Why is this NULL variable truthy?

Question

I've known null to be falsy. Then why does it act as if it is a truthy?

var status = null;

console.log('status:', status);

if(!!status) {
  console.log('status is truthy');   // it should not print
}

if(!!null) {
  console.log('null is truthy');   // it should not print
}

https://stackoverflow.com/questions/27632391/why-null-false-and-null-true-both-return-false answers your question. — Hatted Rooster, Jul 18 '18 at 12:06
@noobie Without looking at documentation. When `!!` converts a value to boolean, how exactly does it do that? What "actions" are performed? — David Tabernero M., Jul 18 '18 at 12:07
Use a different variable name. You are conflicting with [`window.status`](https://developer.mozilla.org/en-US/docs/Web/API/Window/status) which has setters that make it a string. — ASDFGerte, Jul 18 '18 at 12:09
@Federico What Nina means is that the `null` output comes from `console.log(status)`, not from `console.log('null')`. OP's code is misleading — Jeremy Thille, Jul 18 '18 at 12:13
@JeremyThille IMHO it's not. There's an order to those `console.log`s, so it's clear that `null` is printed by the first one and `status` is printed by the second one. — Federico klez Culloca, Jul 18 '18 at 12:14
@JeremyThille then I don't understand what her comment has to do with the question. — Federico klez Culloca, Jul 18 '18 at 12:18
Because OP says `console.log('null') // it should not print`. And it _does not_ print. The `null` we see does not come from here. It comes from `log(status)`. So OP's mislead by his own code — Jeremy Thille, Jul 18 '18 at 12:20
@JeremyThille now I understand, but I didn't read it like this. I think OP *knows* that doesn't print and was just using it to prove that `!!status` should behave like `!!null` and was surprised it didn't. The problem, I think, is that they meant `//does not print` instead. But I'm not in their head, so I'll just let it die here :) — Federico klez Culloca, Jul 18 '18 at 12:31

score 8 · Answer 1 · answered Jul 18 '18 at 12:10

8

The issue is there is already a window.status, with which you conflict. It has setters that always make it a string, which causes your problems.

answered Jul 18 '18 at 12:10

ASDFGerte

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Dear god, I was going nuts why it is initialized as a string when given to variable and stays special value null when not. – DanteTheSmith Jul 18 '18 at 12:13
1

Why status holds "null" as a string? – noobie Jul 18 '18 at 12:13
`window.name` behaves the same way – Adelin Jul 18 '18 at 12:17
see `Object.getOwnPropertyDescriptor(window, "status")`, i am not sure what exactly it does (the setter is native code), but it converts to string, and `String(null) === "null"` – ASDFGerte Jul 18 '18 at 12:20

BlackBeard · Answer 2 · 2018-07-18T12:32:56.127

4

Change variable name status to something else (like status1) and the problem vanishes. This happens due to conflict with status object property of windows.

var status1 = null;


console.log('status1 -->', status1)

if(!!status1) {
  console.log('status')   // it should not print
}

if(!!null) {
  console.log('null')   // it should not print
}

NOTE: No matter what value you assign to window.status it'll get converted back to string. See this:

console.log(typeof window.status)

window.status = 4;   // type Number

console.log(typeof window.status) // still it remains string

edited Jul 18 '18 at 12:32

answered Jul 18 '18 at 12:16

BlackBeard

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1

This is still misleading. The output is `null`, but we can't be sure if it comes from the 1st or the 3rd `console.log`. Change the first one to `console.log("status1 = ", status1)` – Jeremy Thille Jul 18 '18 at 12:18
@JeremyThille edited the answer – BlackBeard Jul 18 '18 at 12:19
2

It adds a working demonstration – Jeremy Thille Jul 18 '18 at 12:22

Why is this NULL variable truthy?

2 Answers2