Keeping only certain properties in a JavaScript object

Question

I have an object. I would like to modify the object (not clone it) by removing all properties except for certain specific properties. For instance, if I started with this o

Answer 1

You could use delete :

for (var k in myObj) {
    if (k !== 'p1' && k !== 'p2' && k !== 'p100') {
        delete myObj[k];
    }
}

An alternative to indexOf :

var keep = /^p(1|2|100)$/;
for (var k in myObj) {
    if (!keep.test(k)) {
        delete myObj[k];
    }
}

Shorter :

var keep = /^p(1|2|100)$/;
for (var k in myObj) {
    keep.test(k) || delete myObj[k];
}

Array to RegExp :

var keep = [1, 2, 100];
keep = new RegExp('^p(' + keep.join('|') + ')$'); // /^p(1|2|100)$/
keep.test('p1'); // true
keep.test('p3'); // false

Useful with a function :

function keep(o, ids) {
    var keep = new RegExp('^p(' + ids.join('|') + ')$');
    for (var k in o) keep.test(k) || delete o[k];
    return o;
}

Usage :

keep(myObj, [1, 2, 100]); // { p1: 123, p2: 321, p100: 3434 }

---

You don't like regular expressions but still want a bullet proof solution :

function keep(o, keys) {
    for (var k in o) contains(keys, k) || delete o[k];
    return o;
}

function contains(array, value) {
    var i = -1, l = array.length;
    while (++i < l) if (array[i] === value) return true;
    return false;
}

function prefix(array, prefix) {
    var i = -1, l = array.length, output = [];
    while (++i < l) output.push(prefix + array[i]);
    return output;
}

Usage :

keep(myObj, ['p1', 'p2', 'p100']);
// with a middleman :
var idsToKeep = [1, 2, 100];
keep(myObj, prefix(idsToKeep, 'p'));

Well, I could go on like this infinitely, but it's time to eat.