Javascript Array, return key:value pairs with specific key

Question

Problem I have a Javascript array and I'm trying to return only the key:value pairs that match a specific key in the most efficient way possible.

I've tried using a Foreach loop and pushing each key:value to a new array but that seems like an inefficient way to do it and overly complex.

Code

initialArray = [ { friends: [],
    otherFriends: [],
    _id: 5c6c7132f9bf4bdab9c906ff,
    user: 5c65d9438e4a834c8e85dd7d,
    password: '$2b$10$LMFB6CBdwxSAfjog/Uo3t.u/G0GBtzfJnYdpvlrSNchA9.jlNOdAa',
    email: '[email protected]',
    createdAt: 2019-02-19T21:12:18.569Z,
    updatedAt: null,
    online: false,
    __v: 0 },
  { friends: [],
    otherFriends: [],
    _id: 5c6ccd6d3a0dc4e4951c2bee,
    user: 5c65d9438e4a834c8e85dd7e,
    password: '$2b$10$gEAbOAdKyHJfAksr5txUNOudCautRs1w/pubplQKzZ5PefhfOOEhq',
    email: '[email protected]',
    createdAt: 2019-02-20T03:45:49.703Z,
    updatedAt: null,
    online: false,
    __v: 0 } ]

Desired Array

[ 
    { user: 5c65d9438e4a834c8e85dd7d },
    { user: 5c65d9438e4a834c8e85dd7e }
]

Current Method

const newArray = []
initialArray.forEach(element => {
            newArray.push({key: element.user});
        });

If anyone has any suggestions that would be amazing! Thanks in advance :)

Are you querying the mongoose model yourself or is it an api response? :) — optimisticupdate
– optimisticupdate, Commented Apr 11, 2019 at 20:33
Then you can use a projection and only query for the user property. MyModel.find({}, 'user'); — optimisticupdate
– optimisticupdate, Commented Apr 11, 2019 at 20:34
If you also want to strip _id do MyModel.find({}, {'user': 1, '_id': 0});' — optimisticupdate
– optimisticupdate, Commented Apr 11, 2019 at 20:42

Nina Scholz · Accepted Answer · 2019-04-11 20:44:08Z

3

You could use a destructuring assignment for user and map this short hand property.

var initialArray = [{ friends: [], otherFriends: [], _id: '5c6c7132f9bf4bdab9c906ff', user: '5c65d9438e4a834c8e85dd7d', password: '$2b$10$LMFB6CBdwxSAfjog/Uo3t.u/G0GBtzfJnYdpvlrSNchA9.jlNOdAa', email: '[email protected]', createdAt: '2019-02-19T21:12:18.569Z', updatedAt: null, online: false, __v: 0 }, { friends: [], otherFriends: [], _id: '5c6ccd6d3a0dc4e4951c2bee', user: '5c65d9438e4a834c8e85dd7e', password: '$2b$10$gEAbOAdKyHJfAksr5txUNOudCautRs1w/pubplQKzZ5PefhfOOEhq', email: '[email protected]', createdAt: '2019-02-20T03:45:49.703Z', updatedAt: null, online: false, __v: 0 }] 
    newArray = initialArray.map(({ user }) => ({ user }));

console.log(newArray);

edited Apr 11, 2019 at 20:44

answered Apr 11, 2019 at 20:33

Nina Scholz

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1 Comment

optimisticupdate Over a year ago

This one will be faster because destructuring and spread is faster than object.assign with a new object

Ele · Accepted Answer · 2019-04-11 20:42:16Z

You can keep the desired keys in an array as follow.

This example extracts two key-values.

var initialArray = [{ friends: [], otherFriends: [], _id: '5c6c7132f9bf4bdab9c906ff', user: '5c65d9438e4a834c8e85dd7d', password: '$2b$10$LMFB6CBdwxSAfjog/Uo3t.u/G0GBtzfJnYdpvlrSNchA9.jlNOdAa', email: '[email protected]', createdAt: '2019-02-19T21:12:18.569Z', updatedAt: null, online: false, __v: 0 }, { friends: [], otherFriends: [], _id: '5c6ccd6d3a0dc4e4951c2bee', user: '5c65d9438e4a834c8e85dd7e', password: '$2b$10$gEAbOAdKyHJfAksr5txUNOudCautRs1w/pubplQKzZ5PefhfOOEhq', email: '[email protected]', createdAt: '2019-02-20T03:45:49.703Z', updatedAt: null, online: false, __v: 0 }],
    keys = ["user", "email"], // for the desired output in the question remove "email"
    newArray = initialArray.map(o => keys.reduce((a, k) => Object.assign({}, a, {[k]: o[k]}), {}));

console.log(newArray);

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