I've created a table with results of different tests for different values I wanted to share

[alinnert]

First of all: Why is there only a "Q&A" section? I hope it's okay if I share a little "cheat sheet" I've created. At least I've also seen other posts like this. So, here I go:

I've created a table that helps you decide what technique you should use to test a variable and what the results are.

Check	Not defined	null	""	0	[]	false	"x"	1	[1]	true
val	💥	❌	❌	❌	❌	❌	🔹	🔹	🔹	🔹
val == true	💥	❌	❌	❌	❌	❌	🔹	🔹	🔹	🔹
val == false	💥	🔹	🔹	🔹	🔹	🔹	❌	❌	❌	❌
val == null	💥	🔹	🔹	🔹	🔹	🔹	❌	❌	❌	❌
val is null	💥	🔹	❌	❌	❌	❌	❌	❌	❌	❌
val is empty	💥	🔹	🔹	❌	🔹	🔹	❌	❌	❌	❌
val is defined	❌	🔹	🔹	🔹	🔹	🔹	🔹	🔹	🔹	🔹
val ?? ...	❌	❌	🔹	🔹	🔹	🔹	🔹	🔹	🔹	🔹
default(...) filter	❌	❌	❌	🔹	❌	❌	🔹	🔹	🔹	🔹

• 🔹 resolves to true, returns the LHS of ??, or doesn't trigger the default filter
• ❌ resolves to false, returns the RHS of ??, or does trigger the default filter
• 💥 throws an error

The first test (val) means using just the value itself as a test like in {% if val %} or {{ val ? 'yes' : 'no' }}.

There are a few things that are noteworthy:

• false is considered "empty" which I didn't expect
• the default filter triggers for false values, which means it cannot be used to provide a default value for bool options. Like, if you embed or include a template that provides an boolean option parameter you cannot set a default value using the default filter in that template, like: {% set someOption = someOption|default(true) %}. If you pass in ... with { someOption: false } it gets defaulted to true.

I hope it's helpful for some of you.

[stof]

• false is considered "empty" which I didn't expect

This is explicitly documented: https://twig.symfony.com/doc/3.x/tests/empty.html

• the default filter triggers for false values, which means it cannot be used to provide a default value for bool options. Like, if you embed or include a template that provides an boolean option parameter you cannot set a default value using the default filter in that template, like: {% set someOption = someOption|default(true) %}. If you pass in ... with { someOption: false } it gets defaulted to true.

The rule is that default triggers for undefined values and any value that is considered "empty" by the is empty test.

That's one of the reason why ?? was introduced as only null and undefined variables trigger the fallback value in it.

[alinnert]

My post is only meant to be a kind of summary of specific parts of the documentation. I would have posted this in the "Show and tell" section that usually exists in GitHub Discussions. The extra info is really helpful though. So, thanks!