cross join equivalent in JavaScript

Question

In an app that's being developed there is a use case whereby dates need to be generated from an array of months and another array of years. Here is an example of representative data in those arrays:

const months= ["1", "4", "7", "9"];
const years = ["2021", "2022", "2027"];

The necessary result is the first day of each month + year combination, like so:

const dates = [
"1/1/2021", "4/1/2021", "7/1/2021", "9/1/2021",
"1/1/2022", "4/1/2022", "7/1/2022", "9/1/2022",
"1/1/2027", "4/1/2027", "7/1/2027", "9/1/2027"
];

The way I would do this in T-SQL is as follows -

select *
into #months
from
(
    values
    (1),
    (4),
    (7),
    (9)
) d (month);

select *
into #years
from
(
    values
    (2021),
    (2022),
    (2027)
) d (year);

select [dates] = datefromparts(year, month, 1)
from #months
cross join #years;

Output:

So, is there a cross join equivalent in Javascript? I'd rather use a concise approach like that rather than nested .forEach() statements.

joshwilsonvu · Accepted Answer · 2021-06-09 13:15:15Z

1

You can build that result by using Array#map() calls together, like so:

const months= ["1", "4", "7", "9"];
const years = ["2021", "2022", "2027"];
const dates = years.flatMap(year => {
  return months.map(month => `${month}/1/${year}`);
});
console.log(dates);

flatMap() is conceptually the same as map() and flat() together (but more performant).

const months= ["1", "4", "7", "9"];
const years = ["2021", "2022", "2027"];
const dates = years.map(year => {
  return months.map(month => `${month}/1/${year}`);
}).flat();
console.log(dates);

edited Jun 9, 2021 at 13:15

answered Jun 4, 2021 at 18:08

joshwilsonvu

2,71911 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Scott Sauyet Over a year ago

I don't see any difference between the support tables for flat and flatMap. Obviously if you need to support older browsers, you can polyfill either of them or use a reduce-concat solution. But I'm not sure there's any good reason for the .flat solution when .flatMap is clearer and more performant.

joshwilsonvu Over a year ago

You're right, thanks. I hoped it would be helpful for understanding .flatMap, edited to explain that better.

Scott Sauyet · Accepted Answer · 2021-06-04 18:07:55Z

0

We can write a fairly simple crossproduct function that takes two arrays and a function and combines each pair of their product with the function.

It might look like this:

const crossproduct = (xs = [], ys = [], fn = (x, y) => [x, y]) =>
  xs .flatMap ((x) => ys .map ((y) => fn(x, y)))

const months= ["1", "4", "7", "9"]
const years = ["2021", "2022", "2027"]

const dates = crossproduct (years, months, (y, m) => `${m}/1/${y}`)

console .log (dates)

.as-console-wrapper {max-height: 100% !important; top: 0}

Note that crossproduct has a default function that just returns an array containing the two values. This is often useful.

answered Jun 4, 2021 at 18:07

Scott Sauyet

50.9k5 gold badges57 silver badges111 bronze badges

Collectives™ on Stack Overflow

cross join equivalent in JavaScript

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related