!!foo applies the unary not operator twice and is used to cast to a Boolean type similar to the use of unary plus +foo to cast to a number and concatenating an empty string ''+foo to cast to a string.

One is simple comparison of bool values, when you want "a == b" be equivalent of "a xor not b" except a=5 and b=7 would both be true but not be equal. Another is when you want to coerce a set of conditions into bits of a variable:

Mar 13, 2025 · To explicitly convert its return value (or any expression in general) to the corresponding boolean value, use a double NOT operator (!!) or the Boolean constructor. The following code shows examples of the ! (logical NOT) operator. Double NOT (!!)

Oct 4, 2024 · The !! (double negation) operator is a repetition of the unary logical “not” (!) operator twice. It is used to determine the truthiness of a value and convert it to a boolean (either true or false). Here’s how it works: The single ! (logical “not”) inverts the truth value of a given expression:

Apr 1, 2014 · Why is it necessary to use the double not operator? Do not the && operator already transform the result to a Boolean?

Jun 30, 2023 · In JavaScript, ! is a logical operator that returns false if its single operand can be converted to true; otherwise, it returns true. It's essentially a way to perform a boolean negation. Here's an example: The Double Not Operator. So what about the double not (!!) operator?

Jan 9, 2021 · Wondering what !! means in JavaScript? It is a convenient way to convert a value to a boolean using a double JavaScript not operator.

Sep 29, 2024 · The double not operator (!!) in JavaScript is a concise way to convert any value to its boolean equivalent. This operator is particularly useful when you need to ensure that a value is...

Dec 26, 2023 · The JavaScript double exclamation mark operator, also known as the logical NOT operator, is a unary operator that reverses the logical value of a boolean expression. In other words, if the expression evaluates to true, the operator will return false, and vice versa.

Jun 26, 2024 · The double NOT (!!) operator in JavaScript is a clever trick that many developers use to convert any value into a boolean. While a single NOT (!) negates a value, applying it twice forces JavaScript to return a strict boolean true or false, depending on the value’s truthiness.