I have JavaScript object array with the following structure:
objArray = [ { foo: 1, bar: 2}, { foo: 3, bar: 4}, { foo: 5, bar: 6} ];
I want to extract a field from each object, and get an array containing the values, for example field foo would give array [ 1, 3, 5 ].
I can do this with this trivial approach:
function getFields(input, field) {
var output = [];
for (var i=0; i < input.length ; ++i)
output.push(input[i][field]);
return output;
}
var result = getFields(objArray, "foo"); // returns [ 1, 3, 5 ]
Is there a more elegant or idiomatic way to do this, so that a custom utility function would be unnecessary?
Note about suggested duplicate, it covers how to convert a single object to an array.
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().
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
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.
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)
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' ]
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]
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.
In ES6, you can do:
const objArray = [{foo: 1, bar: 2}, {foo: 3, bar: 4}, {foo: 5, bar: 6}]
objArray.map(({ foo }) => foo)
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.
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
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)
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}]);
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"]
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)
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
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');
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 :).
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));
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 ]
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!!!
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);
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)]);
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/
Destructure and get specific attributes from array of object:
const customerList = dealerUserData?.partyDetails.map(
({ partyId, custAccountId }) => ({
partyId,
custAccountId,
customerId: dealerUserData?._id,
userId: dealerUserData?.authUserID,
}),
);
I have an object that looks like:
var data = {first: '12/1/2019', second: '12/15/2019'}
I am trying to get into an array of objects using its keys and values like so:
var array = [
{phase: 'first', date: '12/1/2019'},
{phase: 'second', date: '12/15/2019'}
]
I have tried various things, but the closest I have gotten is using something like:
var array = Object.entries(data).map(([key, value]) => ({key,value}));
This gives me an array of objects like:
[
{key: 'first', value: '12/1/2019'},
{key: 'second', value: '12/15/2019'}
]
I'm close! but i can't figure out how to change key and value to be phase and date. Can someone please help me out?
You can actually just rename your key and value parameter names:
var array = Object.entries(data).map(([phrase, date]) => ({phrase,date}));
Try adding labels in object.
var data = {
first: '12/1/2019',
second: '12/15/2019'
}
var array = Object.entries(data).map(([key, value]) => ({
phase: key,
date: value
}))
console.log(array)
You are almost there try by adding the key to return object
var data = {
first: '12/1/2019',
second: '12/15/2019'
}
var array = Object.entries(data).map(([key, value]) => ({
phase: key,
date: value
}));
console.log(array)
Try the following solution using for...in to iterates over all non-Symbol, enumerable properties of an object.
const data = { first: '12/1/2019', second: '12/15/2019' };
const dataset = [];
for (const key in data) {
if (data.hasOwnProperty(key)) {
const element = data[key];
dataset.push({
phase: key,
date: element
});
}
}
console.log(dataset);
You can use map() on Object.keys()
var data = {first: '12/1/2019', second: '12/15/2019'}
let arr = Object.keys(data).map(x => ({phase:x,date:data[x]}))
console.log(arr)
You can also use Object.entries() and map() but give different names to the parameters destructed
var data = {first: '12/1/2019', second: '12/15/2019'}
let arr = Object.entries(data).map(([phase,date]) =>({phase,date}))
console.log(arr)
First extract the key (phase) and value (date) from the data object by Object.entries then use Array.reduce to accumulate and form the new object into an array.
const data = {first: '12/1/2019', second: '12/15/2019'}
const arr = Object.entries(data).reduce((acc, [phase, date]) => acc.concat({phase, date}), []);
console.log(arr);
I am trying to return new object with selected keys - reqProps. I managed to do it with fixes props prop1, prop3 and now want to be able to pass reqProps array values to replace prop1, prop3. I tried function, and string literals and few 'hacks'. None of them worked
const data = [
{
prop1: 1,
prop2: 2,
prop3: 3
},
{
prop1: 10,
prop2: 20,
prop3: 30
},
{
prop2: 200,
prop4: 400
},
{
prop3: 3000
}
];
// to return properties for the following...
const reqProps = ['prop2','prop3','prop4'];
// current implementation fixing return object with prop1, prop3
const obj = data.map(({prop1, prop3}) => {
return {prop1, prop3};
});
The result of obj for the moment is
[{"prop1":1,"prop3":3},{"prop1":10,"prop3":30},{},{"prop3":3000}]
I do not want to use loops, quite like the 'power' of destructuring! ;)
If you insist on destructuring, you have to use eval:
const reqProps = ['prop2','prop3','prop4'];
const literalString = '{'+reqProps.join(',')+'}';
const obj = data.map(new Function(literalString, 'return '+literalString));
You really should use a loop - you can also hide it in a helper function or just use reduce.
