Difference between Object.key and Object.prototype.key

Unlike in languages like Python, static properties of the constructor function are not inherited to instances. Instances only inherit from their prototype, which inherits from the parent type's prototype. Static properties are never inherited.

function Foo() {};
Foo.style = 'bold';

var foo = new Foo();

console.log(Foo.style); // 'bold'
console.log(foo.style); // undefined

Foo.prototype.style = 'italic';

console.log(Foo.style); // 'bold'
console.log(foo.style); // 'italic'