JS ES2015. The syntax looks awful at first glance, but after heavily using it, there's much more power to this language now.
Jokes aside, ES2015 hits the sweet spot for new powerful languages with big-name backers (facebook is compiled in babel).
// 1. [a + b] // [a + b] is an array {[a + b]: c} // [a + b] is not an array
// 2. { x, y: x, y} // inconsistent object fields
and there are more examples. After you get to know the syntax, it can be used for good -- but some decisions are making the language more complex.
As written above, I enjoy its new features
var b = {3: 4}; b[3] // [3] is not an array