How does this object method definition work without the “function” keyword?

How is it possible that this runs at all in any browser? Is is some sort of new ES6 functionality?

Yes.

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Method definitions

A property of an object can also refer to a function or a getter or
setter method.

var o = {
  property: function ([parameters]) {},
  get property() {},
  set property(value) {},
};

In ECMAScript 6, a shorthand notation is available, so that the
keyword “function” is no longer necessary.

// Shorthand method names (ES6)
var o = {
  property([parameters]) {},
  get property() {},
  set property(value) {},
  * generator() {}
};

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