What's New in Windows 10 Build 10.0.10051 ?

Please Note that Build 10.0.10051 is an leaked build which leaked a few days ago, it isn't recommended to install it, so if possible wait until the next preview build which is rumoured to be build 10054. 

Following are the major Improvements seen in this build starting with..

Internet Explorer: Major improvements to ECMAScript 6 support (from 73% to 82%)

• Default function parameters
• Rest parameters
• Spread (...) operator
• RegEx "y" and "u" flags
• Generators
• Proxy
• Function "name" property
• String.prototype methods

Project Spartan: 

• Project Spartan has been updated from version 0.10 to 0.11
• You can now find your downloads in the download manager
• You can now open Internet Explorer from within Spartan
• The "Add to" dialogue has been improved

Apps: Health & Fitness is no longer pinned to start by default

Microsoft Family: Microsoft Family has been added as an app

Outlook: Mail has been replaced by Outlook. It now has a ribbon design.

Outlook Calendar: Calendar has been replaced by Outlook Calendar.