Useful JS Tips

#70 - Picking and rejecting object properties

2017-04-05 by @loverajoel

Sometimes we need to whitelist certain attributes from an object, say we’ve got an array representation of a database table and we need to select just a few fields for some function:

function pick(obj, keys) {
    return keys.map(k => k in obj ? {[k]: obj[k]} : {})
               .reduce((res, o) => Object.assign(res, o), {});
}

const row = {
    'accounts.id': 1,
    'client.name': 'John Doe',
    'bank.code': 'MDAKW213'
};

const table = [
    row,
    {'accounts.id': 3, 'client.name': 'Steve Doe', 'bank.code': 'STV12JB'}
];

pick(row, ['client.name']); // Get client name

table.map(row => pick(row, ['client.name'])); // Get a list of client names

There’s a bit of skulduggery going on in pick. First, we map a function over the keys that will return, each time, an object with only the attribute pointed by the current key (or an empty object if there’s no such attribute in the object). Then, we reduce this collection of single-attribute objects by merging the objects.

But what if we want to reject the attributes? Well, the function changes a bit

function reject(obj, keys) {
    return Object.keys(obj)
        .filter(k => !keys.includes(k))
        .map(k => Object.assign({}, {[k]: obj[k]}))
        .reduce((res, o) => Object.assign(res, o), {});
}

// or, reusing pick
function reject(obj, keys) {
    const vkeys = Object.keys(obj)
        .filter(k => !keys.includes(k));
    return pick(obj, vkeys);
}

reject({a: 2, b: 3, c: 4}, ['a', 'b']); // => {c: 4}
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