length;
+    const numAlternateQuotes = (rawContent.match(alternate.regex) || []).length;
+
+    shouldUseAlternateQuote = numPreferredQuotes > numAlternateQuotes;
   }
 
-  const nextQuote = shouldUseSingleQuote ? "'" : '"';
+  const enclosingQuote = shouldUseAlternateQuote
+    ? alternate.quote
+    : preferred.quote;
 
-  // 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;
+  // It might sound unnecessary to use `makeString` even if `node.raw` already
+  // is enclosed with `enclosingQuote`, but it isn't. `node.raw` could contain
+  // unnecessary escapes (such as in `"\'"`). Always using `makeString` makes
+  // sure that we consistently output the minimum amount of escaped quotes.
+  return makeString(rawContent, enclosingQuote);
+}
 
-      return '\\'.repeat(nextNumBackslash) + quote;
-    }) + nextQuote;
+function makeString(rawContent, enclosingQuote) {
+  const otherQuote = enclosingQuote === '"' ? "'" : '"';
+
+  // Matches _any_ escape and unescaped quotes (both single and double).
+  const regex = /\\([\s\S])|(['"])/g;
+
+  // Escape and unescape single and double quotes as needed to be able to
+  // enclose `rawContent` with `enclosingQuote`.
+  const newContent = rawContent.replace(regex, (match, escaped, quote) => {
+    // If we matched an escape, and the escaped character is a quote of the
+    // other type than we intend to enclose the string with, there's no need for
+    // it to be escaped, so return it _without_ the backslash.
+    if (escaped === otherQuote) {
+      return escaped;
+    }
+
+    // If we matched an unescaped quote and it is of the _same_ type as we
+    // intend to enclose the string with, it must be escaped, so return it with
+    // a backslash.
+    if (quote === enclosingQuote) {
+      return "\\" + quote;
+    }
+
+    // Otherwise return the escape or unescaped quote as-is.
+    return match;
+  });
+
+  return enclosingQuote + newContent + enclosingQuote;
 }
 
 function isFirstStatement(path) {

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

@@ -29,95 +29,259 @@ function b(object, key) {
 `;
 
 exports[`test strings.js 1`] = `
-"\"abc\"
+"// Every string will be changed to double quotes, unless we end up with fewer
+// escaped quotes by using single quotes. (Vice versa if the \"singleQuote\"
+// option is true).
+//
+// Note that even if a string already has the correct enclosing quotes, it is
+// still processed in order to remove unnecessarily escaped quotes within it,
+// for consistency.
+
+// Simple strings.
+\"abc\"
 \'abc\'
 
+// Escape.
+\'\\0\'
+
+// Emoji.
+\'🐶\'
+
+// Empty string.
+\"\"
+\'\'
+
+// Single double quote.
+\"\\\"\"
+\'\"\'
+
+// Single single quote.
+\"\'\"
 \'\\\'\'
 
-\'\"\'
-\'\\\"\'
-\'\\\\\"\'
-
-\"\'\"
+// Unnecessary escapes.
 \"\\\'\"
-\"\\\\\'\"
+\'\\\"\'
 
-\"\'\\\"\"
-\'\\\'\"\'
+// One of each.
+\"\\\"\'\"
+\'\"\\\'\'
 
+// One of each with unnecessary escapes.
+\"\\\"\\\'\"
+\'\\\"\\\'\'
+
+// More double quotes than single quotes.
+\"\\\"\'\\\"\"
+\'\"\\\'\"\'
+
+// More single quotes than double quotes.
+\"\\\"\'\'\"
+\'\"\\\'\\\'\'
+
+// Two of each.
+\"\\\"\\\"\'\'\"
+\'\"\"\\\'\\\'\'
+
+// Single backslash.
 \'\\\\\'
 \"\\\\\"
 
-\'\\0\'
-\'🐶\'
+// Backslases.
+\"\\\"\\\\\\\"\\\\\\\\\\\" \'\\\'\\\\\'\\\\\\\'\\\\\\\\\'\"
+\'\\\'\\\\\\\'\\\\\\\\\\\' \"\\\"\\\\\"\\\\\\\"\\\\\\\\\"\'
+
+// Somewhat more real-word example.
+\"He\'s sayin\': \\\"How\'s it goin\'?\\\" Don\'t ask me why.\"
+\'He\\\'s sayin\\\': \"How\\\'s it goin\\\'?\" Don\\\'t ask me why.\'
+
+// Somewhat more real-word example 2.
+\"var backslash = \\\"\\\\\\\", doubleQuote = \'\\\"\';\"
+\'var backslash = \"\\\\\", doubleQuote = \\\'\"\\\';\'
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// Every string will be changed to double quotes, unless we end up with fewer
+// escaped quotes by using single quotes. (Vice versa if the \"singleQuote\"
+// option is true).
+//
+// Note that even if a string already has the correct enclosing quotes, it is
+// still processed in order to remove unnecessarily escaped quotes within it,
+// for consistency.
+
+// Simple strings.
 \"abc\";
 \"abc\";
 
