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

Question

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  lt  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

User · Accepted Answer

Here is a shorter way of achieving it   let result   objArray map a   gt  a foo     OR  let result   objArray map    foo      gt  foo    You can also check Array prototype map

User · Answer

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   amp  amp  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   amp  amp  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   amp  amp  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   amp  amp  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   amp  amp  field in item   item field     alt   otherwise                         jsbin example

User · Answer

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   gt  a foo     Array prototype map documentation

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In ES6  in case you want to dynamically pass the field as a string  function getFields array  field        return array map a   gt  a field       let result   getFields array   foo

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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

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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     gt    id  name      result will be   id 1  name  one    id 2  name  two    Add or remove properties as needed in the map function

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If you have nested arrays you can make it work like this    x000D   x000D  const objArray      x000D         id  1  items    foo 4  bar  2    x000D         id  2  items    foo 3  bar  2    x000D         id  3  items    foo 1  bar  2    x000D         x000D   x000D      let result   objArray map   id  items   foo      gt    id  foo    x000D       x000D      console log result  x000D   x000D   x000D

User · Answer

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

User · Answer

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   gt   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

User · Answer

It depends of 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   In the other hand  this beauty don t come without additional costs  I m not a big fan of microbench  but I ve put up a small test here  The result 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

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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 lt 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

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In ES6  you can do   const objArray     foo  1  bar  2    foo  3  bar  4    foo  5  bar  6   objArray map    foo      gt  foo

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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      gt     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

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Example to collect the different fields from the object array  x000D   x000D  let inputArray         id  1  name   name1   value   value1         id  2  name   name2   value   value2         let ids   inputArray map   item    gt  item id   let names   inputArray map  item    gt  item name   let values   inputArray map  item    gt  item value    console log ids   console log names   console log values   x000D   x000D   x000D   Result     1  2      name1    name2       value1    value2

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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    gt      acc  Object values obj  map y   gt  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

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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

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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

User · Answer

If you want to also support array-like objects  use Array from  ES2015    Array from arrayLike  x   gt  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

[javascript] From an array of objects, extract value of a property as array

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