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Why won't my JavaScript account system code work or run? [duplicate]

This question already has answers here:
How do I check if an array includes a value in JavaScript?
(60 answers)
Closed 6 years ago.
Is there an easier way to determine if a variable is equal to a range of values, such as:
if x === 5 || 6
rather than something obtuse like:
if x === 5 || x === 6
?

You can stash your values inside an array and check whether the variable exists in the array by using [].indexOf:
if([5, 6].indexOf(x) > -1) {
// ...
}
If -1 is returned then the variable doesn't exist in the array.

Depends on what sort of test you're performing. If you've got static strings, this is very easy to check via regular expressions:
if (/^[56ab]$/.test(item)) {
//-or-
if (/^(foo|bar|baz|fizz|buzz)$/.test(item)) {
doStuff();
} else {
doOtherStuff();
}
If you've got a small set of values (string or number), you can use a switch:
switch (item) {
case 1:
case 2:
case 3:
doStuff();
break;
default:
doOtherStuff();
break;
}
If you've got a long list of values, you should probably use an array with ~arr.indexOf(item), or arr.contains(item):
vals = [1,3,18,3902,...];
if (~vals.indexOf(item)) {
doStuff();
} else {
doOtherStuff();
}
Unfortunately Array.prototype.indexOf isn't supported in some browsers. Fortunately a polyfill is available. If you're going through the trouble of polyfilling Array.prototype.indexOf, you might as well add Array.prototype.contains.
Depending on how you're associating data, you could store a dynamic list of strings within an object as a map to other relevant information:
var map = {
foo: bar,
fizz: buzz
}
if (item in map) {
//-or-
if (map.hasOwnProperty(item)) {
doStuff(map[item]);
} else {
doOtherStuff();
}
in will check the entire prototype chain while Object.prototype.hasOwnProperty will only check the object, so be aware that they are different.

It's perfectly fine. If you have a longer list of values, perhaps you can use the following instead:
if ([5,6,7,8].indexOf(x) > -1) {
}

Yes. You can use your own function. This example uses .some:
var foo = [ 5, 6 ].some(function(val) {
return val === x;
});
foo; // true

This is what I've decided to use:
Object.prototype.isin = function() {
for(var i = arguments.length; i--;) {
var a = arguments[i];
if(a.constructor === Array) {
for(var j = a.length; j--;)
if(a[j] == this) return true;
}
else if(a == this) return true;
}
return false;
}
You would use it like this:
var fav = 'pear',
fruit = ['apple', 'banana', 'orange', 'pear'],
plu = [4152, 4231, 3030, 4409];
if (fav.isin(fruit, plu, 'eggs', 'cheese')) {
//do something cool
}
The advantages are:
it works in IE < 9;
it reads naturally from left to right;
you can feed it arrays or separate values.
If you don't want to allow type coercion (indexOf does not), change the two == to ===. As it stands:
fav = "4231";
plu.indexOf(fav) //-1
fav.isin(plu) //true

no, there might be a few tricks that are case specific but in general i write code like this:
if (someVariable === 1 ||
someVariable === 2 ||
someVariable === 7 ||
someVariable === 12 ||
someVariable === 14 ||
someVariable === 19) {
doStuff();
moreStuff();
} else {
differentStuff();
}

The simple answer is no. You can use a switch statement, which is easier to read if you are comparing a lot of string values, but using it for two values wouldn't look any better.

[Edit] this seems to work, but as Dan pointed out, it is actually a false positive. Do not use this method. I leave it here for educational purposes.
Easiest way I know :
a = [1,2,3,4,5];
if(3 in a) alert("true"); // will alert true
Tested in Chrome console. Not sure if it works in other browsers.

Deeply equal function error in Javascript

I am trying to create a function to check whether two arrays are deeply equal to each other.
An example would be: [1, 2, { a: "hello" }] and [1, 2, { a: "bye" }] would return false.
This is my code so far:
const deeplyEquals = (val1, val2) => {
let counter = 0;
for (var i = 0; i < val1.length; i++) {
if (typeof val1[i] === "object") {
deeplyEquals(JSON.stringify(val1[i]), JSON.stringify(val2[i]));
} else if (typeof val2[i] === "object") {
deeplyEquals(JSON.stringify(val1[i]), JSON.stringify(val2[i]));
} else if (val1[i] !== val2[i]) {
counter++;
}
}
return counter === 0 ? true : false;
};
I implemented a counter so that if it found a value in 1 that was not equal to the same value in 2 then it would increment. If the counter was not 0 then it would return false.
For the example, the counter increments to 7 but then right at the end, changes to 0 and therefore returns true instead of false.
I'm sure there would be an easier way to do this but I was wanting to see whether I could make this work as I am unsure why the counter is changing to 0 right at the end.
Thanks for any help!

