Implement ES6 String.raw behind --harmony-templates

src/harmony-string.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $String = global.String;
 // var $Array = global.Array;
@@ skipping 153 matching lines @@
     } else {
       code -= 0x10000;
       result += %_StringCharFromCode((code >>> 10) & 0x3FF | 0xD800);
       result += %_StringCharFromCode(code & 0x3FF | 0xDC00);
     }
   }
   return result;
 }
 
 
+// ES6 Draft 10-14-2014, section 21.1.2.4
+function StringRaw(callSite) {
+  // TODO(*): Use rest parameters when implemented
+  var substitutions = $Array.prototype.slice.call(arguments, 1);

[arv (Not doing code reviews), 2014/11/14 22:54:25]

This is not safe. Use a c-style for loop using %_ArgumentsLength and %_Arguments
instead.

[caitp (gmail), 2014/11/14 23:29:58]

Done.

+  var numberOfSubstitutions = substitutions.length;
+  var cooked = ToObject(callSite);
+  var raw = ToObject(cooked.raw);
+  var literalSegments = ToLength(raw.length);
+  if (literalSegments <= 0) return "";
+
+  var result = "";
+  var nextIndex = 0;
+
+  while (true) {

[arv (Not doing code reviews), 2014/11/14 22:54:25]

for (var nextIndex = 0; ; nextIndex++) {

[caitp (gmail), 2014/11/14 23:29:58]

I've fixed this slightly differently, so it looks a bit different from the spec,
but I think it works better given the use of the arguments[] index and it may
possibly generate slightly better code.

[arv (Not doing code reviews), 2014/11/14 23:42:17]

I like your new code better. Easier to grok.

+    var next = ToString(raw[nextIndex]);

[arv (Not doing code reviews), 2014/11/14 22:54:25]

Don't we know that these are all strings?

[caitp (gmail), 2014/11/14 23:29:58]

The algorithm specifically calls for ToString(), which would be expected to
throw in the usual cases where it needs to through, I believe.

I'm not positive that the implementation of ToString() in runtime.js is an exact
match with the current draft's version, though.

[arv (Not doing code reviews), 2014/11/14 23:42:17]

I guess it doesn't hurt to call ToString on a string and you are right that raw
might not actually be strings if someone uses String.raw on their own object.

+    result = result + next;

[arv (Not doing code reviews), 2014/11/14 22:54:25]

+=

[caitp (gmail), 2014/11/14 23:29:58]

Acknowledged.

+    if (nextIndex + 1 === literalSegments) return result;
+    if (nextIndex < numberOfSubstitutions) {
+      next = ToString(substitutions[nextIndex]);
+      result = result + next;
+    }
+    nextIndex++;
+  }
+}
+
+
 // -------------------------------------------------------------------
 
 function ExtendStringPrototype() {
   %CheckIsBootstrapping();
 
   // Set up the non-enumerable functions on the String object.
   InstallFunctions($String, DONT_ENUM, $Array(
-    "fromCodePoint", StringFromCodePoint
+    "fromCodePoint", StringFromCodePoint,
+    "raw", StringRaw
   ));
 
   // Set up the non-enumerable functions on the String prototype object.
   InstallFunctions($String.prototype, DONT_ENUM, $Array(
     "codePointAt", StringCodePointAt,
     "contains", StringContains,
     "endsWith", StringEndsWith,
     "repeat", StringRepeat,
     "startsWith", StringStartsWith
   ));
 }
 
 ExtendStringPrototype();