Can a 1 a 2 a 3 ever evaluate to true

Question

Moderator note  Please resist the urge to edit the code or remove this notice  The pattern of whitespace may be part of the question and therefore should not be tampered with unnecessarily  If you are in the  whitespace is insignificant  camp  you should be able to accept the code as is    Is it ever possible that  a   1  amp  amp  a   2  amp  amp  a  3  could evaluate to true in JavaScript   This is an interview question asked by a major tech company  It happened two weeks back  but I m still trying to find the answer  I know we never write such code in our day-to-day job  but I m curious

User · Answer

I think this is the minimal code to implement it:

_x000D_

i=0,a={valueOf:()=>++i}_x000D_
_x000D_
if (a == 1 && a == 2 && a == 3) {_x000D_
  console.log('Mind === Blown');_x000D_
}

_x000D_

Creating a dummy object with a custom valueOf that increments a global variable i on each call. 23 characters!

User · Answer

If you ever get such an interview question  or notice some equally unexpected behavior in your code  think about what kind of things could possibly cause a behavior that looks impossible at first glance    Encoding  In this case the variable you are looking at is not the one you think it is  This can happen if you intentionally mess around with Unicode using homoglyphs or space characters to make the name of a variable look like another one  but encoding issues can also be introduced accidentally  e g  when copying  amp  pasting code from the Web that contains unexpected Unicode code points  e g  because a content management system  did some  auto-formatting  such as replacing fl with Unicode  LATIN SMALL LIGATURE FL   U FB02    Race conditions  A race-condition might occur  i e  a situation where code is not executing in the sequence expected by the developer  Race conditions often happen in multi-threaded code  but multiple threads are not a requirement for race conditions to be possible     asynchronicity is sufficient  and don t get confused  async does not mean multiple threads are used under the hood     Note that therefore JavaScript is also not free from race conditions just because it is single-threaded  See here for a simple single-threaded     but async     example  In the context of an single statement the race condition however would be rather hard to hit in JavaScript   JavaScript with web workers is a bit different  as you can have multiple threads   mehulmpt has shown us a great proof-of-concept using web workers  Side-effects   A side-effect of the equality comparison operation  which doesn t have to be as obvious as in the examples here  often side-effects are very subtle       These kind of issues can appear in many programming languages  not only JavaScript  so we aren t seeing one of the classical JavaScript WTFs here1    Of course  the interview question and the samples here all look very contrived  But they are a good reminder that    Side-effects can get really nasty and that a well-designed program should be free from unwanted side-effects  Multi-threading and mutable state can be problematic  Not doing character encoding and string processing right can lead to nasty bugs    1 For example  you can find an example in a totally different programming language  C   exhibiting a side-effect  an obvious one  here

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This is also possible using a series of self-overwriting getters    This is similar to jontro s solution  but doesn t require a counter variable     x000D   x000D        gt    x000D       use strict   x000D      Object defineProperty this   a     x000D           get        gt    x000D              Object defineProperty this   a     x000D                   get        gt    x000D                      Object defineProperty this   a     x000D                           get        gt    x000D                              return 3  x000D                            x000D                          x000D                      return 2  x000D                     x000D                  configurable  true x000D                  x000D              return 1  x000D             x000D          configurable  true x000D          x000D      if  a    1  amp  amp  a    2  amp  amp  a    3    x000D          document body append  Yes  it   s possible     x000D        x000D        x000D   x000D   x000D

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JavaScript  a    a  1  In JavaScript  there are no integers but only Numbers  which are implemented as double precision floating point numbers   It means that if a Number a is large enough  it can be considered equal to three consecutive integers    x000D   x000D  a   100000000000000000 x000D  if  a    a 1  amp  amp  a    a 2  amp  amp  a    a 3   x000D    console log  Precision loss     x000D    x000D   x000D   x000D    True  it s not exactly what the interviewer asked  it doesn t work with a 0   but it doesn t involve any trick with hidden functions or operator overloading   Other languages  For reference  there are a  1  amp  amp  a  2  amp  amp  a  3 solutions in Ruby and Python  With a slight modification  it s also possible in Java   Ruby  With a custom      class A   def    o      true   end end  a   A new  if a    1  amp  amp  a    2  amp  amp  a    3   puts  Don t do this   end   Or an increasing a   def a    a     0    a    1 end  if a    1  amp  amp  a    2  amp  amp  a    3   puts  Don t do this   end   Python  class A      def   eq   self  who cares           return True a   A    if a    1 and a    2 and a    3      print  Don t do that      Java  It s possible to modify Java Integer cache   package stackoverflow   import java lang reflect Field   public class IntegerMess       public static void main String   args  throws Exception           Field valueField   Integer class getDeclaredField  value            valueField setAccessible true           valueField setInt 1  valueField getInt 42            valueField setInt 2  valueField getInt 42            valueField setInt 3  valueField getInt 42            valueField setAccessible false            Integer a   42           if  a equals 1   amp  amp  a equals 2   amp  amp  a equals 3                 System out println  Bad idea

