Echo JS 0.11.0

Just for reference, jsperf tests a number of variants: 
http://jsperf.com/array-indexof-vs-object-s-in-operator/4

Fastest method being:

if (typeof obj['that'] !== "undefined") {
  // found
}

For very large datasets one could also look at bloom filters, e.g: https://www.jasondavies.com/bloomfilter/