Selecting last element in JavaScript array

Question

I m making an application that updates a user s location and path in real time and displays this on a Google Map  I have functionality that allows multiple users to be tracked at the same time using an object  which is updated every second    Right now  when a user pressed a button in the Android app  the coordinates are sent to a database and each time the location changes  a marker is updated on the map  and a polyline is formed     Since I have multiple users  I send a unique and randomly generated alphanumeric string so that I can display an individual path for each user  When the JS pulls this data from the database  it checks if the user exists  if it does not  it creates a new key with the value being a list  It would look something like this   loc    f096012e-2497-485d-8adb-7ec0b9352c52   new google maps LatLng 39  -86                                                 new google maps LatLng 38  -87                                                 new google maps LatLng 37  -88           44ed0662-1a9e-4c0e-9920-106258dcc3e7   new google maps LatLng 40  -83                                                 new google maps LatLng 41  -82                                                 new google maps LatLng 42  -81      What I m doing is storing a list of coordinates as the value of the key  which is the user s ID  My program keeps updating this list each time the location is changed by adding to the list  this works properly     What I need to do is update the marker s location each time the location changes  I would like to do this by selecting the last item in the array since that would be the last known location  Right now  each time the location is changed a new marker is added to the map  each one of the points in the example would show a marker at that location  so markers continue to be added    I would use a   for  x in loc   statement each time the location updates to grab the last location from the list and use that to update the marker  How do I select this last element from the array within the hash

User · Answer

use es6 deconstruction array with the spread operator  var last       yourArray  pop     note that yourArray doesn t change

User · Answer

Underscore and Lodash have the   last Array  method  that returns the last element in an Array  They both work about the same    last  5  4  3  2  1      gt  1   Ramda also has a   last function  R last   fi    fo    fum         gt   fum

User · Answer

You can define a getter on Array prototype    x000D   x000D  if   Array prototype hasOwnProperty  last      x000D    Object defineProperty Array prototype   last     x000D      get     x000D        return this this length - 1   x000D        x000D        x000D    x000D   x000D  console log  9  8  7  6  last        gt  6 x000D   x000D   x000D    As you can see  access doesn t look like a function call  the getter function is called internally

User · Answer

In case you are using ES6 you can do   const arr     1  2  3         arr   pop       3 arr       1  2  3    wasn t changed

User · Answer

So  a lot of people are answering with pop    but most of them don t seem to realize that s a destructive method    var a    1 2 3  a pop     3   a is now  1 2    So  for a really silly  nondestructive method   var a    1 2 3  a a push a pop   -1    3   a push pop  like in the 90s     push appends a value to the end of an array  and returns the length of the result  so   d    d push  life        gt  1 d      gt   life     pop returns the value of the last item of an array  prior to it removing that value at that index  so   c    1 2 1  c pop        gt  1 c      gt   1 2    arrays are 0 indexed  so c length    3  c c length     undefined  because you re looking for the 4th value if you do that this level of depth is for any hapless newbs that end up here     Probably not the best  or even a good method for your application  what with traffic  churn  blah  but for traversing down an array  streaming it onto another  just being silly with inefficient methods  this  Totally this

User · Answer

Use the slice   method   my array slice -1  0

User · Answer

You can also  pop off the last element  Be careful  this will change the value of the array  but that might be OK for you   var a    1 2 3   a pop       3 a     1 2

User · Answer

var last   function  obj  key          var a   obj key       return a a length - 1       last loc   f096012e-2497-485d-8adb-7ec0b9352c52

User · Answer

This worked   array reverse   0

User · Answer

var arr    1  2  3   arr slice -1  pop       return 3 and arr    1  2  3    This will return undefined if the array is empty and this will not change the value of the array

User · Answer

var last   array slice -1  0     I find slice at -1 useful for getting the last element  especially of an array of unknown length  and the performance is much better than calculating the length less 1   Mozilla Docs on Slice  Performance of the various methods for selecting last array element

User · Answer

How to access last element of an array  It looks like that   var my array      some array here     var last element   my array my array length - 1     Which in your case looks like this   var array1   loc  f096012e-2497-485d-8adb-7ec0b9352c52    var last element   array1 array1 length - 1     or  in longer version  without creating new variables   loc  f096012e-2497-485d-8adb-7ec0b9352c52   loc  f096012e-2497-485d-8adb-7ec0b9352c52   length - 1     How to add a method for getting it simpler  If you are a fan for creating functions shortcuts to fulfill such tasks  the following code   if   Array prototype last       Array prototype last   function            return this this length - 1               will allow you to get the last element of an array by invoking array s last   method  in your case eg    loc  f096012e-2497-485d-8adb-7ec0b9352c52   last      You can check that it works here  http   jsfiddle net D4NRN

User · Answer

Use JavaScript objects if this is critical to your application  You shouldn t be using raw primitives to manage critical parts of your application   As this seems to be the core of your application  you should use objects instead  I ve written some code below to help get you started  The method lastLocation would return the last location     function User id        this id   id       this locations            this getId   function             return this id              this addLocation   function latitude  longitude            this locations this locations length    new google maps LatLng latitude  longitude               this lastLocation   function             return this locations this locations length - 1               this removeLastLocation   function             return this locations pop               function Users         this users            this generateId   function             return Math random                this createUser   function             var id   this generateId            this users id    new User id           return this users id               this getUser   function id            return this users id               this removeUser   function id            var user   this getUser id           delete this users id            return user              var users   new Users     var user   users createUser     user addLocation 0  0   user addLocation 0  1

[javascript] Selecting last element in JavaScript array

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