The problem is that the counter is local to each call to deeplyEquals. There's a different counter for each call, and since you're making the calls recursively, you have lots of different counter variables in memory at the same time.
If you wanted to maintain a counter, you'd have to have each recursive call return the counter value (instead of a flag) so the code calling it could increment its counter by that much.
But there's no need in your code. Instead, just return false the first time you find a difference, either during in the call itself or in one of its recursive calls by checking the return value of the recursive call.
There are other issues with the code. Here's what I notice off-the-cuff:
You're calling JSON.stringify, which returns a string, before passing values to deeplyEquals, which will convert arrays and objects to strings. Comparing the strings won't be reliable (because equivalent objects can have their properties in different orders: JSON.stringify({a:1,b:2}) is the string {"a":1,"b":2}, but JSON.stringify({b:2,a:1}) is the string {"b":2,"a":1}). Instead, pass the actual value.
typeof x returns "object" for arrays and null as well as non-array objects; you need to handle those three cases separately.
When comparing non-array objects, you need to loop through their properties to compare them.
SO has several questions and answers about doing deep equality checks; probably best to search for those, study them to ensure you understand how they work, and go from there.

Why dont you just JSON stringify both arrays and then compare?
const deepEquals(val1, val2){
let v1 = JSON.stringify(val1);
let v2 = JSON.stringify(val2);
return v1 === v2;
}

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

Want to improve this post? Provide detailed answers to this question, including citations and an explanation of why your answer is correct. Answers without enough detail may be edited or deleted.
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 && 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.

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.
const a = {
i: 1,
toString: function () {
return a.i++;
}
}
if(a == 1 && a == 2 && a == 3) {
console.log('Hello World!');
}
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.

I couldn't resist - the other answers are undoubtedly true, but you really can't walk past the following code:
var aﾠ = 1;
var a = 2;
var ﾠa = 3;
if(aﾠ==1 && a== 2 &&ﾠa==3) {
console.log("Why hello there!")
}
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:
var a_ = 1;
var a = 2;
var _a = 3;
if(a_==1 && a== 2 &&_a==3) {
console.log("Why hello there!")
}
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:
var a= 1;
var a‍= 2; //one zero-width character
var a‍‍= 3; //two zero-width characters (or you can use the other one)
if(a==1&&a‍==2&&a‍‍==3) {
console.log("Why hello there!")
}

IT IS POSSIBLE!
var i = 0;
with({
get a() {
return ++i;
}
}) {
if (a == 1 && a == 2 && a == 3)
console.log("wohoo");
}
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 ===.
var i = 0;
with({
get a() {
return ++i;
}
}) {
if (a !== a)
console.log("yep, this is printed.");
}

Example without getters or valueOf:
a = [1,2,3];
a.join = a.shift;
console.log(a == 1 && a == 2 && a == 3);
This works because == invokes toString which calls .join for Arrays.
Another solution, using Symbol.toPrimitive which is an ES6 equivalent of toString/valueOf:
let i = 0;
let a = { [Symbol.toPrimitive]: () => ++i };
console.log(a == 1 && a == 2 && a == 3);

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.
with({
get a() {
return Math.floor(Math.random()*4);
}
}){
for(var i=0;i<1000;i++){
if (a == 1 && a == 2 && a == 3){
console.log("after " + (i+1) + " trials, it becomes true finally!!!");
break;
}
}
}

When you can't do anything without regular expressions:
var a = {
r: /\d/g,
valueOf: function(){
return this.r.exec(123)[0]
}
}
if (a == 1 && a == 2 && a == 3) {
console.log("!")
}
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.

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

It can be accomplished using the following in the global scope. For nodejs use global instead of window in the code below.
var val = 0;
Object.defineProperty(window, 'a', {
get: function() {
return ++val;
}
});
if (a == 1 && a == 2 && a == 3) {
console.log('yay');
}
This answer abuses the implicit variables provided by the global scope in the execution context by defining a getter to retrieve the variable.

