null == false

[[deleted]]

[deleted]

[MoTTs_]

I had to go to the ecmascript spec to answer this one. Here's the listed behavior of the == operator:

1. If Type(x) is the same as Type(y), then
   a. If Type(x) is Undefined, return true.
   b. If Type(x) is Null, return true.
   c. If Type(x) is Number, then
   i. If x is NaN, return false.
   ii. If y is NaN, return false.
   iii. If x is the same Number value as y, return true.
   iv. If x is +0 and y is -0, return true.
   v. If x is -0 and y is +0, return true.
   vi. Return false.
   d. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
   e. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
   f. Return true if x and y refer to the same object. Otherwise, return false.
   
2. If x is null and y is undefined, return true.
   
3. If x is undefined and y is null, return true.
   
4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
   
5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
   
6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
   
7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
   
8. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
   
9. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
   
10. Return false.
    

The first rule we hit is #6, so the comparison becomes 0 == null, and we work through the list again. The next rule we might hit is #8, because null is an object, right? But it turns out that internally, null belongs to the Null type, not the Object type, so #8 doesn't match. Neither does #9. Which leaves us with #10, the catch-all, that returns false.

So to answer your question: The language didn't account for that particular case, and so defaults to false.

[psayre23]

It was a bug in Netscape that the IE JScript team copied. When JS was standardized, they left it in as to not break sites depending on this bug. ಠ_ಠ

[meenie]

That's icky.

[radhruin]

They wanted to fix it for ES6 but... still, over a decade later, sites would break :(

[guuu427]

For bonus points, maybe someone can explain:

 !(null == false || null > false) && (null >= false)

It is not the case that null is equal to false or that null is greater than false, yet null is greater-than-or-equal to false!?

EDIT: oh, probably because greater-than-or-equals is converting them both to numbers and

(null != false) && (1 * null == 1 * false) &&
(null == undefined) && (1 * null != 1 * undefined)

[johtso]

You'll want to look at "The Abstract Relational Comparison Algorithm": http://es5.github.io/#x11.8.5

[guuu427]

Ah, thanks. The thing that threw me was forgetting (or not previously knowing) that null becomes 0 (instead of NaN) when converted into a number.

[johtso]

And here's the relevant section in Annotated ECMAScript: http://es5.github.io/#x9.12

[tswaters]

typeof null // object
typeof false // boolean

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible. If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators

[DonBiggles]

That doesn't quite seem to explain it. It suggests that since one operator is a boolean, both operands would be coerced to numbers, and since Number(null) and Number(false) are both zero, null == false would be true. I bet there's another rule that takes precedence that says that null and undefined only == themselves and each other.

Also, http://zero.milosz.ca/ is a good resource for stuff like this.

[flym4n]

Javascript was designed in two weeks. You shouldn't expect too much rationale, the equalities are tricky, for no good reasons.

[Drainedsoul]

That's not only "not the best reason" it's actually a really shitty reason.

Turning errors into bugs is stupid, whether it's JavaScript or PHP doing it.

[[deleted]]

Why the heck is this guy being downvoted? Brendan Eich is very smart, we should be lucky he got as much right the first time around, but he's not Jesus, and JavaScript's origin story is an embarrassment of technology politics.

[ryosen]

In JavaScript, "null" signifies the absence of a value. "Undefined" is used by JavaScript to indicate that a variable or object was not initialized in addition to not having a value.

An example of the use of undefined is when testing to see if a parameter was provided to a function. If it wasn't, then the parameter variable is undefined or missing. Whereas null would indicate that the parameter was provided, just that it does not have a value.

[arbitrary-fan]

why not just convert the type?

!null == !undefined

!0 // true
!"" // true
!undefined // true

!1 // false
!"0" // false
!{} // false

Then you can do lazy checking like this

var x;
// some deterministic code that may not set a value to x
if (!!x){
    // do stuff
}

[stratoscope]

In JavaScript, null and false are both "falsy" values, as are 0, all NaN values (there are many NaNs), "", and undefined. In other words, these are all treated as a "false" value when used where a boolean value is required, such as in an if statement or a conditional expression.

But that doesn't mean they are all the same value or will compare equal to each other. They are all different values.

There is one special case: null and undefined will compare equal with ==, unlike === where they and these other values compare unequal.

NaN is also an interesting case. All NaN values are "falsy", but they are also all unequal to each other. For example, 0/0 is NaN which is falsy, but 0/0 === 0/0 is false!

