Why use double negation (!!) in JavaScript?
You may have seen the use of !! in JavaScript, here's why.
Double negation is used to convert a "truthy" or "falsy" value into a boolean. It's essentially used to decide if a variable contains a value.
A "truthy" value is something that translates to true when evaluated but might not actually be a boolean.
console.log("Hello" ? true : false); // true
console.log("Hello" === true); // false
The === operator is the "strict equal" operator which only returns true if the operands are equal and of the same type.
console.log(1 == "1"); // true
console.log(1 === "1"); // false
The following values in JavaScript are always "falsy", everything else is "truthy":
- false.
- 0 (zero)
- "" (empty string)
- null
- undefined
- NaN
Some examples:
console.log(!!undefined); // false
console.log(!!null); // false
console.log(!!NaN); // false
console.log(!!1); // true
console.log(!!0); // false
console.log(!!'0'); // true
console.log(!!123); // true
console.log(!!-123); // true
console.log(!!'Hello'); // true
console.log(!!''); // false