As #Bergi suggests, you'd better use loops in some way.
Here's a variant with implicit loops:
data.map(o => reqProps.filter(p => p in o)
.reduce((acc, p) => ({...acc, [p]: o[p]}), {}))
I have 2 objects array as follows:
arr1 = [ { 'v1': 'abcde',
'pv_45': 13018,
'geolocation': '17.340291,76.842807'
}]
arr2 =[{ 'v1':'abcde',
'pv_50': 13010,
geolocation: '17.340291,76.842807'
}]
I want to merge the above 2 array int0 single based on condition that 'v1' and 'geolocation' should be same as follows:
[{'v1':'abcde',
'pv_45': 13018,
'pv_50': 13010,
'geolocation': '17.340291,76.842807'}]
I used _.extend, but its not checking any condition blindly it will merge. Please share your ideas. Thanks in advance.
you can use underscore js union and uniq to do that.
var mergedArray = _.uniq(_.union(c1, c2), false, function(item, key, a){ return item; });
Using pure JavaScript it could be done like this:
var arr1 = [ { 'v1': 'abcde',
'pv_45': 13018,
'geolocation': '17.340291,76.842807'
}],
arr2 =[{ 'v1':'abcde',
'pv_50': 13010,
geolocation: '17.340291,76.842807'
}],
mergeOnV1Geo = function (arr1, arr2) {
var mergeObj = {},
merge = function (item) {
var key = item.v1 + ',' + item.geolocation;
// if this is the first object with this key
// create a new object and copy v1, geolocation info
if (!mergeObj[key]) {
mergeObj[key] = {
v1: item.v1,
geolocation: item.geolocation
};
}
// add other props
Object.keys(item).forEach(function (prop) {
if (!prop.match(/v1|geolocation/)) {
mergeObj[key][prop] = item[prop];
}
});
};
arr1.forEach(merge);
arr2.forEach(merge);
// map back into an array
return Object.keys(mergeObj).map(function (key) {
return mergeObj[key];
});
};
mergeOnV1Geo(arr1, arr2);
You could do the following:
var arr3 = [].concat.apply([], arr1, arr2);
var temp =_.groupBy(arr3, 'geolocation');
var result = Object.keys(_.groupBy(arr3, 'geolocation')).map(function(x) { return _.extend.apply(0, p[x]); })
if you prefer ES-6 arrow functions the result becomes
Object.keys(_.groupBy(arr3, 'geolocation')).map((x) => _.extend.apply(0, p[x]);)
ES6: Using spread operator and reduce.
arr1 = [{ v1: 'abcde',
pv_45: 13018,
geolocation: '17.340291,76.842807'
}]
arr2 =[{ v1:'abcde',
pv_50: 13010,
geolocation: '17.340291,76.842807'
}]
// Keys to be grouped i.e whose values should be equal
groupableKeys = ['v1', 'geolocation'];
// Reducer that creates an Object with Key as the
// groupable key values value1::value2 and Value as
// the list of objects whose v1 and geolocation are same
groupableReducer = (a, b) => {
const uniqKey = groupableKeys.map(key => b[key]).join("::");
a[uniqKey] = [...(a[uniqKey] || []), b];
return a;
}
// Merges two objects using the spread operator
mergableReducer = (a, b) => ({...a, ...b})
// Merge two arrays and start processing
groupableKeyObject = [...arr1, ...arr2].reduce(groupableReducer, {})
output = Object.keys(groupableKeyObject)
.map(key =>
groupableKeyObject[key].reduce(mergableReducer, {})
)
console.log(output);