js - double negation !!

double-negation-in-javascript-what-is-the-purpose : stackoverflow.com

It casts to boolean. The first ! negates it once, converting values like so:

• undefined to true
• null to true
• +0 to true
• -0 to true
• '' to true
• NaN to true
• false to true
• All other expressions to false

Then the other ! negates it again. A concise cast to boolean, exactly equivalent to ToBoolean simply because ! is defined as its negation. It’s unnecessary here, though, because it’s only used as the condition of the conditional operator, which will determine truthiness in the same way.