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

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Is it ever possible that (a== 1 && a ==2 && 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.

This question is related to javascript ecmascript-6

The answer is


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_
(() => {_x000D_
    "use strict";_x000D_
    Object.defineProperty(this, "a", {_x000D_
        "get": () => {_x000D_
            Object.defineProperty(this, "a", {_x000D_
                "get": () => {_x000D_
                    Object.defineProperty(this, "a", {_x000D_
                        "get": () => {_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 && a == 2 && a == 3) {_x000D_
        document.body.append("Yes, it’s possible.");_x000D_
    }_x000D_
})();
_x000D_
_x000D_
_x000D_


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 && a == 2 && 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.


Using Proxies:

var a = new Proxy({ i: 0 }, {
    get: (target, name) => name === Symbol.toPrimitive ? () => ++target.i : target[name],
});
console.log(a == 1 && a == 2 && 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:

  1. We create a target object, { i: 0 }, where the i property is our counter
  2. We create a Proxy for the target object and assign it to a
  3. For each a == comparison, a's type is coerced to a primitive value
  4. This type coercion results in calling a[Symbol.toPrimitive]() internally
  5. The Proxy intercepts getting the a[Symbol.toPrimitive] function using the "get handler"
  6. 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 && // a == ++target.i == 1
a == 2 && // 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.


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 && a ==?2 && 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 && a? == 2 && a? == 3);_x000D_
_x000D_
/****_x000D_
var a = 1;_x000D_
var a\u200c = 2;_x000D_
var a\u200d = 3;_x000D_
console.log(a == 1 && a\u200c == 2 && 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 && a? == 2; // true).


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 && a == 2 && 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 && a == 2 && a == 3) {_x000D_
  console.log('bingo!');_x000D_
}
_x000D_
_x000D_
_x000D_


Yes, it is possible!

» JavaScript

_x000D_
_x000D_
if?=()=>!0;_x000D_
var a = 9;_x000D_
_x000D_
if?(a==1 && a== 2 && a==3)_x000D_
{_x000D_
    document.write("<h1>Yes, it is possible!</h1>")_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 && a== 2 && a==3)_x000D_
{_x000D_
    //console.log("Yes, it is possible!")_x000D_
    document.write("<h1>Yes, it is possible!</h1>")_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 => ++_a;

public static void Main()
{
    if(a==1 && a==2 && a==3)
    {
        Console.WriteLine("Yes, it is possible!");
    }
}

Live Demo


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:

  1. 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 & 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)).

  2. 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.

  3. 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.


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 && a == 2 && 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.


Same, but different, but still same (can be "tested" multiple times):

_x000D_
_x000D_
const a = { valueOf: () => this.n = (this.n || 0) % 3 + 1}_x000D_
    _x000D_
if(a == 1 && a == 2 && a == 3) {_x000D_
  console.log('Hello World!');_x000D_
}_x000D_
_x000D_
if(a == 1 && a == 2 && a == 3) {_x000D_
  console.log('Hello World!');_x000D_
}
_x000D_
_x000D_
_x000D_

My idea started from how Number object type equation works.


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 && a == 2 && a == 3) {_x000D_
  console.log('Yes');_x000D_
}
_x000D_
_x000D_
_x000D_


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 && a == 2 && a == 3 ){_x000D_
    alert( 'Oh dear, what have we done?' );_x000D_
}
_x000D_
_x000D_
_x000D_


An ECMAScript 6 answer that makes use of Symbols:

const a = {value: 1};
a[Symbol.toPrimitive] = function() { return this.value++ };
console.log((a == 1 && a == 2 && 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.


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 && a== 2 &&?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 && a== 2 &&_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&&a?==2&&a??==3) {_x000D_
    console.log("Why hello there!")_x000D_
}
_x000D_
_x000D_
_x000D_


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 => m.postMessage(sab))
worker.postMessage(sab)

worker.js

let array

Object.defineProperty(self, 'a', {
  get() {
    return array[0]
  }
});

addEventListener('message', ({data}) => {
    array = new Uint8Array(data)
    let count = 0
    do {
        var res = a == 1 && a == 2 && 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}) => {
    setInterval( () => {
        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:

enter image description here

Second attempt:

enter image description here

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 && a == 2 and change Math.random()*3 to Math.random()*2. Adding more and more to list drops the chance of hitting.


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<1000;i++){_x000D_
    if (a == 1 && a == 2 && a == 3){_x000D_
      console.log("after " + (i+1) + " trials, it becomes true finally!!!");_x000D_
      break;_x000D_
    }_x000D_
  }_x000D_
}
_x000D_
_x000D_
_x000D_


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 && a == a+2 && 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 && a==2 && 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 && a == 2 && a == 3
  puts "Don't do this!"
end

Or an increasing a:

def a
  @a ||= 0
  @a += 1
end

if a == 1 && a == 2 && 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) && a.equals(2) && a.equals(3)) {
            System.out.println("Bad idea.");
        }
    }
}

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 && a == 2 && 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.


I think this is the minimal code to implement it:

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

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


Example without getters or valueOf:

_x000D_
_x000D_
a = [1,2,3];_x000D_
a.join = a.shift;_x000D_
console.log(a == 1 && a == 2 && 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]: () => ++i };_x000D_
_x000D_
console.log(a == 1 && a == 2 && a == 3);
_x000D_
_x000D_
_x000D_


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 && a == 2 && ? == 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.


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.


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": () => {_x000D_
    return _a++;_x000D_
  },_x000D_
  configurable: true_x000D_
});_x000D_
_x000D_
console.log(a)_x000D_
console.log(a)_x000D_
console.log(a)
_x000D_
_x000D_
_x000D_


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 && a== 2 && a==3) {
    std::cout << "Yes, it's possible!" << std::endl;
} else {
    std::cout << "it's impossible!" << std::endl;
}

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 && a === 2 && a === 3) {_x000D_
  console.log('yo!');_x000D_
}
_x000D_
_x000D_
_x000D_


IT IS POSSIBLE!

_x000D_
_x000D_
var i = 0;_x000D_
_x000D_
with({_x000D_
  get a() {_x000D_
    return ++i;_x000D_
  }_x000D_
}) {_x000D_
  if (a == 1 && a == 2 && 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_