This one weird trick... with JS object keys

I’ve been reading the source of https://github.com/kirm/sip.js recently, and I stumbled upon a JS idiom which I had no idea about:

function makeContextId(msg) {
  var via = msg.headers.via[0];
  return [via.params.branch, via.protocol, via.host, via.port, msg.headers['call-id'], msg.headers.cseq.seq];
}

// ... 
// Else where in the code 
var ctx = contexts[makeContextId(msg)];

REF: https://github.com/kirm/sip.js/blob/master/proxy.js

I’ve known for a while JS objects must be keys or symbols, I even remember a particular developer poo pooing JS for the lack of flexibility. Well, that contraints still holds, but it seems the bracket operators [] coerces the value to a string for you! and so the above code works, both marvellous and terrifying :)

This is because the toString function of an array returns something useful:

let list = [1,2,3,4,5]
list.toString()
>>> '1,2,3,4,5'

but beware, this is not always the case…

let obj = { a:1 }
obj.toString()
>>> '[object Object]'