Providing a full answer to this question.
You can use Terraformer (http://terraformer.io/) to do these conversions in Javascript.
Console:
$ npm install terraformer
$ node
Node:
> var t = require('terraformer')
> extent = { "type": "extent", "xmin": -8591193.02145689, "ymin": 4686637.938322207, "xmax": -8560023.564034801, "ymax": 4726686.262984599, "spatialReference": { "wkid": 102100 } }
{ type: 'extent',
xmin: -8591193.02145689,
ymin: 4686637.938322207,
xmax: -8560023.564034801,
ymax: 4726686.262984599,
spatialReference: { wkid: 102100 } }
> [t.Tools.positionToGeographic([extent.xmin, extent.ymin]), t.Tools.positionToGeographic([extent.xmax, extent.ymax])]
[ [ -77.17599999995697, 38.754999999978295 ],
[ -76.89599999995721, 39.03499999997806 ] ]