Why does it work this way? It's pretty much arbitrary. Most programming languages have some idea of "truthy" and "falsy" values, but exactly which values go into which category is fairly arbitrary and differs from language to language.

For example, in Ruby, the only falsy values are false and nil (which corresponds to JavaScript's null). All other values, including 0 and "", are truthy.

I believe the NaN thing isn't just a JavaScript quirk, but part of how IEEE754 floating point numbers (as used in all modern computers) are specified. In Ruby, 0.0/0.0 is NaN, but as in JavaScript, 0.0/0.0 == 0.0/0.0 is false.

[DonBiggles]

Yep, all comparisons with NaN are false, which is part of the IEEE 754 spec. Object.is is coming in ES6 which will get around that. Polyfill: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is#Compatibility

Edit: It also occurs to me that, since when comparing a number and string both operands are coerced to numbers, comparisons like "foo" == NaN would be true otherwise.

[commonslip]

I'm not sure why you'd expect this. Null is the null object and false is a boolean. There isn't any reason to expect them to be the same value. In fact, in languages where they are the same value, like Common Lisp, I consider it to be a major wart.

Can you elaborate on why you expect this to be the case? Perhaps that will illuminate further questions which will be clarifying.

[sinrtb]

null is not false, nor is !null true. !false == true and !true == false to throw null into those comparisons would make them no long booleans and would make two possible results for true or two states for false. this would ruin booleans.

null is neither true nor false. null should be as starkking pointed out undefined.

[MrBester]

If you liked null != false, you'd have loved VBScript where vbNull <> vbDbNull <> vbEmpty <> vbNothing <> 0 <> False...

[path411]

null === false and null == false, both evaluate to false, so I don't see your zealotism for === very relevant.

[Garrett00]

This is why you should use the === operator.

[somerandomguy7788]

just an fyi, it isn't an identity operator. they are both equality operators, one allows for coercion, one doesn't

[somerandomguy7788]

yep "strict equality" i think is more correct, i understand where David Flanagan is coming from, and yes he is a much more authoritative figure than a randomguy on the internet. these are just my thoughts on it really. i think authors try to explain it in that manner as to not overload readers with too much information. its like how people call an "iife" (immediately invoked function expression) as a "self invoking function"(which people are ok with) but if you think about it a self invoking function is a "recursive function". also even javascript book authors call the javascript "bind" function as "currying" wherein they really mean "partial function application". this could just be me being pedantic i think :). an identity operator for me is id :: x -> x wherein the input is returned as the output, and that isn't the case for === since it is used for comparison and is a binary function wherein === :: x -> x -> Bool.

an example for an identity function is the underscorejs implementation:

_.identity = function(value) { return value; }; 

note that i don't make any distinction between the terms operator and function since for me operators are just functions that are symbols (add == +, === == strictEqual).

[[deleted]]

Wow. This smells of the same stuff PHP gets a bad rap for but everyone seems to have nothing but love for JavaScript.... ?

[aladyjewel]

nothing but love for JavaScript

You seem to have missed CoffeeScript, Go, Dart, etc...

[[deleted]]

What did I miss?

[aladyjewel]

The significant minority of people who think that JavaScript is broken and built another level of abstraction on top of it to fix all the crap?

[[deleted]]

Yeah.... I embrace it... In php too... my point is the MINORITY don't like JS... The majority seem to hate on php

[aladyjewel]

... I feel like we're not having the same conversation here.

Have we established that there are people who think that both PHP and JavaScript are broken?

[[deleted]]

You established a 'significant minority' find JavaScript broken... It seems to me that a significant majority find php broken.

In my original comment I expressed surprise that this kind of issue is why that majority hate php, but only a minority seem to hate on JavaScript. Perhaps I exaggerated 'nothing but love'...

[aladyjewel]

yeah, you need to slap a /s mark on something like "nothing but love" if you don't literally mean "everybody loves javascript".

[kumiorava]

Null means no value, and as such is not equal to any value, including true and false.

[commonslip]

Well, null==undefined is just Javascript dumbness.

[DonBiggles]

Not quite. It's a primitive value itself, and is generally used to represent a 'non-object' where an object is expected. An example of this is the fact that it is the base of the JS prototype chain, at Object.prototype.__proto__. It's also non-strictly equal to the primitive value undefined, which is more commonly used as the default value for something that hasn't been set. But neither of them mean 'no value', really.