From an array of objects, extract value of a property as array

Question 1

Here is a shorter way of achieving it:

let result = objArray.map(a => a.foo);

OR

let result = objArray.map(({ foo }) => foo)

You can also check Array.prototype.map().

Question 2

Yes, but it relies on an ES5 feature of JavaScript. This means it will not work in IE8 or older.

var result = objArray.map(function(a) {return a.foo;});

On ES6 compatible JS interpreters you can use an arrow function for brevity:

var result = objArray.map(a => a.foo);

Array.prototype.map documentation

Question 3

Speaking for the JS only solutions, I've found that, inelegant as it may be, a simple indexed for loop is more performant than its alternatives.

Extracting single property from a 100000 element array (via jsPerf)

Traditional for loop 368 Ops/sec

var vals=[];
for(var i=0;i<testArray.length;i++){
   vals.push(testArray[i].val);
}

ES6 for..of loop 303 Ops/sec

var vals=[];
for(var item of testArray){
   vals.push(item.val); 
}

Array.prototype.map 19 Ops/sec

var vals = testArray.map(function(a) {return a.val;});

TL;DR - .map() is slow, but feel free to use it if you feel readability is worth more than performance.

Edit #2: 6/2019 - jsPerf link broken, removed.

Question 4

Check out Lodash's _.pluck() function or Underscore's _.pluck() function. Both do exactly what you want in a single function call!

var result = _.pluck(objArray, 'foo');

Update: _.pluck() has been removed as of Lodash v4.0.0, in favour of _.map() in combination with something similar to Niet's answer. _.pluck() is still available in Underscore.

Update 2: As Mark points out in the comments, somewhere between Lodash v4 and 4.3, a new function has been added that provides this functionality again. _.property() is a shorthand function that returns a function for getting the value of a property in an object.

Additionally, _.map() now allows a string to be passed in as the second parameter, which is passed into _.property(). As a result, the following two lines are equivalent to the code sample above from pre-Lodash 4.

var result = _.map(objArray, 'foo');
var result = _.map(objArray, _.property('foo'));

_.property(), and hence _.map(), also allow you to provide a dot-separated string or array in order to access sub-properties:

var objArray = [
    {
        someProperty: { aNumber: 5 }
    },
    {
        someProperty: { aNumber: 2 }
    },
    {
        someProperty: { aNumber: 9 }
    }
];
var result = _.map(objArray, _.property('someProperty.aNumber'));
var result = _.map(objArray, _.property(['someProperty', 'aNumber']));

Both _.map() calls in the above example will return [5, 2, 9].

If you're a little more into functional programming, take a look at Ramda's R.pluck() function, which would look something like this:

var result = R.pluck('foo')(objArray);  // or just R.pluck('foo', objArray)

Question 5

It is better to use some sort of libraries like lodash or underscore for cross browser assurance.

In Lodash you can get values of a property in array by following method

_.map(objArray,"foo")

and in Underscore

_.pluck(objArray,"foo")

Both will return

[1, 2, 3]

Question 6

The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

let kvArray = [{key: 1, value: 10},
               {key: 2, value: 20},
               {key: 3, value: 30}]

let reformattedArray = kvArray.map(obj => {
   return obj.value
})

Or

const kvArray = [['key1', 'value1'], ['key2', 'value2']]

// Use the regular Map constructor to transform a 2D key-value Array into a map
const myMap = new Map(kvArray)

myMap.get('key1') // returns "value1"

// Use Array.from() to transform a map into a 2D key-value Array
console.log(Array.from(myMap)) // Will show you exactly the same Array as kvArray

// A succinct way to do the same, using the spread syntax
console.log([...myMap])

// Or use the keys() or values() iterators, and convert them to an array
console.log(Array.from(myMap.keys())) // ["key1", "key2"]

Question 7

Example to collect the different fields from the object array

let inputArray = [
  { id: 1, name: "name1", value: "value1" },
  { id: 2, name: "name2", value: "value2" },
];

let ids = inputArray.map( (item) => item.id);
let names = inputArray.map((item) => item.name);
let values = inputArray.map((item) => item.value);

console.log(ids);
console.log(names);
console.log(values);

Result :

[ 1, 2 ]
[ 'name1', 'name2' ]
[ 'value1', 'value2' ]

Question 8

Using Array.prototype.map:

function getFields(input, field) {
    return input.map(function(o) {
        return o[field];
    });
}

See the above link for a shim for pre-ES5 browsers.

