log-a-variable-in-an-arrow-function.md

title	category	date	topics
Log a variable in an arrow function	Trick	2021-02-25 11:57:00 +7	JavaScript

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

• Conditional logging in the Console
• Log a value to the Console
• Log a variable to the console using conditional breakpoints
• Log an array to the Console