//variables, with no value assigned, are undefined var foo; console.log(foo); //logs undefined //Attempting to access properties that do not exist returns undefined console.log(this.foo); //logs undefined //Passing missing arguments to function, expecting them, results in undefined parameters var myFunction = function f(x){return x} console.log(myFunction()); //logs undefined //Functions return undefined, if no return value is specified var myFunc = function f(){}; console.log(myFunc()); //logs undefined