What does = +_ mean in JavaScript

javascript

unary-plus-operator

javascript-basics

code-tuning

byAnton Shumikhin·Jan 10, 2025

=_ is not valid in JavaScript and seems like a typo. To add to a variable and assign it simultaneously, we use +=. The unary + is used to convert values to numbers. Here's the correct usage:

let num = 10;
let str = '5';
num += +str; // num becomes 15, str secretly going to the gym, getting all buffed up as a number.

In the above example, += adds 5 to 10 after +str transforms '5' into a number.

Reading the cryptic

The unusual suspect r = +_ looks confusing but it's a cinch once broken down:

r is your chosen variable for the transformed outcome.
+ is the unary plus operator, aiming to convert any following value into a number.
_ represents your source value that ought to be converted.

The _, a string, boolean, or null, will be converted into a number, or NaN if it's unconvertable.

When things go `NaN`

Converting undefined values, non-numeric strings, or complex objects with the unary plus will yield NaN. To catch such confounding results, use this:

let result = +_ // `_` seen at the gym, str spotting...does he have it in him?
if (isNaN(result)) {
  console.log('Resistance was futile, gym membership revoked!');
}

Better variable naming

While _ is used as a variable name here, it doesn't beat expressive names for readable and maintainable code. Avoid being 'under_scored' by more refined variable names.

Tricky mix-ups

Watch out for mix-ups between + and ++ the increment operator, or += which augments and reassigns. Misunderstanding these can bring about unexpected results akin to pouring salt instead of sugar. 😅

Code-tuning tricks

The unary + provides a more concise cast than parseInt() or parseFloat(), potentially trimming code fat and boosting performance.
~ should be avoided for string to integer conversion; it's less conventional and more of a "wild west" approach in JavaScript.
Creative use of comments and eloquent variable names can spare future you from puzzlement over shorthand operations like unary plus.