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If you take advantage of how    works  you could simply create an object with a custom toString  or valueOf  function that changes what it returns each time it is used such that it satisfies all three conditions    x000D   x000D  const a     x000D    i  1  x000D    toString  function      x000D      return a i    x000D      x000D    x000D   x000D  if a    1  amp  amp  a    2  amp  amp  a    3    x000D    console log  Hello World     x000D    x000D   x000D   x000D      The reason this works is due to the use of the loose equality operator  When using loose equality  if one of the operands is of a different type than the other  the engine will attempt to convert one to the other  In the case of an object on the left and a number on the right  it will attempt to convert the object to a number by first calling valueOf if it is callable  and failing that  it will call toString  I used toString in this case simply because it s what came to mind  valueOf would make more sense  If I instead returned a string from toString  the engine would have then attempted to convert the string to a number giving us the same end result  though with a slightly longer path

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By overriding valueOf in a class declaration  it can be done   class Thing       constructor             this value   1             valueOf             return this value             const a   new Thing     if a    1  amp  amp  a    2  amp  amp  a    3        console log a       What happens is that valueOf is called in each comparison operator   On the first one  a will equal 1  on the second  a will equal 2  and so on and so forth  because each time valueOf is called  the value of a is incremented   Therefore the console log will fire and output  in my terminal anyways  Thing    value  4   indicating the conditional was true

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This is possible in case of variable a being accessed by  say 2 web workers through a SharedArrayBuffer as well as some main script  The possibility is low  but it is possible that when the code is compiled to machine code  the web workers update the variable a just in time so the conditions a  1  a  2 and a  3 are satisfied   This can be an example of race condition in multi-threaded environment provided by web workers and SharedArrayBuffer in JavaScript   Here is the basic implementation of above   main js     Main Thread  const worker   new Worker  worker js   const modifiers    new Worker  modifier js    new Worker  modifier js       Let s use 2 workers const sab   new SharedArrayBuffer 1   modifiers forEach m   gt  m postMessage sab   worker postMessage sab    worker js  let array  Object defineProperty self   a       get         return array 0           addEventListener  message     data     gt        array   new Uint8Array data      let count   0     do           var res   a    1  amp  amp  a    2  amp  amp  a    3           count       while res    false     just for clarity   res is fine     console log  It happened after   count  iterations       console log  You should  ve never seen this        modifier js  addEventListener  message      data     gt        setInterval       gt            new Uint8Array data  0    Math floor Math random   3    1             On my MacBook Air  it happens after around 10 billion iterations on the first attempt     Second attempt     As I said  the chances will be low  but given enough time  it ll hit the condition   Tip  If it takes too long on your system  Try only a    1  amp  amp  a    2 and change Math random   3 to Math random   2  Adding more and more to list drops the chance of hitting

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Using Proxies   var a   new Proxy   i  0          get   target  name    gt  name     Symbol toPrimitive        gt    target i   target name       console log a    1  amp  amp  a    2  amp  amp  a    3     Proxies basically pretend to be a target object  the first parameter   but intercept operations on the target object  in this case the  get property  operation  so that there is an opportunity to do something other than the default object behavior  In this case the  get property  action is called on a when    coerces its type in order to compare it to each number  This happens    We create a target object    i  0    where the i property is our counter We create a Proxy for the target object and assign it to a For each a    comparison  a s type is coerced to a primitive value This type coercion results in calling a Symbol toPrimitive    internally The Proxy intercepts getting the a Symbol toPrimitive  function using the  get handler  The Proxy s  get handler  checks that the property being gotten is Symbol toPrimitive  in which case it increments and then returns the counter from the target object    target i  If a different property is being retrieved  we just fall back to returning the default property value  target name    So   var a           a valueOf    target i    0 a    1  amp  amp     a      target i    1 a    2  amp  amp     a      target i    2 a    3       a      target i    3   As with most of the other answers  this only works with a loose equality check       because strict equality checks       do not do type coercion that the Proxy can intercept