-\"\'\";
-
-\'\"\';
-\'\\\"\';
-\'\\\\\"\';
-
-\"\'\";
-\"\\\'\";
-\"\\\\\'\";
-
-\"\'\\\"\";
-\"\'\\\"\";
-
-\"\\\\\";
-\"\\\\\";
-
+// Escape.
 \"\\0\";
-\"🐶\";
+
+// Emoji.
+\"🐶\"; // Empty string.
+\"\";
+
+\"\"; // Single double quote.
+\'\"\';
+
+\'\"\'; // Single single quote.
+\"\'\";
+
+\"\'\"; // Unnecessary escapes.
+\"\'\";
+
+\'\"\'; // One of each.
+\"\\\"\'\";
+
+\"\\\"\'\"; // One of each with unnecessary escapes.
+\"\\\"\'\";
+
+\"\\\"\'\"; // More double quotes than single quotes.
+\'\"\\\'\"\';
+
+\'\"\\\'\"\'; // More single quotes than double quotes.
+\"\\\"\'\'\";
+
+\"\\\"\'\'\"; // Two of each.
+\"\\\"\\\"\'\'\";
+
+\"\\\"\\\"\'\'\"; // Single backslash.
+\"\\\\\";
+
+\"\\\\\"; // Backslases.
+\"\\\"\\\\\\\"\\\\\\\\\\\" \'\'\\\\\'\\\\\'\\\\\\\\\'\";
+
+\'\\\'\\\\\\\'\\\\\\\\\\\' \"\"\\\\\"\\\\\"\\\\\\\\\"\'; // Somewhat more real-word example.
+\"He\'s sayin\': \\\"How\'s it goin\'?\\\" Don\'t ask me why.\";
+
+\"He\'s sayin\': \\\"How\'s it goin\'?\\\" Don\'t ask me why.\"; // Somewhat more real-word example 2.
+\'var backslash = \"\\\\\", doubleQuote = \\\'\"\\\';\';
+\'var backslash = \"\\\\\", doubleQuote = \\\'\"\\\';\';
 "
 `;
 
 exports[`test strings.js 2`] = `
-"\"abc\"
+"// Every string will be changed to double quotes, unless we end up with fewer
+// escaped quotes by using single quotes. (Vice versa if the \"singleQuote\"
+// option is true).
+//
+// Note that even if a string already has the correct enclosing quotes, it is
+// still processed in order to remove unnecessarily escaped quotes within it,
+// for consistency.
+
+// Simple strings.
+\"abc\"
 \'abc\'
 
+// Escape.
+\'\\0\'
+
+// Emoji.
+\'🐶\'
+
+// Empty string.
+\"\"
+\'\'
+
+// Single double quote.
+\"\\\"\"
+\'\"\'
+
+// Single single quote.
+\"\'\"
 \'\\\'\'
 
-\'\"\'
-\'\\\"\'
-\'\\\\\"\'
-
-\"\'\"
+// Unnecessary escapes.
 \"\\\'\"
-\"\\\\\'\"
+\'\\\"\'
 
-\"\'\\\"\"
-\'\\\'\"\'
+// One of each.
+\"\\\"\'\"
+\'\"\\\'\'
 
+// One of each with unnecessary escapes.
+\"\\\"\\\'\"
+\'\\\"\\\'\'
+
+// More double quotes than single quotes.
+\"\\\"\'\\\"\"
+\'\"\\\'\"\'
+
+// More single quotes than double quotes.
+\"\\\"\'\'\"
+\'\"\\\'\\\'\'
+
+// Two of each.
+\"\\\"\\\"\'\'\"
+\'\"\"\\\'\\\'\'
+
+// Single backslash.
 \'\\\\\'
 \"\\\\\"
 
-\'\\0\'
-\'🐶\'
+// Backslases.
+\"\\\"\\\\\\\"\\\\\\\\\\\" \'\\\'\\\\\'\\\\\\\'\\\\\\\\\'\"
+\'\\\'\\\\\\\'\\\\\\\\\\\' \"\\\"\\\\\"\\\\\\\"\\\\\\\\\"\'
+
+// Somewhat more real-word example.
+\"He\'s sayin\': \\\"How\'s it goin\'?\\\" Don\'t ask me why.\"
+\'He\\\'s sayin\\\': \"How\\\'s it goin\\\'?\" Don\\\'t ask me why.\'
+
+// Somewhat more real-word example 2.
+\"var backslash = \\\"\\\\\\\", doubleQuote = \'\\\"\';\"
+\'var backslash = \"\\\\\", doubleQuote = \\\'\"\\\';\'
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// Every string will be changed to double quotes, unless we end up with fewer
+// escaped quotes by using single quotes. (Vice versa if the \"singleQuote\"
+// option is true).
+//
+// Note that even if a string already has the correct enclosing quotes, it is
+// still processed in order to remove unnecessarily escaped quotes within it,
+// for consistency.
+
+// Simple strings.
 \'abc\';
 \'abc\';
 
