Add `optional` predicate about ow

[sindresorhus]

I would prefer ow.optional.string. Maybe we should just release ow without the ability to use optional and see what the community can come up with? Maybe someone has a clever solution.

Have you tried it with TS 2.8? I see they introduced conditional types there: https://blogs.msdn.microsoft.com/typescript/2018/03/15/announcing-typescript-2-8-rc/ I'm not sure it helps at all, but thought I would mention it, just in case.

[SamVerschueren]

We really need this :D. I have to use the following construct for now which is ugly.

if (foo.bar) {
    ow(foo.bar, ow.string);
}

if (unicorn.rainbow) {
    ow(unicorn.rainbow, ow.string);
}

Would be much better if we could write

ow(foo.bar, ow.optional.string);
ow(unicorn.rainbow, ow.optional.string);

[SamVerschueren]

@Martin-Pitt can you create a new issue so we can discuss this separately? I'm close to landing a PR for optional so it will get lost.

[sindresorhus]

@hamxabaig There's an open PR for it here: #82 You could help test it and add your feedback. That would help move it along.

[SamVerschueren]

I think I prefer ow.optional.string as well. I will have a look at the conditional feature, looks pretty neat! Thanks for sharing!

[Martin-Pitt]

On the other side of the coin for non-optional, could there be a ow.mandatory that throws by using it as default arg?

function getPage(identifier = ow.mandatory()) {
    ow(identifier, ow.string);
}

Reference: 16. Mandatory Parameter Shorthand

mandatory = () => {
  throw new Error('Missing parameter!');
}

foo = (bar = mandatory()) => {
  return bar;
}

[hamxabaig]

Is this gonna land anytime soon in the library? Definitely makes sense otherwise makes the code very ugly with ifs & elses.

[hamxabaig]

@sindresorhus sure thing, But i'm not that proficient in TypeScript. I'll try to see what i can do 👍

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