Short circuit Array forEach like calling break

Question

1 2 3  forEach function el        if el     1  break        How can I do this using the new forEach method in JavaScript  I ve tried return   return false  and break  break crashes and return does nothing but continue iteration

User · Answer

Yes it is possible to continue and to exit of a forEach loop.

To continue, you can use return, the loop will continue but the current function will end.

To exit of the loop, you can set the third parameter to 0 length, set to empty array. The loop will not continue, the current function do, so you can use "return" to finish, like exit in a normal for loop...

This:

[1,2,3,4,5,6,7,8,9,10].forEach((a,b,c) => {
    console.log(a);
    if(b == 2){return;}
    if(b == 4){c.length = 0;return;}
    console.log("next...",b);
});

will print this:

1
next... 0
2
next... 1
3
4
next... 3
5

User · Answer

Another concept I came up with    x000D   x000D  function forEach array  cb    x000D    var shouldBreak  x000D    function  break     shouldBreak   true    x000D    for  var i   0  bound   array length  i  lt  bound    i    x000D      if  shouldBreak    break    x000D      cb array i   i  array   break   x000D      x000D    x000D   x000D     Usage x000D   x000D  forEach   a   b   c   d   e   f    function  char  i  array   break    x000D    console log i  char   x000D    if  i     2     break      x000D      x000D   x000D   x000D

User · Answer

If you would like to use Dean Edward s suggestion and throw the StopIteration error to break out of the loop without having to catch the error  you can use the following the function  originally from here       Use a closure to prevent the global namespace from be polluted   function          Define StopIteration as part of the global scope if it      isn t already defined    if typeof StopIteration     undefined         StopIteration   new Error  StopIteration              The original version of Array prototype forEach    var oldForEach   Array prototype forEach        If forEach actually exists  define forEach so you can      break out of it by throwing StopIteration   Allow      other errors will be thrown as normal    if oldForEach        Array prototype forEach   function           try           oldForEach apply this     slice call arguments  0                  catch e            if e     StopIteration              throw e                                       The above code will give you the ability to run code such as the following without having to do your own try-catch clauses      Show the contents until you get to  2    0 1 2 3 4  forEach function val      if val    2      throw StopIteration    alert val         One important thing to remember is that this will only update the Array prototype forEach function if it already exists   If it doesn t exist already  it will not modify the it

User · Answer

Agree with  bobince  upvoted   Also  FYI   Prototype js has something for this purpose    lt script type  text javascript  gt        a   each function el  idx        if      break condition      throw  break         do something        lt  script gt     break will be catched and handled by Prototype js internally  breaking the  each  cycle but not generating external errors   See Prototype JS API for details   jQuery also has a way  just return false in the handler to break the loop early    lt script type  text javascript  gt    jQuery  a   each  function idx        if      break condition      return false         do something         lt  script gt    See jQuery API for details

User · Answer

Quoting from the MDN documentation of Array prototype forEach        There is no way to stop or break a forEach   loop other than    by throwing an exception  If you need such behaviour  the  forEach   method is the wrong tool  use a plain loop instead  If you are testing the array elements for a predicate and need a boolean return value  you can use every   or some   instead    For your code  in the question   as suggested by  bobince  use Array prototype some   instead  It suits very well to your usecase      Array prototype some   executes the callback function once for each element present in the array until it finds one where callback returns a truthy value  a value that becomes true when converted to a Boolean   If such an element is found  some   immediately returns true  Otherwise  some   returns false  callback is invoked only for indexes of the array which have assigned values  it is not invoked for indexes which have been deleted or which have never been assigned values

User · Answer

If you don t need to access your array after iteration you can bail out by setting the array s length to 0  If you do still need it after your iteration you could clone it using slice     1 3 4 5 6 7 8 244 3 5 2  forEach function  item  index  arr      if  index     3  arr length   0        Or with a clone   var x    1 3 4 5 6 7 8 244 3 5 2    x slice   forEach function  item  index  arr      if  index     3  arr length   0        Which is a far better solution then throwing random errors in your code

User · Answer

Why don t you try wrapping the function in a Promise  The only reason I bring it up is that I am using a function in an API that acts in a similar manner to forEach  I don t want it to keep iterating once it finds a value  and I need to return something so I am simply going to resolve a Promise and do it that way  traverseTree doc   Promise lt any gt      return new Promise lt any gt   resolve  reject    gt        this gridOptions api forEachNode  node  index    gt          the above function is the one I want to short circuit        if node data id     doc id            return resolve node                            Then all you need to do is do something with the result like this traverseTree doc  then  result    gt       this doSomething result        My above example is in typescript  simply ignore the types  The logic should hopefully help you  quot break quot  out of your loop

