mapIndexed transducer
npm install map-indexed-xf   
This module defines a function that returns a transducer which calls a given
function with each value and its index. This transducer may be used with
Javascript transducer libraries such as
jlongster/transducers.js and
cognitect-labs/transducers-js.
``js
const t = require('transducers.js');
const mapIndexed = require('map-indexed-xf');
const result = t.toArray([5,6,7], t.compose(
t.map(x => x*10),
mapIndexed((x, i) => [i, x])
));
console.log(result); // [[0, 50], [1, 60], [2, 70]]
``
TypeScript type definitions for this module
are included! The type definitions won't require any configuration to use.