Implement Array.from()

src/runtime.js

 // Copyright 2006-2008 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.
 
 // This files contains runtime support implemented in JavaScript.
 
 // CAUTION: Some of the functions specified in this file are called
 // directly from compiled code. These are the functions with names in
 // ALL CAPS. The compiled code passes the first argument in 'this' and
 // it does not push the function onto the stack. This means that you
@@ skipping 446 matching lines @@
   return %ToNumber(this);
 }
 
 
 // Convert the receiver to a string - forward to ToString.
 function TO_STRING() {
   return %ToString(this);
 }
 
 
+// ES6, draft 05-22-14, section 7.1.15
+function TO_LENGTH(arg) {
+  arg = ToInteger(arg);
+  if (arg < +0) {
+    return 0;
+  }
+  return arg < $Number.MAX_SAFE_INTEGER ? arg : $Number.MAX_SAFE_INTEGER;
+}
+
+
 /* -------------------------------------
    - - -   C o n v e r s i o n s   - - -
    -------------------------------------
 */
 
 // ECMA-262, section 9.1, page 30. Use null/undefined for no hint,
 // (1) for number hint, and (2) for string hint.
 function ToPrimitive(x, hint) {
   // Fast case check.
   if (IS_STRING(x)) return x;
@@ skipping 170 matching lines @@
   return i;
 }
 
 
 // NOTE: Setting the prototype for Array must take place as early as
 // possible due to code generation for array literals.  When
 // generating code for a array literal a boilerplate array is created
 // that is cloned when running the code.  It is essential that the
 // boilerplate gets the right prototype.
 %FunctionSetPrototype($Array, new $Array(0));