filter_groupBy.js

let array = [
    { name: "Test Name 1", gender: "M", age: "28"},
    { name: "Test Name 2", gender: "F", age: "29"},
    { name: "Test Name 3", gender: "M", age: "35"},
    { name: "Test Name 4", gender: "F", age: "38"}
    ];
    
Array.prototype.groupBy = function(prop) {
  return this.reduce((groups, item) => {
    const val = item[prop];
    console.log("asdf",groups, item);
    groups[val] = groups[val] || []
    groups[val].push(item)
    return groups
  }, {})
}
    
function someFunction(arr) {
    return arr.filter(f => {
        return f.age > 30 && f.age < 40;
    })
    .groupBy('gender');
}

console.log(someFunction(array));