JavaScript syntax
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• Numbers: 0, -0, NaN,
• Special: null, undefined,
• Boolean: false.
The Boolean function can be used to explicitly convert to a primitive of type Boolean:
// Only empty strings return false
console.log(Boolean("") === false);
console.log(Boolean("false") === true);
console.log(Boolean("0") === true);
// Only zero and NaN return false
console.log(Boolean(NaN) === false);
console.log(Boolean(0) === false);
console.log(Boolean(-0) === false); // equivalent to -1*0
console.log(Boolean(-2) === true);
// All objects return true
console.log(Boolean(this) === true);
console.log(Boolean({}) === true);
console.log(Boolean([]) === true);
// These types return false
console.log(Boolean(null) === false);
console.log(Boolean(undefined) === false); // equivalent to Boolean()
The NOT operator evaluates its operand as a Boolean and returns the negation. Using the operator twice in a row, as a double negative, explicitly converts an expression to a primitive of type Boolean:
console.log( !0 === Boolean(!0));
console.log(Boolean(!0) === !!1);
console.log(!!1 === Boolean(1));
console.log(!!0 === Boolean(0));
console.log(Boolean(0) === !1);
console.log(!1 === Boolean(!1));
console.log(!"" === Boolean(!""));
console.log(Boolean(!"") === !!"s");
console.log(!!"s" === Boolean("s"));
console.log(!!"" === Boolean(""));
console.log(Boolean("") === !"s");
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