How to filter an object using values of an array in Javascript?

Question

i have an object like this (this is how we see the object in chrome dev tools for example):

obj: {
  1: {...},
  2: {...},
  3: {...},
  4: {...},
  5: {...},
}

And i have a simple array like this:

arr: [1,3,5,7]

Basically i want my object to stay just with the keys that are in the array, like this:

obj: {
  1: {...},
  3: {...},
  5: {...},
}

At the moment my code is this:

var select = (arr, obj) => arr.reduce((r, e) => 
  Object.assign(r, obj[e] ? { [e]: obj[e] } : null)
, {});

var output = select(arr, obj);

I dont know why but this sometimes works and other times don´t. I can not use Jquery. Can anyone help me?

whats the enter code here? And when does it not work? you have example data? the code looks good — Lux
– Lux, Commented Jan 4, 2020 at 14:37
Your code is fine. What errors do you encountered when it doesn't work? — Ori Drori
– Ori Drori, Commented Jan 4, 2020 at 14:47
obj[e] ? is not a proper check (cause falsy values might not be what you want to check for). e in obj ? would be more accurate. — Jonas Wilms
– Jonas Wilms, Commented Jan 4, 2020 at 15:02

Danziger · Accepted Answer · 2020-01-04 15:08:44Z

You could use Object.fromEntries(), Object.entries(), Array.prototype.filter() and Array.prototype.includes() to filter out the keys that are not inside arr:

const obj ={
  1: {},
  2: {},
  3: {},
  4: {},
  5: {},
};

const arr = [1, 3, 5, 7];

const filtered = Object.fromEntries(
  // Note `key` is an `string` here, thus the `+`:
  Object.entries(obj).filter(([key]) => arr.includes(+key))
);

console.log(filtered);

Or a simple for loop, for...of in this case, again with Array.prototype.includes() to pick the ones you want (rather than filtering those you don't) and Object.prototype.hasOwnProperty() to avoid adding keys that don't exist in obj:

const obj ={
  1: {},
  2: {},
  3: {},
  4: {},
  5: {},
};

const arr = [1, 3, 5, 7];

const filtered = {};

for (const key of arr) { 
  // Note the `hasOwnProperty` check here to avoid adding the key `7` with a value of `undefined`:
  if (obj.hasOwnProperty(key)) filtered[key] = obj[key];
}

console.log(filtered);

Or you could do this same thing using Array.prototype.reduce():

const obj ={
  1: {},
  2: {},
  3: {},
  4: {},
  5: {},
};

const arr = [1, 3, 5, 7];

const filtered = arr.reduce((newObj, key) => {
  // Note we add the values to `newObj` rather than `filtered` now:
  if (obj.hasOwnProperty(key)) newObj[key] = obj[key];
  
  // And you always need to return `newObj`:
  return newObj;
}, { })

console.log(filtered);

Lastly, if you are already using Lodash, you could use _.pick:

const obj ={
  1: {},
  2: {},
  3: {},
  4: {},
  5: {},
};

const arr = [1, 3, 5, 7];

const filtered = _.pick(obj, arr);

console.log(filtered);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.core.min.js"></script>

T.J. Crowder · Accepted Answer · 2020-01-04 14:38:23Z

In a modern environment (or with appropriate polyfills), you'd use Object.entries, Array.prototype.filter, and Object.fromEntries, like this:

const result = Object.fromEntries(
    Object.entries(obj)
        .filter(([key, value]) => arr.includes(+key))
);

Live Example:

const obj = {
  1: {name: "one"},
  2: {name: "two"},
  3: {name: "three"},
  4: {name: "four"},
  5: {name: "five"},
};

const arr = [1,3,5,7];

const result = Object.fromEntries(
    Object.entries(obj)
        .filter(([key, value]) => arr.includes(+key))
);

console.log(result);

...but you can also take a simple loop approach:

const result = {};
for (const [key, value] of Object.entries(obj)) {
    if (arr.includes(+key)) {
        result[key] = value;
    }
}

Live Example:

const obj = {
  1: {name: "one"},
  2: {name: "two"},
  3: {name: "three"},
  4: {name: "four"},
  5: {name: "five"},
};

const arr = [1,3,5,7];

const result = {};
for (const [key, value] of Object.entries(obj)) {
    if (arr.includes(+key)) {
        result[key] = value;
    }
}

console.log(result);

In both of the above, note that + before key in the filter call. The array contains numbers, but the property keys of objects are always either strings or Symbols (strings, in your case). So we have to convert for includes to find them.

Collectives™ on Stack Overflow

How to filter an object using values of an array in Javascript?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related