Why is this an issue?

Importing an external library for lightweight operations increases overall size of the program. Using native methods instead reduces the amount of memory and storage to run and store the application. This is especially critical in environments with limited resources, such as on mobile devices or in web applications where bandwidth and download times matter.

Smaller programs generally have better runtime performance. Reducing the number of unnecessary modules minimizes the amount of code that needs to be interpreted or compiled, leading to faster execution and improved overall performance.

Examples

Example with the numbro library, when you use format method.

// Example with numbro (not compliant)
import numbro from "numbro";

numbro.setLanguage('en-GB');
var string = numbro(1000).format({
  thousandSeparated: true,
}); // '1,000'

// Example with numerable (not compliant)
import { format } from "numerable";
format(1000, '0,0');

// Example with Intl (compliant)
new Intl.NumberFormat("en-GB").format(1000); // '1,000'

Limitations

As for now, only two libraries are handled by this rule :

Some candidates for the future developments are :

It’s more likely this rule won’t ever be exhaustive.

Resources

Documentation