is.defined about is HOT 13 CLOSED

If we're seeking readability, how about a not modifier?

is.not.undefined
is.not.null
is.not.arrayLike

Etc...

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The is.undefined() method is clear, since JS has the undefined primitive, but is.defined() could in my mind both mean that it's defined, as in not undefined, but also defined, as in not null or undefined. So I don't really find it clearer than !is.undefined() which negates the name of the primitive.

@ltetzlaff @SamVerschueren @brandon93s Thoughts?

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What would be the expected behavior for is.defined(null)?

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To be true. Essentially same as expect(foo).toBeDefined() when testing

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Thanks for considering it. I agree that there’s definitely some ambiguity, but on the other hand:

• there is also ambiguity in other functions in this package (eg you probably need to read the docs to know what isArrayLike means)
• using “defined” to mean “!undefined” has some precedent as I mentioned, with expect

I found that the “double negative” of “not undefined” still makes me pause and re-read source code so was hoping to streamline it.

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I think it'd be too easy to accidentally misuse an is.defined method regardless of which implementation we choose. Being "defined" doesn't seem to have a concrete definition - at least not amongst the JS community.

falsey truthy nullOrUndefinrd already cover the other use cases, so this would just be a negated version of an existing check which in general I'd like to avoid to minimize the API surface.

"Array-like" isn't the most well known term, but it does have a concrete definition in the community. It's used in the slice documents, for example.

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@brandon93s I like the idea of .not, if it's agreed that defined is too ambiguous.

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is.defined() could in my mind both mean that it's defined, as in not undefined, but also defined, as in not null or undefined.

Also note that undefined is a defined variable. So here is another ambiguity where is.defined() always returns true unless the given variable does not exist

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@brandon93s this seems like the cleanest solution. I support it. At the same time, it is an answer to all use cases where you need to check a negative of an existing predicate.

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I'd probably not use it myself and just go with the good ol' !is.nullOrUndefined() or so but I see the point of the not function for some people I guess, at least given the nature of this repo.

Btw ArrayLike is also defined in lib.d.ts as

interface ArrayLike<T> {
    readonly length: number;
    readonly [n: number]: T;
}

and used pretty widely when "softly" referring to Enumerables but not Iterables or the like.

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I'm going to pass on this as it's too ambiguous.

As for is.not.string, I'm not sure it makes sense, since you can just use !is.string. If someone feels strongly for is.not, convince me otherwise in a new issue ;)

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@sindresorhus How do you assert that a value is defined? ! won't work for assertions, I think?

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@bartzy Assertions did not exist when this issue was discussed. It's generally better to open a new issue than to comment on a very old closed issue.

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