Map / Reducing in Javascript yourself.

PeopleFilter.js

var people = [
	  { name:"dom", age:22}
	, { name:"paddy", age:22}
	, { name:"pete", age:22}
	, { name:"dick", age:100}
]

var query1 = { field: "age", found: function(field) { 
		if(field > 50) { 
			return true;
		} else { 
			return false;
		}
	}
};

var query2 = { field: "age", found: function(field) { 
		if(field < 23) { 
			return true;
		} else { 
			return false;
		}
	}
}; 

var query3 = { field: "name", found: function(field) { 
		if(field == 'dom') { 
			return true;
		} else { 
			return false;
		}
	}
}; 

var filter = function(people, query) { 
	var peopleFound = []; 
	
	for(var i = 0; i < people.length; i++) { 
		if(query.found(people[i][query.field])) { 
			peopleFound.push(people[i]); 
		} 
	}
	return peopleFound;
}

var peopleOver50 = filter(people, query1);
var peopleUnder23 = filter(people, query2);
var peopleCalledDom = filter(people, query3);
console.log(peopleOver50);
console.log(peopleUnder23);
console.log(peopleCalledDom);