Is it possible to get the index you re sorting over in Underscore js

Question

I m using the JS library Underscore and in particular using the   each and   sortby library calls   I m wondering if there s any possible way to get the index of the value within the iterator delegate    sortBy  1  4  2  66  444  9   function num          It d be great to have access to the index in here        return Math sin num

User · Accepted Answer

Index is actually available like     sortBy  1  4  2  66  444  9   function num  index

User · Answer

You can get the index of the current iteration by adding another parameter to your iterator function  e g     each   foo    bar    baz    function  val  i        console log i          val      0  foo  1  bar  2  baz

User · Answer

When available  I believe that most lodash array functions will show the iteration  But sorting isn t really an iteration in the same way  when you re on the number 66  you aren t processing the fourth item in the array until it s finished  A custom sort function will loop through an array a number of times  nudging adjacent numbers forward or backward  until the everything is in its proper place

User · Answer

More generally  under most circumstances  underscore functions that take a list and argument as the first two arguments   provide access to the list index as the next to last argument to the iterator   This is an important distinction when it comes to the two underscore functions    reduce and   reduceRight  that take  memo  as their third argument -- in the case of these two the index will not be the second argument  but the third   var destination    function         var fields      333st    offroad    fbi        return   reduce waybillInfo destination split       function destination  segment  index            destination fields index     segment          return destination                     console log destination                   333st   NYARFTW  TX  fbi   FTWUP  offroad   UP   The following is better of course but not demonstrate my point  var destination     object    333st    offroad    fbi    waybillInfo destination split             So if you wanted you could get the index using underscore itself    last   initial arguments     A possible exception  I haven t tried  is   map  as it can take an object instead of a list   If list is a JavaScript object  iterator s arguments will be  value  key  list    -- see  http   underscorejs org  map

User · Answer

The iterator of   each is called with 3 parameters  element  index  list   So yes  for   each you cab get the index   You can do the same in sortBy

User · Answer

If you d rather transform your array  then the iterator parameter of underscore s map function is also passed the index as a second argument  So     map  1  4  2  66  444  9   function value  index   return index         value            returns     0 1    1 4    2 2    3 66    4 444    5 9

User · Answer

I think it s worth mentioning how the Underscore s   each   works internally  The   each list  iteratee  checks if the passed list is an array object  or an object   In the case that the list is an array  iteratee arguments will be a list element and index as in the following example   var a     I    like    pancakes    a    lot           each  a  function v  k    console log  k         v        0 I 1 like 2 pancakes 3 a 4 lot 5     On the other hand  if the list argument is an object the iteratee will take a list element and a key   var o    name   mike   lastname   doe   age  21     each  o  function v  k    console log  k         v        name mike lastname doe age 21   For reference this is the   each   code from Underscore js 1 8 3    each     forEach   function obj  iteratee  context       iteratee   optimizeCb iteratee  context      var i  length     if  isArrayLike obj           for  i   0  length   obj length  i  lt  length  i               iteratee obj i   i  obj                else         var keys     keys obj         for  i   0  length   keys length  i  lt  length  i               iteratee obj keys i    keys i   obj                   return obj

[javascript] Is it possible to get the index you're sorting over in Underscore.js?

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