User · Answer

There is now an even better way to do this in ECMAScript2015  aka ES6  using the new for of loop  For example  this code does not print the array elements after the number 5    x000D   x000D  let arr    0  1  2  3  4  5  6  7  8  9  10   x000D  for  let el of arr    x000D    console log el   x000D    if  el     5    x000D      break  x000D      x000D    x000D   x000D   x000D    From the docs       Both for   in and for   of statements iterate over something  The main difference between them is in what they iterate over  The for   in statement iterates over the enumerable properties of an object  in original insertion order  The for   of statement iterates over data that iterable object defines to be iterated over    Need the index in the iteration  You can use Array entries     for  const  index  el  of arr entries        if   index     5   break

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As mentioned before  you can t break  forEach     Here s a slightly more modern way of doing a foreach with ES6 Iterators  Allows you to get direct access to index value when iterating   const array     one    two    three     for  const  index  val  of array entries        console log  item      index  val       if  index     1        console log  break         break          Output   item    index  0  val   one    item    index  1  val   two    break    Links   Array prototype entries   Iterators and generators Destructuring assignment

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Consider to use jquery s each method  since it allows to return false inside callback function     each function e  i        if  i   2  return false     console log e       Lodash libraries also provides takeWhile method that can be chained with map reduce fold etc   var users          user    barney     active   false         user    fred       active   false         user    pebbles    active   true         takeWhile users  function o    return  o active           gt  objects for   barney    fred       The    matches  iteratee shorthand    takeWhile users     user    barney    active   false          gt  objects for   barney       The    matchesProperty  iteratee shorthand    takeWhile users    active   false         gt  objects for   barney    fred       The    property  iteratee shorthand    takeWhile users   active         gt

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try with  find     var myCategories       category   start   name   Start   color    AC193D      category   action   name   Action   color    8C0095      category   exit   name   Exit   color    008A00       function findCategory category      return myCategories find function element        return element category     category           console log findCategory  start        output    category   start   name   Start   color    AC193D

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There s no built-in ability to break in forEach  To interrupt execution you would have to throw an exception of some sort  eg    x000D   x000D  var BreakException       x000D   x000D  try   x000D     1  2  3  forEach function el    x000D      console log el   x000D      if  el     2  throw BreakException  x000D        x000D    catch  e    x000D    if  e     BreakException  throw e  x000D    x000D   x000D   x000D    JavaScript exceptions aren t terribly pretty  A traditional for loop might be more appropriate if you really need to break inside it    Use Array some  Instead  use Array some    x000D   x000D   1  2  3  some function el    x000D    console log el   x000D    return el     2  x000D      x000D   x000D   x000D    This works because some returns true as soon as any of the callbacks  executed in array order  return true  short-circuiting the execution of the rest   some  its inverse every  which will stop on a return false   and forEach are all ECMAScript Fifth Edition methods which will need to be added to the Array prototype on browsers where they re missing

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If you need to break based on the value of elements that are already in your array as in your case  i e  if break condition does not depend on run-time variable that may change after array is assigned its element values  you could also use combination of slice   and indexOf   as follows   If you need to break when forEach reaches  Apple  you can use  var fruits     Banana    Orange    Lemon    Apple    Mango    var fruitsToLoop   fruits slice 0  fruits indexOf  Apple        fruitsToLoop   Banana Orange Lemon  fruitsToLoop forEach function el           no need to break       As stated in W3Schools com the slice   method returns the selected elements in an array  as a new array object  The original array will not be changed   See it in JSFiddle  Hope it helps someone

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var Book     Titles                                         Book3     BULLETIN 3                            Book1     BULLETIN 1                         Book2     BULLETIN 2                var findbystr   function str         var return val      Book Titles forEach function data            if typeof data str      undefined                         return val   data str                     str        return return val     book   findbystr  Book1    console log book

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This is a for loop  but maintains the object reference in the loop just like a forEach   but you can break out   var arr    1 2 3   for  var i   0  el  el   arr i   i          if el     1  break

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You can create a variant of forEach that allows for break  continue  return  and even async await   example written in TypeScript   export type LoopControlOp    break     continue      return   any   export type LoopFunc lt T gt     value  T  index  number  array  T     gt LoopControlOp   Array prototype ForEach   function ForEach lt T gt  this  T    func  LoopFunc lt T gt         for  let i   0  i  lt  this length  i              const controlOp   func this i   i  this           if  controlOp     break   break          if  controlOp     continue   continue          if  controlOp instanceof Array  return controlOp 1                this variant lets you use async await in the loop-func  with the loop  awaiting  for each entry Array prototype ForEachAsync   async function ForEachAsync lt T gt  this  T    func  LoopFunc lt T gt         for  let i   0  i  lt  this length  i              const controlOp   await func this i   i  this           if  controlOp     break   break          if  controlOp     continue   continue          if  controlOp instanceof Array  return controlOp 1              Usage   function GetCoffee         const cancelReason   peopleOnStreet ForEach  person  index   gt            if  index    0  return  continue           if  person type     friend   return  break           if  person type     boss   return   return    nevermind                if  cancelReason  console log  Coffee canceled because      cancelReason

