javascript - double negation

javascript 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.

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