Edit in JSFiddle

//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