Truthy & Falsy Values

The result of !!document.all in modern browsers is false. This is a special historical case in JavaScript. document.all is an array-like object that was used in older browsers to access all elements in a document. Modern browsers still support it for backward compatibility, but they make it falsy (despite being an object, which would normally be truthy). This is one of the very rare exceptions to JavaScript's truthy/falsy rules. According to the ECMAScript specification, document.all has a special [[IsHTMLDDA]] internal slot that makes it behave like undefined in boolean contexts and when compared with undefined, but it still functions as an object when its properties or methods are accessed.