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It can be accomplished using the following in the global scope  For nodejs use global instead of window in the code below    x000D   x000D  var val   0  x000D  Object defineProperty window   a     x000D    get  function     x000D      return   val  x000D      x000D      x000D  if  a    1  amp  amp  a    2  amp  amp  a    3    x000D    console log  yay    x000D    x000D   x000D   x000D    This answer abuses the implicit variables provided by the global scope in the execution context by defining a getter to retrieve the variable

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I couldn t resist - the other answers are undoubtedly true  but you really can t walk past the following code    x000D   x000D  var a    1  x000D  var a   2  x000D  var  a   3  x000D  if a   1  amp  amp  a   2  amp  amp  a  3    x000D      console log  Why hello there    x000D    x000D   x000D   x000D    Note the weird spacing in the if statement  that I copied from your question   It is the half-width Hangul  that s Korean for those not familiar  which is an Unicode space character that is not interpreted by ECMA script as a space character - this means that it is a valid character for an identifier  Therefore there are three completely different variables  one with the Hangul after the a  one with it before and the last one with just a  Replacing the space with   for readability  the same code would look like this    x000D   x000D  var a    1  x000D  var a   2  x000D  var  a   3  x000D  if a   1  amp  amp  a   2  amp  amp  a  3    x000D      console log  Why hello there    x000D    x000D   x000D   x000D    Check out the validation on Mathias  variable name validator  If that weird spacing was actually included in their question  I feel sure that it s a hint for this kind of answer   Don t do this  Seriously   Edit  It has come to my attention that  although not allowed to start a variable  the Zero-width joiner and Zero-width non-joiner characters are also permitted in variable names - see Obfuscating JavaScript with zero-width characters - pros and cons    This would look like the following    x000D   x000D  var a  1  x000D  var a   2    one zero-width character x000D  var a    3    two zero-width characters  or you can use the other one  x000D  if a  1 amp  amp a   2 amp  amp a    3    x000D      console log  Why hello there    x000D    x000D   x000D   x000D

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This is an inverted version of  Jeff s answer  where a hidden character  U 115F  U 1160 or U 3164  is used to create variables that look like 1  2 and 3    x000D   x000D  var  a   1  x000D  var  1   a  x000D  var  2   a  x000D  var  3   a  x000D  console log  a    1  amp  amp  a    2  amp  amp  a    3    x000D   x000D   x000D      That answer can be simplified by using zero width non-joiner  U 200C  and zero width joiner  U 200D   Both of these characters are allowed inside identifiers but not at the beginning    x000D   x000D  var a   1  x000D  var a    2  x000D  var a    3  x000D  console log a    1  amp  amp  a     2  amp  amp  a     3   x000D   x000D        x000D  var a   1  x000D  var a u200c   2  x000D  var a u200d   3  x000D  console log a    1  amp  amp  a u200c    2  amp  amp  a u200d    3   x000D        x000D   x000D   x000D    Other tricks are possible using the same idea e g  by using Unicode variation selectors to create variables that look exactly alike  a    1  a    2  a     1  amp  amp  a     2     true

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Alternatively  you could use a class for it and an instance for the check    x000D   x000D  function A     x000D      var value   0  x000D      this valueOf   function      return   value     x000D    x000D   x000D  var a   new A  x000D   x000D  if  a    1  amp  amp  a    2  amp  amp  a    3    x000D      console log  bingo     x000D    x000D   x000D   x000D    EDIT  Using ES6 classes it would look like this   x000D   x000D  class A   x000D    constructor     x000D      this value   0  x000D      this valueOf    x000D      x000D    valueOf     x000D      return this value    x000D       x000D    x000D   x000D  let a   new A  x000D   x000D  if  a    1  amp  amp  a    2  amp  amp  a    3    x000D    console log  bingo     x000D    x000D   x000D   x000D

