Lodash has a _.pickBy
function which does exactly what you’re looking for.
var thing = {
"a": 123,
"b": 456,
"abc": 6789
};
var result = _.pickBy(thing, function(value, key) {
return _.startsWith(key, "a");
});
console.log(result.abc) // 6789
console.log(result.b) // undefined
<script src="https://cdn.jsdelivr.net/lodash/4.16.4/lodash.min.js"></script>