Log a variable in an arrow function

Have you ever had to debug an 1 line arrow function using console.log? It usually requires us to switch to a multiple lines version such as:
const formatYmd = (date) => {
console.log(date.toISOString());
return date.toISOString().slice(0, 10);
};
You can get rid of that conversion by using the || operator. It works because console.log() returns undefined, so the || will enforce the function to evaluate and return the right side which is our actual function.
const formatYmd = (date) => console.log(date.toISOString()) || date.toISOString().slice(0, 10);
formatYmd(new Date());
// Print something like `2021-02-25T04:52:39.720Z` in the Console
There is another, less known tip which uses the comma operator:
const formatYmd = date => (console.log(...), date.toISOString().slice(0, 10));
You will find more useful 1 line-of-code functions on 1 LOC

See also