Question 9

In ES6, you can do:

const objArray = [{foo: 1, bar: 2}, {foo: 3, bar: 4}, {foo: 5, bar: 6}]
objArray.map(({ foo }) => foo)

Question 10

While map is a proper solution to select 'columns' from a list of objects, it has a downside. If not explicitly checked whether or not the columns exists, it'll throw an error and (at best) provide you with undefined. I'd opt for a reduce solution, which can simply ignore the property or even set you up with a default value.

function getFields(list, field) {
    //  reduce the provided list to an array only containing the requested field
    return list.reduce(function(carry, item) {
        //  check if the item is actually an object and does contain the field
        if (typeof item === 'object' && field in item) {
            carry.push(item[field]);
        }

        //  return the 'carry' (which is the list of matched field values)
        return carry;
    }, []);
}

jsbin example

This would work even if one of the items in the provided list is not an object or does not contain the field.

It can even be made more flexible by negotiating a default value should an item not be an object or not contain the field.

function getFields(list, field, otherwise) {
    //  reduce the provided list to an array containing either the requested field or the alternative value
    return list.reduce(function(carry, item) {
        //  If item is an object and contains the field, add its value and the value of otherwise if not
        carry.push(typeof item === 'object' && field in item ? item[field] : otherwise);

        //  return the 'carry' (which is the list of matched field values)
        return carry;
    }, []);
}

jsbin example

This would be the same with map, as the length of the returned array would be the same as the provided array. (In which case a map is slightly cheaper than a reduce):

function getFields(list, field, otherwise) {
    //  map the provided list to an array containing either the requested field or the alternative value
    return list.map(function(item) {
        //  If item is an object and contains the field, add its value and the value of otherwise if not
        return typeof item === 'object' && field in item ? item[field] : otherwise;
    }, []);
}

jsbin example

And then there is the most flexible solution, one which lets you switch between both behaviours simply by providing an alternative value.

function getFields(list, field, otherwise) {
    //  determine once whether or not to use the 'otherwise'
    var alt = typeof otherwise !== 'undefined';

    //  reduce the provided list to an array only containing the requested field
    return list.reduce(function(carry, item) {
        //  If item is an object and contains the field, add its value and the value of 'otherwise' if it was provided
        if (typeof item === 'object' && field in item) {
            carry.push(item[field]);
        }
        else if (alt) {
            carry.push(otherwise);
        }

        //  return the 'carry' (which is the list of matched field values)
        return carry;
    }, []);
}

jsbin example

As the examples above (hopefully) shed some light on the way this works, lets shorten the function a bit by utilising the Array.concat function.

function getFields(list, field, otherwise) {
    var alt = typeof otherwise !== 'undefined';

    return list.reduce(function(carry, item) {
        return carry.concat(typeof item === 'object' && field in item ? item[field] : (alt ? otherwise : []));
    }, []);
}

jsbin example

Question 11

In general, if you want to extrapolate object values which are inside an array (like described in the question) then you could use reduce, map and array destructuring.

ES6

let a = [{ z: 'word', c: 'again', d: 'some' }, { u: '1', r: '2', i: '3' }];
let b = a.reduce((acc, obj) => [...acc, Object.values(obj).map(y => y)], []);

console.log(b)

The equivalent using for in loop would be:

for (let i in a) {
  let temp = [];
  for (let j in a[i]) {
    temp.push(a[i][j]);
  }
  array.push(temp);
}

Produced output: ["word", "again", "some", "1", "2", "3"]

Question 12

If you want to also support array-like objects, use Array.from (ES2015):

Array.from(arrayLike, x => x.foo);

The advantage it has over Array.prototype.map() method is the input can also be a Set:

let arrayLike = new Set([{foo: 1}, {foo: 2}, {foo: 3}]);

Question 13

If you want multiple values in ES6+ the following will work

objArray = [ { foo: 1, bar: 2, baz: 9}, { foo: 3, bar: 4, baz: 10}, { foo: 5, bar: 6, baz: 20} ];

let result = objArray.map(({ foo, baz }) => ({ foo, baz }))

This works as {foo, baz} on the left is using object destructoring and on the right side of the arrow is equivalent to {foo: foo, baz: baz} due to ES6's enhanced object literals.

