!) is a unary operator, used to invert the truth value of its operand. When used twice, known as the double negation operator (
!!), it can be used to convert a value to a boolean.
const x = 1; const y = null; !!x; // true !!y; // false
Using the double negation operator is functionally equivalent to using the
Boolean() function, which we explored in depth in a previous article. In terms of readability and usability, I would still suggest using the
Boolean() function. It conveys the intent of the operation more clearly, and it's easier to understand at a glance.
const x = 1; const y = null; Boolean(x); // true Boolean(y); // false const values = [0, 0, 2, 0, 3]; // Kinda readable, but not great values.filter(x => !!x); // [2, 3] // Arguably more readable values.filter(Boolean); // [2, 3]