change values in array when doing foreach

Question

example   var arr     one   two   three     arr forEach function part     part    four     return  four       alert arr     The array is still with it s original values  is there any way to have writing access to array s elements from iterating function

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Here s a similar answer using using a   gt  style function   var data    1 2 3 4   data forEach   item  i  self    gt  self i    item   10      gives the result    11 12 13 14    The self parameter isn t strictly necessary with the arrow style function  so  data forEach   item i    gt  data i    item   10     also works

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With the Array object methods you can modify the Array content yet compared to the basic for loops  these methods lack one important functionality  You can not modify the index on the run   For example if you will remove the current element and place it to another index position within the same array you can easily do this  If you move the current element to a previous position there is no problem in the next iteration you will get the same next item as if you hadn t done anything   Consider this code where we move the item at index position 5 to index position 2 once the index counts up to 5   var ar    0 1 2 3 4 5 6 7 8 9   ar forEach  e i a    gt    i    5  amp  amp  a splice 2 0 a splice i 1  0   console log i e          0 0 - 1 1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 6 - 7 7 - 8 8 - 9 9   However if we move the current element to somewhere beyond the current index position things get a little messy  Then the very next item will shift into the moved items position and in the next iteration we will not be able to see or evaluate it   Consider this code where we move the item at index position 5 to index position 7 once the index counts up to 5   var a    0 1 2 3 4 5 6 7 8 9   a forEach  e i a    gt    i    5  amp  amp  a splice 7 0 a splice i 1  0   console log i e          0 0 - 1 1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 7 - 7 5 - 8 8 - 9 9   So we have never met 6 in the loop  Normally in a for loop you are expected decrement the index value when you move the array item forward so that your index stays at the same position in the next run and you can still evaluate the item shifted into the removed item s place  This is not possible with array methods  You can not alter the index  Check the following code  var a    0 1 2 3 4 5 6 7 8 9   a forEach  e i a    gt    i    5  amp  amp   a splice 7 0 a splice i 1  0    i--   console log i e          0 0 - 1 1 - 2 2 - 3 3 - 4 4 - 4 5 - 6 7 - 7 5 - 8 8 - 9 9   As you see when we decrement i it will not continue from 5 but 6  from where it was left   So keep this in mind

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You can try this if you want to override   var newArray   444 555 666   var oldArray   11 22 33   oldArray forEach  name  index    gt  oldArray  index    newArray index    console log newArray

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Array   1  2  3  4  Result    quot foo1 quot    quot foo2 quot    quot foo3 quot    quot foo4 quot   Array prototype map   Keep original array  x000D   x000D  const originalArr     Iron    Super    Ant    Aqua    const modifiedArr   originalArr map name   gt     name man     console log   Original   s   originalArr    console log   Modified   s   modifiedArr    x000D   x000D   x000D   Array prototype forEach   Override original array  x000D   x000D  const originalArr     Iron    Super    Ant    Aqua    originalArr forEach  name  index    gt  originalArr index       name man     console log   Overridden   s   originalArr    x000D   x000D   x000D

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The  forEach function can have a callback function eachelement  elementIndex  So basically what you need to do is    arr forEach function element index       arr index     four       set the value       console log arr     the array has been overwritten    Or if you want to keep the original array  you can make a copy of it before doing the above process  To make a copy  you can use   var copy   arr slice

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Javascript is pass by value  and which essentially means part is a copy of the value in the array   To change the value  access the array itself in your loop   arr index     new value

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To add or delete elements entirely which would alter the index  by way of extension of zhujy 8833 suggestion of slice   to iterate over a copy  simply count the number of elements you have already deleted or added and alter the index accordingly  For example  to delete elements   let values     A0    A1    A2    A3    A4    A5    A6    A7    A8    let count   0  values slice   forEach  value  index    gt        if  value      A2     value      A5             values splice index - count    1              console log values       Expected     A0    A1    A3    A4    A6    A7    A8      To insert elements before   if  value      A0     value      A6     value      A8         values splice index - count--  0   newVal           Expected    newVal   A0   A1    A2    A3    A4    A5    newVal    A6    A7    newVal    A8      To insert elements after   if  value      A0     value      A6     value      A8         values splice index - --count  0   newVal           Expected    A0    newVal    A1    A2    A3    A4    A5    A6    newVal    A7    A8    newVal     To replace an element   if  value      A3     value      A4     value      A7         values splice index  1   newVal           Expected     A0    A1    A2    newVal    newVal    A5    A6    newVal    A8      Note  if implementing both  before  and  after  inserts  code should handle  before  inserts first  other way around would not be as expected

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replace it with index of the array   array index    new value

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The callback is passed the element  the index  and the array itself   arr forEach function part  index  theArray      theArray index     hello world         edit     as noted in a comment  the  forEach   function can take a second argument  which will be used as the value of this in each call to the callback   arr forEach function part  index      this index     hello world      arr      use arr as this   That second example shows arr itself being set up as this in the callback One might think that the array involved in the  forEach   call might be the default value of this  but for whatever reason it s not  this will be undefined if that second argument is not provided    Note  the above stuff about this does not apply if the callback is a   gt  function  because this is never bound to anything when such functions are invoked    Also it s important to remember that there is a whole family of similar utilities provided on the Array prototype  and many questions pop up on Stackoverflow about one function or another such that the best solution is to simply pick a different tool  You ve got    forEach for doing a thing with or to every entry in an array  filter for producing a new array containing only qualifying entries  map for making a one-to-one new array by transforming an existing array  some to check whether at least one element in an array fits some description  every to check whether all entries in an array match a description  find to look for a value in an array   and so on  MDN link

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Let s try to keep it simple and discuss how it is actually working  It has to do with variable types and function parameters   Here is your code we are talking about   var arr     one   two   three     arr forEach function part      part    four     return  four       alert arr     First off  here is where you should be reading about Array prototype forEach     https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Array forEach  Second  let s talk briefly about value types in JavaScript   Primitives  undefined  null  String  Boolean  Number  store an actual value   ex  var x   5   Reference Types  custom objects  store the memory location of the object   ex  var xObj     x   5     And third  how function parameters work   In functions  parameters are always passed by value   Because arr is an array of Strings  it s an array of primitive objects  which means they are stored by value   So for your code above  this means that each time the forEach   iterates  part is equal to the same value as arr index   but not the same object   part    four   will change the part variable  but will leave arr alone   The following code will change the values you desire   var arr     one   two   three     arr forEach function part  index      arr index     four        alert arr     Now if array arr was an array of reference types  the following code will work because reference types store a memory location of an object instead of the actual object   var arr      num    one       num    two      num    three      arr forEach function part  index         part and arr index  point to the same object      so changing the object that part points to changes the object that arr index  points to    part num    four        alert arr 0  num   alert arr 1  num   alert arr 2  num     The following illustrates that you can change part to point to a new object while leaving the objects stored in arr alone   var arr      num    one       num    two      num    three      arr forEach function part  index         the following will not change the object that arr index  points to because part now points at a new object   part   5       alert arr 0  num   alert arr 1  num   alert arr 2  num

[javascript] change values in array when doing foreach

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