Why does javascript map function return undefined

Question

My code   var arr     a   b  1    var results   arr map function item                   if typeof item     string   return item                          This gives the following results    a   b  undefined    I don t want undefined in the results array How can I do it

User · Answer

You can implement like a below logic  Suppose you want an array of values   let test      name  test  lastname  kumar  age 30                 name  test  lastname  kumar  age 30                 name  test3  lastname  kumar  age 47                 name  test  lastname  kumar  age 28                 name  test4  lastname  kumar  age 30                 name  test  lastname  kumar  age 29    let result1   test map element   gt                                     if  element age     30                                          return element lastname                                      filter notUndefined   gt  notUndefined     undefined    output     kumar   kumar   kumar

User · Answer

My solution would be to use filter after the map   This should support every JS data type   example   const notUndefined   anyValue   gt  typeof anyValue      undefined      const noUndefinedList   someList            map    mapping condition             filter notUndefined      by doing this                           you can ensure what s returned is not undefined

User · Answer

var arr     a   b  1    var results   arr filter function item                   if typeof item     string   return item

User · Answer

Since ES6 filter supports pointy arrow notation  like LINQ    So it can be boiled down to following one-liner     a   b  1  filter item   gt  typeof item     string

User · Answer

Filter works for this specific case where the items are not modified  But in many cases when you use map you want to make some modification to the items passed   if that is your intent  you can use reduce   var arr     a   b  1   var results   arr reduce  results  item    gt        if  typeof item      string   results push modify item      modify is a fictitious function that would apply some change to the items in the array     return results

User · Answer

You aren t returning anything in the case that the item is not a string  In that case  the function returns undefined  what you are seeing in the result   The map function is used to map one value to another  but it looks you actually want to filter the array  which a map function is not suitable for   What you actually want is a filter function  It takes a function that  returns true or false based on whether you want the item in the resulting array or not   var arr     a   b  1   var results   arr filter function item       return typeof item     string

User · Answer

You only return a value if the current element is a string  Perhaps assigning an empty string otherwise will suffice   var arr     a   b  1   var results   arr map function item       return  typeof item     string     item               Of course  if you want to filter any non-string elements  you shouldn t use map    Rather  you should look into using the filter   function

[javascript] Why does javascript map function return undefined?

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