Having sorted out the ES5 situation yesterday, I decided to take a look at ES6: what is shimable and what already has broad implementation (Chrome, Opera and Firefox).

You can easily use these string and number methods now, anywhere you have ES5 or an ES5 shim:

String.prototype.repeat
String.prototype.startsWith
String.prototype.endsWith
String.prototype.includes

Number.isFinite
Number.isInteger
Number.isNaN

I have collected polyfills for these here: tiny-es6

The ES6 features below are also broadly available and can be used via es6-shim. Unfortutuately es6-shim does things that are not compatible with Chrome Apps and WinJS Apps.

Map
Set
Promise

Math.clz32
Math.imul
Math.sign
Math.log10
Math.log2
Math.log1p
Math.expm1
Math.cosh
Math.sinh
Math.tanh
Math.acosh
Math.asinh
Math.atanh
Math.trunc
Math.fround
Math.cbrt
Math.hypot