Best way to get intersection of keys of two objects?

A solution without indexOf.

var firstObject = { x: 0, y: 1, z: 2, a: 10, b: 20, e: 30 },
    secondObject = { x: 0, y: 1, z: 2, a: 10, c: 20, d: 30 };

function intersection(o1, o2) {
    return Object.keys(o1).concat(Object.keys(o2)).sort().reduce(function (r, a, i, aa) {
        if (i && aa[i - 1] === a) {
            r.push(a);
        }
        return r;
    }, []);
}

document.write('<pre>' + JSON.stringify(intersection(firstObject, secondObject), 0, 4) + '</pre>');

Second attempt with O(n).

var firstObject = { x: 0, y: 1, z: 2, a: 10, b: 20, e: 30 },
    secondObject = { x: 0, y: 1, z: 2, a: 10, c: 20, d: 30 };

function intersection(o1, o2) {
    return Object.keys(o1).filter({}.hasOwnProperty.bind(o2));
}

document.write('<pre>' + JSON.stringify(intersection(firstObject, secondObject), 0, 4) + '</pre>');

The given answers are nice and astonishing but there could be a problem in void's answer and that is: "What if one of property values intentionally set to undefined."

Nina's answer is good (really fantastic) but as we are in era of fun JavaScript I think mine wont be too bad:

var a = { x: undefined, y: 1, z: 2, a: 10, b: 20, e: 30 }
var b = { x: 0, y: 1, z: 2, a: 10, c: 20, d: 30 }

function intersect(o1, o2){
    return Object.keys(o1).filter(k => k in o2)
}

document.write('<pre>' + JSON.stringify(intersect(a, b)) + '</pre>');

Update

onalbi mentioned some performance issue in comments which is rational and therefore the code bellow seems to be a better way to handle the problem: 

var a = { x: undefined, y: 1, z: 2, a: 10, b: 20, e: 30};
var b = { x: 0, y: 1, z: 2, a: 10, c: 20, d: 30};

function intersect(o1, o2) {

  const [k1, k2] = [Object.keys(o1), Object.keys(o2)];
  const [first, next] = k1.length > k2.length ? [k2, o1] : [k1, o2];
  return first.filter(k => k in next);
}

document.write('<pre>' + JSON.stringify(intersect(a, b)) + '</pre>');

The procedure i will suggest is:

  1. Get the array of keys using Object.keys() for one of the objects.
  2. Find the intersection the array using .filter and checking if the second object contains a key matching the first array.

var firstObject = {
  x: 0,
  y: 1,
  z: 2,

  a: 10,
  b: 20,
  e: 30
}

var secondObject = {
  x: 0,
  y: 1,
  z: 2,

  a: 10,
  c: 20,
  d: 30
}

function getIntKeys(obj1, obj2){

    var k1 = Object.keys(obj1);
    return k1.filter(function(x){
        return obj2[x] !== undefined;
    });
  
}

alert(getIntKeys(firstObject, secondObject));

Tags:

Javascript

Object

Big O

Functional Programming

Intersection

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