When coding in JavaScript, I use Eslint like everybody else. (Let’s set the discussion about Eslint vs. Oxlint for another time.) One problem I have is that sometimes (rarely, but not never) I need to tell Eslint that I broke one of the rules intentionally and I don’t want it to nag me about it. The easiest way is to put a comment saying
// eslint-disable-next-line <name of the rule I broke>
directly above the offending line. Trouble is, I never remember the exact syntax of that line (is it eslint-disable-next-line? or disable-eslint-next-line, or maybe eslint-disable-line…?), not to mention the name of the rule. (Yes, it appears in the echo area. No, you can’t easily copy it from there. Well, you can say C-u C-x =, press RET on the [Show] button next to the flycheck-error property name. Or you can just head to the *Messages* buffer after seeing the rule name in the echo area and copy it from there. Either way, it’s not exactly convenient.)
I figured that having a function to add such a comment automatically would be pretty cool. Of course, I would have to dive in Flycheck internals. As usual in such cases, I decided to start with checking if such a function exists already. Also as usual, I was not disappointed! I have Tide installed, and it turns out that it defines a command tide-add-eslint-disable-next-line which does exactly what I need! One caveat is that the Eslint checker must be enabled in Flycheck for this to work.
That’s it for today, come next week for another post!
CategoryEnglish, CategoryBlog, CategoryEmacs, CategoryJavaScript
Today I have a nice SQL tip about sampling tables in SQL. (I’m writing about PostgreSQL as usual but this applies to other databases, too, since this feature is in the SQL standard.) Imagine you have a large table and you want to get a general feeling about what its contents look like. Saying select * from my_table limit 10 may be a nice idea, but it doesn’t always suffice. For example, if you have a table where you never delete or update rows, only insert them, that query will often just show the earliest 10 rows, and you might want to see how the contents of the table changed over time.
You can say, for example,
select * from my_table tablesample bernoulli (5)
and see about 5 percent of rows from table my_table. (It’s approximate because each row is included in the result with a probability of 5%, so depending on the RNG there may be more or fewer rows in the result.) The mysterious bernoulli keyword tells Postgres that the decision whether to include a row or not must be made for each row individually; if you use system instead, these decisions are made for pages (by default, a page contains 8kB of data, so if your row contains, say, half a kilobyte of data on average, then a page may have about 16 rows – or probably 15, since there is some overhead for the page header), which is “less random” but much faster.
One more thing worth knowing is the repeatable keyword, which allows to provide a random seed. If you say
select * from my_table tablesample bernoulli (5) repeatable (1337)
you will get the exact same 5% of rows of the underlying table every time (provided that you don’t make any changes to its contents, of course).
That’s it for today, see you next time!
Sometimes you need to create an array with the sequence of numbers, say from 0 to 9. Popular libraries like Lodash or Ramda have a function for that (in both cases, it’s called range), but how to do that concisely in vanilla JS? In case you’re wondering, both Lodash and Ramda use a while loop for this task; can we do better? (In this case “better” does not necessarily mean “faster”, but “shorter”, and – let’s be honest — “in a more clever/hackish/crazy way”.) It turns out that the answer is yes. The idea is to use Array.from with an array-like object possessing only the length property. Array.from has an optional parameter – a mapping function, which is called for every element of the “source array”. Of course, in the case of the {length: ...} object there are no elements, so the argument to the mapping function is effectively undefined. However, this function also receives a second argument, which is the index of the element in the “source array” (much like in the regular Array.prototype.map). This means that we can do this:
Array.from({length: 8}, (_, i) => i)
Interestingly, unlike the regular map, the mapping function in Array.from does not receive the whole “source array” as the third argument. Go figure.
Of course, it’s still possible to use this trick to generate sequences not starting at 0 and with step different than 1 – but the code becomes longer. Here is a snippet generating the sequence of all two-digit odd numbers:
Array.from({length: 45}, (_, i) => 11 + i * 2)
Is this all Array.from can do? Of course not. Another use-case is deduplication (via a set):
Array.from(new Set([1, 1, 2, 3, 5, 8])) // => [ 1, 2, 3, 5, 8 ]
Yet another is splitting strings into individual characters. A popular way to achieve that is to say string.split(''), but this won’t work with certain Unicode characters:
'I🧡JS🤣'.split('') // => [ 'I', '\ud83e', '\udde1', 'J', 'S', '\ud83e', '\udd23' ]
Array.from('I🧡JS🤣') // => [ 'I', '🧡', 'J', 'S', '🤣' ]
In this case, Array.from is not indispensable – a similar effect can be achieved with the spread operator:
[...'I🧡JS🤣'] // => [ 'I', '🧡', 'J', 'S', '🤣' ]
but you could argue that Array.from is slightly more idiomatic since it makes the intent of “converting into an array” more explicit.
That’s it for today – see you next week!