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Example without getters or valueOf    x000D   x000D  a    1 2 3   x000D  a join   a shift  x000D  console log a    1  amp  amp  a    2  amp  amp  a    3   x000D   x000D   x000D    This works because    invokes toString which calls  join for Arrays   Another solution  using Symbol toPrimitive which is an ES6 equivalent of toString valueOf     x000D   x000D  let i   0  x000D  let a      Symbol toPrimitive        gt    i    x000D   x000D  console log a    1  amp  amp  a    2  amp  amp  a    3   x000D   x000D   x000D

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IT IS POSSIBLE    x000D   x000D  var i   0  x000D   x000D  with   x000D    get a     x000D      return   i  x000D      x000D       x000D    if  a    1  amp  amp  a    2  amp  amp  a    3  x000D      console log  wohoo    x000D    x000D   x000D   x000D    This uses a getter inside of a with statement to let a evaluate to three different values       this still does not mean this should be used in real code     Even worse  this trick will also work with the use of        x000D   x000D    var i   0  x000D   x000D    with   x000D      get a     x000D        return   i  x000D        x000D         x000D      if  a     a  x000D        console log  yep  this is printed     x000D      x000D   x000D   x000D

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Honestly though  whether there is a way for it to evaluate to true or not  and as others have shown  there are multiple ways   the answer I d be looking for  speaking as someone who has conducted hundreds of interviews  would be something along the lines of    Well  maybe yes under some weird set of circumstances that aren t immediately obvious to me    but if I encountered this in real code then I would use common debugging techniques to figure out how and why it was doing what it was doing and then immediately refactor the code to avoid that situation    but more importantly  I would absolutely NEVER write that code in the first place because that is the very definition of convoluted code  and I strive to never write convoluted code    I guess some interviewers would take offense to having what is obviously meant to be a very tricky question called out  but I don t mind developers who have an opinion  especially when they can back it up with reasoned thought and can dovetail my question into a meaningful statement about themselves

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Yes  it is possible       JavaScript   x000D   x000D  if      gt  0  x000D  var a   9  x000D   x000D  if  a  1  amp  amp  a   2  amp  amp  a  3  x000D    x000D      document write   lt h1 gt Yes  it is possible  lt  h1 gt    x000D    x000D   x000D   x000D    The above code is a short version  thanks to   Forivin for its note in comments  and the following code is original    x000D   x000D  var a   9  x000D   x000D  if  a  1  amp  amp  a   2  amp  amp  a  3  x000D    x000D        console log  Yes  it is possible    x000D      document write   lt h1 gt Yes  it is possible  lt  h1 gt    x000D    x000D   x000D    -------------------------------------------- x000D   x000D  function if    return true   x000D   x000D   x000D       If you just see top side of my code and run it you say WOW  how       So I think it is enough to say Yes  it is possible to someone that said to   you  Nothing is impossible      Trick  I used a hidden character after if to make a function that its name is similar to if  In JavaScript we can not override keywords so I forced to use this way  It is a fake if  but it works for you in this case         C   Also I wrote a C  version  with increase property value technic    static int  a  public static int a   gt     a   public static void Main         if a  1  amp  amp  a  2  amp  amp  a  3                Console WriteLine  Yes  it is possible               Live Demo

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Same  but different  but still same  can be  tested  multiple times     x000D   x000D  const a     valueOf       gt  this n    this n    0    3   1  x000D       x000D  if a    1  amp  amp  a    2  amp  amp  a    3    x000D    console log  Hello World     x000D    x000D   x000D  if a    1  amp  amp  a    2  amp  amp  a    3    x000D    console log  Hello World     x000D    x000D   x000D   x000D    My idea started from how Number object type equation works