Question 14

If you have nested arrays you can make it work like this:

const objArray = [ 
     { id: 1, items: { foo:4, bar: 2}},
     { id: 2, items: { foo:3, bar: 2}},
     { id: 3, items: { foo:1, bar: 2}} 
    ];

    let result = objArray.map(({id, items: {foo}}) => ({id, foo}))
    
    console.log(result)

Question 15

The above answer is good for a single property but when select multiple properties from an array use this

var arrayObj=[{Name,'A',Age:20,Email:'a.gmail.com'},{Name,'B',Age:30,Email:'b.gmail.com'},{Name,'C',Age:40,Email:'c.gmail.com'}]

now I select only two fields

 var outPutArray=arrayObj.map(( {Name,Email} ) =>  ({Name,Email}) )
 console.log(outPutArray)

Question 16

It depends on your definition of "better".

The other answers point out the use of map, which is natural (especially for guys used to functional style) and concise. I strongly recommend using it (if you don't bother with the few IE8- IT guys). So if "better" means "more concise", "maintainable", "understandable" then yes, it's way better.

On the other hand, this beauty doesn't come without additional costs. I'm not a big fan of microbench, but I've put up a small test here. The results are predictable, the old ugly way seems to be faster than the map function. So if "better" means "faster", then no, stay with the old school fashion.

Again this is just a microbench and in no way advocating against the use of map, it's just my two cents :).

Question 17

Easily extracting multiple properties from array of objects:

let arrayOfObjects = [
  {id:1, name:'one', desc:'something'},
  {id:2, name:'two', desc:'something else'}
];

//below will extract just the id and name
let result = arrayOfObjects.map(({id, name}) => ({id, name}));

result will be [{id:1, name:'one'},{id:2, name:'two'}]

Add or remove properties as needed in the map function

Question 18

In ES6, in case you want to dynamically pass the field as a string:

function getFields(array, field) {
    return array.map(a => a[field]);
}

let result = getFields(array, 'foo');

Question 19

create an empty array then forEach element from your list, push what you want from that object into your empty array.

 let objArray2 = [];
 objArray.forEach(arr => objArray2.push(arr.foo));

Question 20

Above provided answer is good for extracting single property, what if you want to extract more than one property from array of objects. Here is the solution!! In case of that we can simply use _.pick(object, [paths])

_.pick(object, [paths])

Lets assume objArray has objects with three properties like below

objArray = [ { foo: 1, bar: 2, car:10}, { foo: 3, bar: 4, car:10}, { foo: 5, bar: 6, car:10} ];

Now we want to extract foo and bar property from every object and store them in a separate array. First we will iterate array elements using map and then we apply Lodash Library Standard _.pick() method on it.

Now we are able to extract 'foo' and 'bar' property.

var newArray = objArray.map((element)=>{ return _.pick(element, ['foo','bar'])}) console.log(newArray);

and result would be [{foo: 1, bar: 2},{foo: 3, bar: 4},{foo: 5, bar: 6}]

enjoy!!!

Question 21

Function map is a good choice when dealing with object arrays. Although there have been a number of good answers posted already, the example of using map with combination with filter might be helpful.

In case you want to exclude the properties which values are undefined or exclude just a specific property, you could do the following:

    var obj = {value1: "val1", value2: "val2", Ndb_No: "testing", myVal: undefined};
    var keysFiltered = Object.keys(obj).filter(function(item){return !(item == "Ndb_No" || obj[item] == undefined)});
    var valuesFiltered = keysFiltered.map(function(item) {return obj[item]});

https://jsfiddle.net/ohea7mgk/

Question 22

Here is another shape of using map method on array of objects to get back specific property:

const objArray = [ { foo: 1, bar: 2}, { foo: 3, bar: 4}, { foo: 5, bar: 6} ];

const getProp = prop => obj => obj[prop];
const getFoo = getProp('foo');
const fooes = objArray.map(getFoo);
console.log(fooes);

Question 23

From an array of objects, extract the value of a property as an array with for loop.

//input
objArray = [ { foo: 1, bar: 2}, { foo: 3, bar: 4}, { foo: 5, bar: 6} ]; 

//Code
let output=[];
for(let item of objArray){
    output.push(item.foo); 
}

// Output
[ 1, 3, 5 ]

Question 24

Destructure and get specific attributes from array of object:

const customerList = dealerUserData?.partyDetails.map(
  ({ partyId, custAccountId }) => ({
    partyId,
    custAccountId,
    customerId: dealerUserData?._id,
    userId: dealerUserData?.authUserID,
  }),
);

Question 25

I would only improve one of the answers if you even don't know the exact property of the object you'r playing with use below:

let result = objArray.map(a => a[Object.getOwnPropertyNames(a)]);