JavaScript Difference between forEach and map

Question

I know that there were a lot of topics like this  And I know the basics   forEach   operates on original array and  map   on the new one   In my case   function practice  i       return i 1      var a     -1  0  1  2  3  4  5    var b     0    var c     0    console log a   b   a forEach practice   console log           console log a   console log b   c   a map practice   console log           console log a   console log c     And this is output     -1  0  1  2  3  4  5           -1  0  1  2  3  4  5   undefined         -1  0  1  2  3  4  5     0  1  2  3  4  5  6     I can t understand why using practice changes value of b to undefined  I m sorry if this is silly question  but I m quite new in this language and answers I found so far didn t satisfy me

User · Answer

Diffrence between Foreach  amp  map    Map     If you use map then map can return new array by iterating main array   Foreach     If you use Foreach then it can not return anything for each can iterating main array   useFul link   use this link for understanding diffrence  https   codeburst io javascript-map-vs-foreach-f38111822c0f

User · Answer

forEach  If you want to perform an action on the elements of an Array and it is same as you use for loop  The result of this method does not give us an output buy just loop through the elements   map  If you want to perform an action on the elements of an array and also you want to store the output of your action into an Array  This is similar to for loop within a function that returns the result after each iteration   Hope this helps

User · Answer

The main difference that you need to know is  map   returns a new array while  forEach   doesn t  That is why you see that difference in the output   forEach   just operates on every value in the array   Read up    Array prototype forEach   - JavaScript   MDN Array prototype map   - JavaScript   MDN   You might also want to check out   - Array prototype every   - JavaScript   MDN

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Array forEach    executes a provided function once per array element     Array map    creates a new array with the results of calling a provided function on every element in this array       So  forEach doesn   t actually return anything  It just calls the function for each array element and then it   s done  So whatever you return within that called function is simply discarded   On the other hand  map will similarly call the function for each array element but instead of discarding its return value  it will capture it and build a new array of those return values   This also means that you could use map wherever you are using forEach but you still shouldn   t do that so you don   t collect the return values without any purpose  It   s just more efficient to not collect them if you don   t need them

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Difference between forEach    amp  map     forEach   just loop through the elements  It s throws away return values and always returns undefined The result of this method does not give us an output    map   loop through the elements allocates memory and stores return values by iterating main array  Example      var numbers    2 3 5 7       var forEachNum   numbers forEach function number         return number          console log forEachNum       output undefined     var mapNum   numbers map function number         return number          console log mapNum       output  2 3 5 7       map   is faster than forEach

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----------------------------------------------------------------------------------------------                       foreach                                map                                        ---------------- ------------------------------------- ---------------------------------------      Functionality     Performs given operation on each       Performs given  quot transformation quot  on                            element of the array                    quot copy quot  of each element                                                                                                                                                                                                                                                                                                                 Return value      Returns undefined                      Returns new array with tranformed                                                                    elements leaving back original array                                                                 unchanged                                                                                                                                                                                                                                                                                                                              Preferrable       Performing non   transformation like     Obtaining array containing output of        usage scenario    processing on each element             some processing done on each element        and example                                              of the array                                                  For example  saving all elements in                                                                  the database                           For example  obtaining array of                                                                      lengths of each string in the                                                                        array                                     ----------------------------------------------------------------------------------------------

User · Answer

The difference lies in what they return  After execution   arr map     returns an array of elements resulting from  the processed function  while    arr forEach     returns undefined

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forEach     return value   undefined originalArray   not modified after the method call newArray is not created after the end of method call  map     return value   new Array populated with the results of calling a provided function on every element in the calling array originalArray   not modified after the method call newArray is created after the end of method call  Since map builds a new array  using it when you aren t using the returned array is an anti-pattern  use forEach or for-of instead

User · Answer

One thing to point out is that foreach skips uninitialized values while map does not   var arr    1    3    arr forEach function element        console log element         Expected output  1 3  console log arr map element   gt  element      Expected output   1  undefined  3

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Map implicitly returns while forEach does not   This is why when you re coding a JSX application  you almost always use map instead of forEach to display content in React

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Performance Analysis For loops performs faster than map or foreach as number of elements in a array increases   let array       for  var i   0  i  lt  20000000  i        array push i     console time  map    array map num   gt      return num   4      console timeEnd  map      console time  forEach    array forEach  num  index    gt      return array index    num   4      console timeEnd  forEach     console time  for    for  i   0  i  lt  array length  i        array i    array i    2     console timeEnd  for

User · Answer

They are not one and the same  Let me explain the difference  forEach  This iterates over a list and applies some operation with side effects to each list member  example  saving every list item to the database  map  This iterates over a list  transforms each member of that list  and returns another list of the same size with the transformed members  example  transforming list of strings to uppercase   It does not mutate the array on which it is called  although if passed a callback function  it may do so   References Array prototype forEach   - JavaScript   MDN Array prototype map   - JavaScript   MDN

[javascript] JavaScript: Difference between .forEach() and .map()

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