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.)
(() => {
"use strict";
Object.defineProperty(this, "a", {
"get": () => {
Object.defineProperty(this, "a", {
"get": () => {
Object.defineProperty(this, "a", {
"get": () => {
return 3;
}
});
return 2;
},
configurable: true
});
return 1;
},
configurable: true
});
if (a == 1 && a == 2 && a == 3) {
document.body.append("Yes, it’s possible.");
}
})();

Alternatively, you could use a class for it and an instance for the check.
function A() {
var value = 0;
this.valueOf = function () { return ++value; };
}
var a = new A;
if (a == 1 && a == 2 && a == 3) {
console.log('bingo!');
}
EDIT
Using ES6 classes it would look like this
class A {
constructor() {
this.value = 0;
this.valueOf();
}
valueOf() {
return this.value++;
};
}
let a = new A;
if (a == 1 && a == 2 && a == 3) {
console.log('bingo!');
}

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.
var a = 1;
var ａ = 2;
var а = 3;
if(a == 1 && ａ == 2 && а == 3) {
console.log("Why hello there!")
}
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
а - 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, Α, А, and Ꭺ 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: α,ꭺ, 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. wikipediа.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.

Yes, it is possible! 😎
» JavaScript
if‌=()=>!0;
var a = 9;
if‌(a==1 && a== 2 && a==3)
{
document.write("<h1>Yes, it is possible!😎</h1>")
}
The above code is a short version (thanks to #Forivin for its note in comments) and the following code is original:
var a = 9;
if‌(a==1 && a== 2 && a==3)
{
//console.log("Yes, it is possible!😎")
document.write("<h1>Yes, it is possible!😎</h1>")
}
//--------------------------------------------
function if‌(){return true;}
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

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 four consecutive integers:
a = 100000000000000000
if (a == a+1 && a == a+2 && a == a+3){
console.log("Precision loss!");
}
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
You can either define == for a new class:
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!")
or, if you're feeling adventurous, redefine the values of integers:
import ctypes
def deref(addr, typ):
return ctypes.cast(addr, ctypes.POINTER(typ))
deref(id(2), ctypes.c_int)[6] = 1
deref(id(3), ctypes.c_int)[6] = 1
deref(id(4), ctypes.c_int)[6] = 1
print(1 == 2 == 3 == 4)
# True
It might segfault, depending on your system/interpreter.
The python console crashes with the above code, because 2 or 3 are probably used in the background. It works fine if you use less-common integers:
>>> import ctypes
>>>
>>> def deref(addr, typ):
... return ctypes.cast(addr, ctypes.POINTER(typ))
...
>>> deref(id(12), ctypes.c_int)[6] = 11
>>> deref(id(13), ctypes.c_int)[6] = 11
>>> deref(id(14), ctypes.c_int)[6] = 11
>>>
>>> print(11 == 12 == 13 == 14)
True
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.");
}
}
}

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.
var a = 1;
var ﾠ1 = a;
var ﾠ2 = a;
var ﾠ3 = a;
console.log( a ==ﾠ1 && a ==ﾠ2 && a ==ﾠ3 );
* 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:
var a = 1;
var a‌ = 2;
var a‍ = 3;
console.log(a == 1 && a‌ == 2 && a‍ == 3);
/****
var a = 1;
var a\u200c = 2;
var a\u200d = 3;
console.log(a == 1 && a\u200c == 2 && a\u200d == 3);
****/
Other tricks are possible using the same idea e.g. by using Unicode variation selectors to create variables that look exactly alike (a︀ = 1; a︁ = 2; a︀ == 1 && a︁ == 2; // true).

Rule number one of interviews; never say impossible.
No need for hidden character trickery.
window.__defineGetter__( 'a', function(){
if( typeof i !== 'number' ){
// define i in the global namespace so that it's not lost after this function runs
i = 0;
}
return ++i;
});
if( a == 1 && a == 2 && a == 3 ){
console.log( 'Oh dear, what have we done?' );
}

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.

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 & 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.