-\"\'\";
-
-\'\"\';
-\'\\\"\';
-\'\\\\\"\';
-
-\"\'\";
-\"\\\'\";
-\"\\\\\'\";
-
-\'\\\'\"\';
-\'\\\'\"\';
-
-\'\\\\\';
-\'\\\\\';
-
+// Escape.
 \'\\0\';
-\'🐶\';
+
+// Emoji.
+\'🐶\'; // Empty string.
+\'\';
+
+\'\'; // Single double quote.
+\'\"\';
+
+\'\"\'; // Single single quote.
+\"\'\";
+
+\"\'\"; // Unnecessary escapes.
+\"\'\";
+
+\'\"\'; // One of each.
+\'\"\\\'\';
+
+\'\"\\\'\'; // One of each with unnecessary escapes.
+\'\"\\\'\';
+
+\'\"\\\'\'; // More double quotes than single quotes.
+\'\"\\\'\"\';
+
+\'\"\\\'\"\'; // More single quotes than double quotes.
+\"\\\"\'\'\";
+
+\"\\\"\'\'\"; // Two of each.
+\'\"\"\\\'\\\'\';
+
+\'\"\"\\\'\\\'\'; // Single backslash.
+\'\\\\\';
+
+\'\\\\\'; // Backslases.
+\"\\\"\\\\\\\"\\\\\\\\\\\" \'\'\\\\\'\\\\\'\\\\\\\\\'\";
+
+\'\\\'\\\\\\\'\\\\\\\\\\\' \"\"\\\\\"\\\\\"\\\\\\\\\"\'; // Somewhat more real-word example.
+\"He\'s sayin\': \\\"How\'s it goin\'?\\\" Don\'t ask me why.\";
+
+\"He\'s sayin\': \\\"How\'s it goin\'?\\\" Don\'t ask me why.\"; // Somewhat more real-word example 2.
+\'var backslash = \"\\\\\", doubleQuote = \\\'\"\\\';\';
+\'var backslash = \"\\\\\", doubleQuote = \\\'\"\\\';\';
 "
 `;

tests/quotes/strings.js

@@ -1,21 +1,69 @@
+// Every string will be changed to double quotes, unless we end up with fewer
+// escaped quotes by using single quotes. (Vice versa if the "singleQuote"
+// option is true).
+//
+// Note that even if a string already has the correct enclosing quotes, it is
+// still processed in order to remove unnecessarily escaped quotes within it,
+// for consistency.
+
+// Simple strings.
 "abc"
 'abc'
 
+// Escape.
+'\0'
+
+// Emoji.
+'🐶'
+
+// Empty string.
+""
+''
+
+// Single double quote.
+"\""
+'"'
+
+// Single single quote.
+"'"
 '\''
 
-'"'
-'\"'
-'\\"'
-
-"'"
+// Unnecessary escapes.
 "\'"
-"\\'"
+'\"'
 
-"'\""
-'\'"'
+// One of each.
+"\"'"
+'"\''
 
+// One of each with unnecessary escapes.
+"\"\'"
+'\"\''
+
+// More double quotes than single quotes.
+"\"'\""
+'"\'"'
+
+// More single quotes than double quotes.
+"\"''"
+'"\'\''
+
+// Two of each.
+"\"\"''"
+'""\'\''
+
+// Single backslash.
 '\\'
 "\\"
 
-'\0'
-'🐶'
+// Backslases.
+"\"\\\"\\\\\" '\'\\'\\\'\\\\'"
+'\'\\\'\\\\\' "\"\\"\\\"\\\\"'
+
+// Somewhat more real-word example.
+"He's sayin': \"How's it goin'?\" Don't ask me why."
+'He\'s sayin\': "How\'s it goin\'?" Don\'t ask me why.'
+
+// Somewhat more real-word example 2.
+"var backslash = \"\\\", doubleQuote = '\"';"
+'var backslash = "\\", doubleQuote = \'"\';'

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

@@ -30,10 +30,10 @@ exports[`test strings.js 1`] = `
   \"abc\",
   \"\'\",
   \'\"\',
-  \'\\\"\',
+  \'\"\',
   \'\\\\\"\',
   \"\'\",
-  \"\\\'\",
+  \"\'\",
   \"\\\\\'\",
   \"\'\\\"\",
   \"\'\\\"\",