What is the ES6 equivalent of Python 'enumerate' for a sequence?

Yes there is, check out Array.prototype.entries().

const foobar = ['A', 'B', 'C'];

for (const [index, element] of foobar.entries()) {
  console.log(index, element);
}

Array.prototype.map

Array.prototype.map already gives you the index as the second argument to the callback procedure... And it's supported almost everywhere.

['a','b'].map(function(element, index) { return index + ':' + element; });
//=> ["0:a", "1:b"]

I like ES6 too

['a','b'].map((e,i) => `${i}:${e}`)
//=> ["0:a", "1:b"]

make it lazy

However, python's enumerate is lazy and so we should model that characteristic as well -

function* enumerate (it, start = 0)
{ let i = start
  for (const x of it)
    yield [i++, x]
}

for (const [i, x] of enumerate("abcd"))
  console.log(i, x)

0 a
1 b
2 c
3 d

Specifying the second argument, start, allows the caller to control the transform of the index -

for (const [i, x] of enumerate("abcd", 100))
  console.log(i, x)

100 a
101 b
102 c
103 d