Here's another variation, using an array to pop off whatever values you want.
const a = {
n: [3,2,1],
toString: function () {
return a.n.pop();
}
}
if(a == 1 && a == 2 && a == 3) {
console.log('Yes');
}

Okay, another hack with generators:
const value = function* () {
let i = 0;
while(true) yield ++i;
}();
Object.defineProperty(this, 'a', {
get() {
return value.next().value;
}
});
if (a === 1 && a === 2 && a === 3) {
console.log('yo!');
}

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:
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 && // 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.

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;
}

Same, but different, but still same (can be "tested" multiple times):
const a = { valueOf: () => this.n = (this.n || 0) % 3 + 1}
if(a == 1 && a == 2 && a == 3) {
console.log('Hello World!');
}
if(a == 1 && a == 2 && a == 3) {
console.log('Hello World!');
}
My idea started from how Number object type equation works.

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 think this is the minimal code to implement it:
i=0,a={valueOf:()=>++i}
if (a == 1 && a == 2 && a == 3) {
console.log('Mind === Blown');
}
Creating a dummy object with a custom valueOf that increments a global variable i on each call. 23 characters!

This one uses the defineProperty with a nice side-effect causing global variable!
var _a = 1
Object.defineProperty(this, "a", {
"get": () => {
return _a++;
},
configurable: true
});
console.log(a)
console.log(a)
console.log(a)

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.

As we already know that the secret of loose equality operator (==) will try to convert both values to a common type. As a result, some functions will be invoked.
ToPrimitive(A) attempts to convert its object argument to a primitive
value, by invoking varying sequences of A.toString and A.valueOf
methods on A.
So as other answers using Symbol.toPrimitive, .toString, .valueOf from integer. I would suggest the solution using an array with Array.pop like this.
let a = { array: [3, 2, 1], toString: () => a.array.pop() };
if(a == 1 && a == 2 && a == 3) {
console.log('Hello World!');
}
In this way, we can work with text like this
let a = { array: ["World", "Hello"], toString: () => a.array.pop() };
if(a == "Hello" && a == "World") {
console.log('Hello World!');
}

Surprisingly, yes. The == loose equality operator in JS calls the valueOf() method of the object that's being compared. Therefore, you can create a class that returns an internal value, then increments that interval value every time it's called. Like this:
class AClass {
constructor(initalVal) {
this.val = initalVal;
}
valueOf() {
return this.val++;
}
}
const a = new AClass(1);
console.log(a==1 && a==2 && a==3)
I know that there are a lot of other answers to this question, but this is how you'd do it with ES6 syntax.
Note: If you don't want this to happen, then you should use the === operator to check for strict instead. Like this:
class AClass {
constructor(initalVal) {
this.val = initalVal;
}
valueOf() {
return this.val++;
}
}
const a = new AClass(1);
console.log(a===1 && a===2 && a===3)

Yes, you can Do that, see the following JavaScript code:
let a = 0 // Create a variable and give it a value
if( a !== 1 && a !== 2 && a !== 3 )
{
console.log("true")
}
Explanation of the solution:
Simply , we add the not equal sign
before the == sign so that we tell the language that these values are
not equal to the value in the variable

Create instance methods in JS that apply function with a dot [duplicate]

I want to extend the number class to have instance functions such as odd and even so I can do something like this:
2.odd() => false
2.even() => true
1.even() => false
1.odd() => true
Extending classes is a good Ruby practise: "Ruby check if even number, float".
Is the same true in JavaScript, or does it cause performance issues or some other problem?
Anyway, I can't extend despite my best efforts:
var NumberInstanceExtensions = {
accuracy: function(){
return 'This is cool ' + this
}
}
$.extend(Number.prototype,NumberInstanceExtensions);
alert( $.type(5) ); //-> number
//alert( 5.accuracy() ); //-> Uncaught SyntaxError: Unexpected token ILLEGAL
http://jsfiddle.net/VLPTb/2/
How can I get this to work? The syntax error makes me think this isn't how JavaScript works on a fundamental level. Is my best bet extending the Math class and doing this instead:
Math.odd(2) => false
Math.even(2) => true
Math.even(1) => false
Math.odd(1) => true
That seems far more inelegant than 2.odd().