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I don t see this answer already posted  so I ll throw this one into the mix too  This is similar to Jeff s answer with the half-width Hangul space    x000D   x000D  var a   1  x000D  var a   2  x000D  var     3  x000D  if a    1  amp  amp  a    2  amp  amp       3    x000D      console log  Why hello there    x000D    x000D   x000D   x000D    You might notice a slight discrepancy with the second one  but the first and third are identical to the naked eye  All 3 are distinct characters   a - Latin lower case A a - Full Width Latin lower case A   - Cyrillic lower case A  The generic term for this is  homoglyphs   different unicode characters that look the same  Typically hard to get three that are utterly indistinguishable  but in some cases you can get lucky  A        and   would work better  Latin-A  Greek Alpha  Cyrillic-A  and Cherokee-A respectively  unfortunately the Greek and Cherokee lower-case letters are too different from the Latin a  a    and so doesn t help with the above snippet    There s an entire class of Homoglyph Attacks out there  most commonly in fake domain names  eg  wikipedi  org  Cyrillic  vs wikipedia org  Latin    but it can show up in code as well  typically referred to as being underhanded  as mentioned in a comment   underhanded  questions are now off-topic on PPCG  but used to be a type of challenge where these sorts of things would show up    I used this website to find the homoglyphs used for this answer

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When you can t do anything without regular expressions    x000D   x000D  var a     x000D    r    d g   x000D    valueOf  function    x000D      return this r exec 123  0  x000D      x000D    x000D   x000D  if  a    1  amp  amp  a    2  amp  amp  a    3    x000D      console log      x000D    x000D   x000D   x000D    It works because of custom valueOf method that is called when Object compared with primitive  such as Number   Main trick is that a valueOf returns new value every time because it s calling exec on regular expression with g flag  which causing updating lastIndex of that regular expression every time match is found  So first time this r lastIndex    0  it matches 1 and updates lastIndex  this r lastIndex    1  so next time regex will match 2 and so on

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Actually the answer to the first part of the question is  Yes  in every programming language  For example  this is in the case of C C      define a    b    int b   1  if  a   1  amp  amp  a   2  amp  amp  a  3        std  cout  lt  lt   Yes  it s possible    lt  lt  std  endl    else       std  cout  lt  lt   it s impossible    lt  lt  std  endl

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Okay  another hack with generators    x000D   x000D  const value   function       x000D    let i   0  x000D    while true  yield   i  x000D       x000D   x000D  Object defineProperty this   a     x000D    get     x000D      return value next   value  x000D      x000D      x000D   x000D  if  a     1  amp  amp  a     2  amp  amp  a     3    x000D    console log  yo     x000D    x000D   x000D   x000D

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An ECMAScript nbsp 6 answer that makes use of Symbols   const a    value  1   a Symbol toPrimitive    function     return this value      console log  a    1  amp  amp  a    2  amp  amp  a    3      Due to    usage  JavaScript is supposed to coerce a into something close to the second operand  1  2  3 in this case   But before JavaScript tries to figure coercing on its own  it tries to call Symbol toPrimitive  If you provide Symbol toPrimitive JavaScript would use the value your function returns  If not  JavaScript would call valueOf

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Here s another variation  using an array to pop off whatever values you want    x000D   x000D  const a     x000D    n   3 2 1   x000D    toString  function      x000D      return a n pop    x000D      x000D    x000D   x000D  if a    1  amp  amp  a    2  amp  amp  a    3    x000D    console log  Yes    x000D    x000D   x000D   x000D

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Rule number one of interviews  never say impossible   No need for hidden character trickery    x000D   x000D  window   defineGetter     a   function    x000D      if  typeof i      number     x000D             define i in the global namespace so that it s not lost after this function runs x000D          i   0  x000D        x000D      return   i  x000D      x000D   x000D  if  a    1  amp  amp  a    2  amp  amp  a    3    x000D      alert   Oh dear  what have we done      x000D    x000D   x000D   x000D

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If it is asked if it is possible  not MUST   it can ask  a  to return a random number  It would be true if it generates 1  2  and 3 sequentially    x000D   x000D  with   x000D    get a     x000D      return Math floor Math random   4   x000D      x000D      x000D    for var i 0 i lt 1000 i     x000D      if  a    1  amp  amp  a    2  amp  amp  a    3   x000D        console log  after      i 1      trials  it becomes true finally       x000D        break  x000D        x000D      x000D    x000D   x000D   x000D

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This one uses the defineProperty with a nice side-effect causing global variable    x000D   x000D  var  a   1 x000D   x000D  Object defineProperty this   a     x000D     get        gt    x000D      return  a    x000D       x000D    configurable  true x000D      x000D   x000D  console log a  x000D  console log a  x000D  console log a  x000D   x000D   x000D

[javascript] Can (a== 1 && a ==2 && a==3) ever evaluate to true?

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