Using array map to filter results with if conditional

Question

I am trying to use an array map to filter a object a bit further to prepare it to send to the server to for saving  I can filter to 1 key value  which is great  but I want to take it 1 step further and check them against a boolean inside   So  right now this is what I have -    scope appIds    scope applicationsHere map  function obj           if obj selected    true               return obj id                      This works great for pulling out the id s  however I don t want to push them in this new array if they their selected value    false  so I put a conditional to filter further  This somewhat works  I get an array of id s  but the id s that have  selected    false are still in the array  just with the value of null  So If  I have 4 items in the object and 2 of them are false it looks like this  -    appIds    id1  id2  null  null     My question is - is there a way to do this without the nulls being put in there  Thanks for reading

User · Answer

You could use flatMap  It can filter and map in one   scope appIds    scope applicationsHere flatMap obj   gt  obj selected   obj id

User · Answer

You re looking for the  filter   function      scope appIds    scope applicationsHere filter function obj        return obj selected          That ll produce an array that contains only those objects whose  selected  property is true  or truthy    edit sorry I was getting some coffee and I missed the comments - yes  as jAndy noted in a comment  to filter and then pluck out just the  id  values  it d be      scope appIds    scope applicationsHere filter function obj        return obj selected       map function obj    return obj id        Some functional libraries  like Functional  which in my opinion doesn t get enough love  have a  pluck   function to extract property values from a list of objects  but native JavaScript has a pretty lean set of such tools

User · Answer

You should use Array prototype reduce to do this  I did do a little JS perf test to verify that this is more performant than doing a  filter    map    scope appIds    scope applicationsHere reduce function ids  obj       if obj selected     true           ids push obj id             return ids            Just for the sake of clarity  here s the sample  reduce I used in the JSPerf test    x000D   x000D    var things     x000D       id  1  selected  true   x000D       id  2  selected  true   x000D       id  3  selected  true   x000D       id  4  selected  true   x000D       id  5  selected  false   x000D       id  6  selected  true   x000D       id  7  selected  false   x000D       id  8  selected  true   x000D       id  9  selected  false   x000D       id  10  selected  true   x000D       x000D     x000D      x000D  var ids   things reduce  ids  thing    gt    x000D    if  thing selected    x000D      ids push thing id   x000D      x000D    return ids  x000D          x000D   x000D  console log ids  x000D   x000D   x000D      EDIT 1  Note  As of 2 2018 Reduce   Push is fastest in Chrome and Edge  but slower than Filter   Map in Firefox

User · Answer

Here s some info if someone comes upon this in 2019   I think reduce vs map   filter might be somewhat dependent on what you need to loop through  Not sure on this but reduce does seem to be slower   One thing is for sure - if you re looking for performance improvements the way you write the code is extremely important   Here a JS perf test that shows the massive improvements when typing out the code fully rather than checking for  falsey  values  e g  if  string         or returning  falsey  values where a boolean is expected   Hope this helps someone

[javascript] Using array map to filter results with if conditional

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