I think as long as you understand the side-effects of your "extension" then you're okay. I often modify the String prototype to add an "elipsis" method so I can do things like
"SomeString".elipsis()
But start at the beginning. You're not "extending classes" in JavaScript. JavaScript is a prototype-based language. You can modify prototypes to do what you need.
You won't be able to add a method directly to the number itself. You can, however modify the prototype of the Number object:
Number.prototype.even = function(){
return this.valueOf() % 2 === 0;
}
With this, you won't be able to use the following syntax:
10.even();
But, since you aren't hard-coding stuff, otherwise you wouldn't need this function anyways, you CAN do the following:
var a = 10;
a.even(); //true
I might say that you could consider adding a utilities object to do these things, because modifying primitive prototypes is not always guaranteed to be side-effect free.
This function does not really provide any gain for you. You're checking for odd and even, replacing one line of code with another. Think about the difference:
var a = 10;
var aIsEven = a.even();
vs:
var a = 10;
var aIsEven = a % 2 === 0;
You gain three characters of code, and the second option is less likely to break your "JavaScript".

You can extend natives JS objects by using (for example) Number.prototype.myFn = function(){}.
So you could do :
Math.prototype.odd = function(n){
return n % 2 === 0;
};
Math.prototype.even = function(n){
return n % 2 === 1;
};
And then use it like so :
var two = 2;
console.log(Math.odd(2)); // true
BUT I would strongly advise you against extending natives in JavaScript.
You can read more about it here
EDIT : After trying my code on JSFiddle, it appears the Math object has no prototype, you can read more about it here. The code above won't work !
Instead, you could do :
Math.odd = function(n){
return n % 2 === 0;
};
Math.even = function(n){
return n % 2 === 1;
};
console.log(Math.odd(2)); // true
or :
Number.prototype.odd = function(){
return this % 2 === 0;
};
Number.prototype.even = function(){
return this % 2 === 1;
};
console.log(new Number(2).odd()); // true

I'd like to point out that that is already available in the numbers class.
Just use the boolean methods, odd? and even?
2.odd?
=> false
2.even?
=> true
Hope this helps.
No need to create a new class, it already exists in the numbers class.

JavaScript conditional - how to test if a variable matches any of a list of values?

Does JavaScript have a convenient way to test if a variable matches one of many values?
This is my code,
function Start()
{
if(number==(0||3||6||8||9||11||13||14||15||18||19||22||23||25||27||28||31||34||43||46||47||49||54||58||59||62||63||68||71||74||75))
{
FirstFunction();
}
if(number==(1||4||5||7||12||16||17||20||21||26||29||32||33||42||45||48||50||51||53||55||56||57||60||61||64||65||67||69||70||73||76))
{
SecondFunction();
}
}
as you can see, I tried to use the "or" operator to check if number equals ANY of the listed. this, unfortunately, did not work. I know I can just code:
if(number==0||number==3||number==6....)
I think there should be an alternative to that, is there?
Thank you in advance.

You should insert all your elements in an array and use arr.indexOf(element)
It will return -1 if the element doesn't exist which you can use for your if logic
This is better than having lot of if statements
var x = new Array(1,7,15,18);
if ( x.indexOf(31) != -1 )
{
// Add your logic here
}

You can write something like this, which looks a bit nicer:
This Array prototype function will allow you check if an element exists in a JS array:
Array.prototype.exists = function (x) {
for (var i = 0; i < this.length; i++) {
if (this[i] == x) return true;
}
return false;
}
Then:
function Start()
{
var values1 =[0,3,6,8,9,11,13,14,15,18,19,22,23,25,27,28,31,34,43,46,47,49,54,58,59,62,63,68,71,74,75];
var values2 = [1,4,5,7,12,16,17,20,21,26,29,32,33,42,45,48,50,51,53,55,56,57,60,61,64,65,67,69,70,73,76];
if( values1.exists(number) )
{
FirstFunction();
} else if ( values2.exists(number) )
{
SecondFunction();
}
}

The array techniques already mentioned are good, e.g., [0, 3, 6, 8].indexOf(number) != -1, but note that not all browsers support .indexOf() on arrays (think older IE). If you have a look at the MDN page on .indexOf() you'll see they've provided an implementation of .indexOf() that you can add to the Array.prototype if it doesn't already exist.
But here's a non-array method that will work in older browsers at least as far back as IE6 without needing to add your own functions or modify the prototype of any built-in objects:
if (/^(0|3|6|8|9|11)$/.test(number)) {
// matched, so do something
}
The regex .text() method is expecting a string, but if you give it a number it'll cope.
I'd probably still recommend the array method, but it can't hurt to have another option.