Roughly every six weeks, we create a new branch of V8 as part of our release process. Each version is branched from V8’s Git master immediately before Chrome branches for a Chrome Beta milestone. Today we’re pleased to announce our newest branch, V8 version 4.8, which will be in beta until it is released in coordination with Chrome 48 Stable. V8 4.8 contains a handful of developer-facing features, so we’d like to give you a preview of some of the highlights in anticipation of the release in several weeks.

Improved ECMAScript 2015 (ES6) support #

This release of V8 provides support for two well-known symbols, built-in symbols from the ES2015 spec that allow developers to leverage several low-level language constructs which were previously hidden.

@@isConcatSpreadable #

The name for a boolean-valued property that if true indicates an object should be flattened to its array elements by Array.prototype.concat.

(function() {
  'use strict';
  class AutomaticallySpreadingArray extends Array {
    get [Symbol.isConcatSpreadable]() {
      return true;
    }
  }
  const first = [1];
  const second = new AutomaticallySpreadingArray();
  second[0] = 2;
  second[1] = 3;
  const all = first.concat(second);
  // Outputs [1, 2, 3]
  console.log(all);
}());

@@toPrimitive #

The name for a method to invoke on an object for implicit conversions to primitive values.

(function(){
  'use strict';
  class V8 {
    [Symbol.toPrimitive](hint) {
      if (hint === 'string') {
        console.log('string');
        return 'V8';
      } else if (hint === 'number') {
        console.log('number');
        return 8;
      } else {
        console.log('default:' + hint);
        return 8;
      }
    }
  }

  const engine = new V8();
  console.log(Number(engine));
  console.log(String(engine));
}());

ToLength #

The ES2015 spec adjusts the abstract operation for type conversion to convert an argument to an integer suitable for use as the length of an array-like object. (While not directly observable, this change might be indirectly visible when dealing with array-like objects with negative length.)

V8 API #

Please check out our summary of API changes. This document gets regularly updated a few weeks after each major release.

Developers with an active V8 checkout can use git checkout -b 4.8 -t branch-heads/4.8 to experiment with the new features in V8 v4.8. Alternatively you can subscribe to Chrome's Beta channel and try the new features out yourself soon.