var list1 = [{"rid":1, "bet":100, "fold":false},
{"rid":2, "bet":200, "fold":false},
{"rid":4, "bet":200, "fold":false},
{"rid":3, "bet":300, "fold":false}];
list1 = _(list1)
.filter({fold:false})
.groupBy('bet')
.keys()
.map(_.toInteger)
.
$("#output").html(JSON.stringify(list1));
<div id="output"></div>