example.js

a = [
	{_id: "123", val: "123"},
	{_name: "456", val: "456"}
];

result = a.reduce(function(previousValue, currentValue) {
	if (currentValue._id) {
		previousValue[currentValue._id] = currentValue;
	}
	
	if (currentValue._name) {
		previousValue[currentValue._name] = currentValue;
	}
	
	return previousValue;
}, {});

console.log("a: %j", result);