User · Answer

This isn t the most efficient  since you still cycle all the elements  but I thought it might be worth considering the very simple   let keepGoing   true  things forEach   thing    gt      if  noMore  keepGoing   false    if  keepGoing            do things with thing

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I use nullhack for that purpose  it tries to access property of null  which is an error   try      1 2 3 4 5     forEach      function   val  idx  arr           if   val    3   null NULLBREAK               catch  e         e  lt   gt  TypeError  null has no properties

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Found this solution on another site  You can wrap the forEach in a try   catch scenario   if typeof StopIteration     undefined      StopIteration   new Error  StopIteration       try      1 2 3  forEach function el       alert el       if el     1  throw StopIteration          catch error    if error    StopIteration  throw error      More details here  http   dean edwards name weblog 2006 07 enum

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you can follow the code below which works for me    var     loopStop   false  YOUR ARRAY forEach function loop        if loopStop   return        if condition   loopStop   true

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You can use every method   1 2 3  every function el        return   el     1        ES6  1 2 3  every  el   gt  el     1    for old browser support use  if   Array prototype every      Array prototype every   function fun     thisp            var len   this length      if  typeof fun     quot function quot         throw new TypeError         var thisp   arguments 1       for  var i   0  i  lt  len  i                if  i in this  amp  amp             fun call thisp  this i   i  this           return false             return true          more details here

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This is just something I came up with to solve the problem    I m pretty sure it fixes the problem that the original asker had   Array prototype each   function callback       if  callback  return false      for var i 0  i lt this length  i             if callback this i   i     false  break             And then you would call it by using   var myarray    1 2 3   myarray each function item  index          do something with the item        if item    somecondition  return false         Returning false inside the callback function will cause a break  Let me know if that doesn t actually work

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I know it not right way  It is not break the loop  It is a Jugad  x000D   x000D  let result   true  x000D   1  2  3  forEach function el    x000D      if result   x000D        console log el   x000D        if  el     2   x000D          result   false  x000D          x000D        x000D      x000D   x000D   x000D

User · Answer

If you want to keep your forEach syntax  this is a way to keep it efficient  although not as good as a regular for loop   Check immediately for a variable that knows if you want to break out of the loop    This example uses a anonymous function for creating a function scope around the forEach which you need to store the done information    x000D   x000D   function    x000D      var element   document getElementById  printed-result    x000D      var done   false  x000D       1 2 3 4  forEach function item   x000D          if done   return    x000D          var text   document createTextNode item   x000D          element appendChild text   x000D          if  item     2   x000D            done   true  x000D            return  x000D            x000D          x000D        x000D   lt div id  printed-result  gt  lt  div gt  x000D   x000D   x000D    My two cents

User · Answer

Use the array prototype every function  which provide you the utility to break the looping  See example here Javascript documentation on Mozilla developer network

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Short answer  use for   break for this or change your code to avoid breaking of forEach  Do not use  some   or  every   to emulate for   break  Rewrite your code to avoid for   break loop  or use for   break  Every time you use these methods as for   break alternative God kills kitten    Long answer    some   and  every   both return boolean value   some   returns true if there any element for which passed function returns true  every returns false if there any element for which passed function returns false  This is what that functions mean  Using functions for what they doesn t mean is much worse then using tables for layout instead of CSS  because it frustrates everybody who reads your code   Also  the only possible way to use these methods as for   break alternative is to make side-effects  change some vars outside of  some   callback function   and this is not much different from for   break   So  using  some   or  every   as for   break loop alternative isn t free of side effects  this isn t much cleaner then for   break  this is frustrating  so this isn t better    You can always rewrite your code so that there will be no need in for   break  You can filter array using  filter    you can split array using  slice   and so on  then use  forEach   or  map   for that part of array

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Unfortunately in this case it will be much better if you don t use forEach   Instead use a regular for loop and it will now work exactly as you would expect    var array    1  2  3   for  var i   0  i  lt  array length  i        if  array i      1       break

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From your code example  it looks like Array prototype find is what you are looking for  Array prototype find   and Array prototype findIndex      1  2  3  find function el        return el     2         returns 2

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Yet another approach          var wageType   types filter function element               if e params data text    element name                    return element                                    console dir wageType

[javascript] Short circuit Array.forEach like calling break

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