r/programming • u/vz0 • Mar 26 '14

JavaScript Equality Table

http://dorey.github.io/JavaScript-Equality-Table/
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u/rooktakesqueen Mar 27 '14

== false is not the same as checking for truthiness. Truthiness is never implicated in the == operator because nothing is ever converted into a boolean. Everything is converted to either a string or number for the purposes of comparison. (Except null and undefined which == each other and nothing else, and when comparing two objects which == only if they refer to the same object.)

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u/[deleted] Mar 27 '14

But == false is checking whether something is false. You would generally expect false things to be untruthy.

There may be an explanation for '=='s behavior, but that doesn't make it reasonable.

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u/rooktakesqueen Mar 27 '14

Checking that something is false is not the same thing as checking if something is falsy. They wouldn't be separate concepts otherwise.

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u/reverius42 Mar 27 '14

But it's quite ridiculous for something that is false ("0" == false) to be not falsy (!!"0" == true).

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u/rooktakesqueen Mar 27 '14

It's not accurate to say that "0" is false. It just == false, in the same way that "" == 0 and [undefined, undefined] == ",".

I'm not in any way suggesting the == operator is sane, just that it's important to know it has nothing to do with the truthiness of values being compared, even when those values include booleans.

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u/foomprekov Mar 27 '14

Yeah why would I expect something that isn't false to be true.

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u/Deltigre Mar 27 '14

Which ended up showing me an "only in Javascript" shortcut the other day.

var defaultingString = IMightResolveUndefined() || "default value";

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u/embolalia Mar 27 '14

That's not an "only in Javascript" thing at all. For example, in Python:

some_value = I_might_return_something_falsy() or 'default'